Merge branch 'main' into sameh/route-corridor

src/data/motionModel.js

@@ -198,6 +198,157 @@ export function staleCoastLimitSeconds({
   return Math.min(ceiling, Math.max(floor, contactLeadSec + grace));
 }
 
+/** @constant {number} Dead reckoning never moves a contact faster than this
+ *  (1,500 kt, the trail's continuity limit), so a corrupt reported speed
+ *  cannot fly a coasting plane hundreds of kilometres. */
+export const COAST_MAX_SPEED_MPS = (1_500 * 1852) / 3600;
+
+/** @constant {number} A contact with no fresh report coasts at most this long past it, whatever its source limit; there is no distance cap. */
+export const STALE_COAST_MAX_SEC = 300;
+
+/** @constant {number} A report this close to a known ground height holds
+ *  instead of coasting. */
+export const STALE_HOLD_MAX_HEIGHT_M = 150;
+
+/**
+ * Whether a contact whose reports have stopped holds at its last reported
+ * position instead of coasting: it reported on the ground, or a known ground
+ * floor puts it within STALE_HOLD_MAX_HEIGHT_M of the ground (landing or
+ * taking off). A plane that low has no evidence of being anywhere but where
+ * it was last heard. Where the ground is unknown, barometric altitude is not
+ * read as height above it; the contact coasts under the stale cap.
+ * @param {object} report
+ * @param {boolean} [report.onGround]
+ * @param {number} [report.heightM] - Render height (ellipsoidal metres).
+ * @param {number} [report.groundM] - Ellipsoidal ground under the report, if known.
+ * @returns {boolean}
+ */
+export function staleHoldsAtLastReport({ onGround, heightM, groundM } = {}) {
+  if (onGround === true) return true;
+  return (
+    Number.isFinite(heightM) &&
+    Number.isFinite(groundM) &&
+    heightM - groundM < STALE_HOLD_MAX_HEIGHT_M
+  );
+}
+
+/**
+ * The hold-or-coast decision for a report, made once and kept on the report,
+ * so a ground floor that resolves while the contact coasts cannot flip it and
+ * snap the icon back. Height and position come from the report itself.
+ * @param {{position: Cesium.Cartesian3, staleHolds?: boolean}} fix - History
+ *   fix; `staleHolds` is stored on it.
+ * @param {object} args
+ * @param {boolean} [args.onGround] - The contact's ground flag.
+ * @param {(latDeg: number, lonDeg: number) => number|null} [args.cachedGroundFloor]
+ * @returns {boolean}
+ */
+export function staleHoldsForFix(fix, { onGround, cachedGroundFloor } = {}) {
+  if (typeof fix.staleHolds === 'boolean') return fix.staleHolds;
+  const carto = Cesium.Cartographic.fromCartesian(
+    fix.position,
+    Cesium.Ellipsoid.WGS84,
+    _holdCarto,
+  );
+  const groundM =
+    carto && typeof cachedGroundFloor === 'function'
+      ? cachedGroundFloor(
+          Cesium.Math.toDegrees(carto.latitude),
+          Cesium.Math.toDegrees(carto.longitude),
+        )
+      : null;
+  fix.staleHolds = staleHoldsAtLastReport({
+    onGround,
+    heightM: carto?.height,
+    groundM,
+  });
+  return fix.staleHolds;
+}
+
+const _holdCarto = new Cesium.Cartographic();
+
+/**
+ * Seconds a contact coasts past `newest` at a display time `elapsedSec`
+ * after it, counting any coast a synthetic fix already took since the real
+ * report it was built from (`reportEpochMs`), so a chain of synthetic fixes
+ * never coasts further than one report would.
+ * @param {object} args
+ * @param {{epochMs: number, reportEpochMs?: number, velocity?: number}} args.newest
+ * @param {number} args.elapsedSec - Display time past `newest`.
+ * @param {number} args.coastLimitSec - Source limit past `newest`.
+ * @param {boolean} args.holds
+ * @returns {number}
+ */
+export function staleCoastSecondsFrom({
+  newest,
+  elapsedSec,
+  coastLimitSec,
+  holds,
+}) {
+  const alreadySec = Number.isFinite(newest?.reportEpochMs)
+    ? Math.max(0, (newest.epochMs - newest.reportEpochMs) / 1000)
+    : 0;
+  return Math.max(
+    0,
+    staleCoastSeconds({
+      elapsedSec: elapsedSec + alreadySec,
+      coastLimitSec: coastLimitSec + alreadySec,
+      holds,
+    }) - alreadySec,
+  );
+}
+
+/** @constant {number} A coasted position sits at least this far (m) above the ground. */
+export const COAST_FLOOR_LIFT_M = 1.5;
+
+/**
+ * Lift a coasted position to COAST_FLOOR_LIFT_M above the ground where the
+ * ground is known; a coast runs level from the report and terrain may rise
+ * under it.
+ * @param {Cesium.Cartesian3} position - Mutated in place.
+ * @param {(latDeg: number, lonDeg: number) => number|null} [cachedGroundFloor]
+ * @returns {Cesium.Cartesian3}
+ */
+export function floorCoastedPosition(position, cachedGroundFloor) {
+  if (typeof cachedGroundFloor !== 'function') return position;
+  const carto = Cesium.Cartographic.fromCartesian(
+    position,
+    Cesium.Ellipsoid.WGS84,
+    _coastCarto,
+  );
+  if (!carto) return position;
+  const ground = cachedGroundFloor(
+    Cesium.Math.toDegrees(carto.latitude),
+    Cesium.Math.toDegrees(carto.longitude),
+  );
+  if (!Number.isFinite(ground) || carto.height >= ground + COAST_FLOOR_LIFT_M)
+    return position;
+  return Cesium.Cartesian3.fromRadians(
+    carto.longitude,
+    carto.latitude,
+    ground + COAST_FLOOR_LIFT_M,
+    Cesium.Ellipsoid.WGS84,
+    position,
+  );
+}
+
+const _coastCarto = new Cesium.Cartographic();
+
+/**
+ * Seconds to coast past the last report: the elapsed time, bounded by the
+ * source's coast limit and STALE_COAST_MAX_SEC; none for a contact that
+ * holds.
+ * @param {object} args
+ * @param {number} args.elapsedSec - Display time past the last report.
+ * @param {number} args.coastLimitSec - From `staleCoastLimitSeconds`.
+ * @param {boolean} args.holds - From `staleHoldsAtLastReport`.
+ * @returns {number}
+ */
+export function staleCoastSeconds({ elapsedSec, coastLimitSec, holds }) {
+  if (holds || !(elapsedSec > 0)) return 0;
+  return Math.max(0, Math.min(elapsedSec, coastLimitSec, STALE_COAST_MAX_SEC));
+}
+
 /**
  * Shortest-arc rate limiter: step `prevDeg` toward `targetDeg` by at most
  * `maxDegPerSec * dtSec`. Seeds directly on the target when prev is null.
@@ -446,12 +597,21 @@ const _forwardFixOffset = new Cesium.Cartesian3();
  * kinematics. This prevents a late course change from reprojecting the entire
  * stale interval and snapping the rendered contact by kilometres.
  *
+ * The same stale rules as the display apply: no fix for a contact that holds
+ * at its report (`staleHoldsForFix`, decided before calling), a coast of at
+ * most STALE_COAST_MAX_SEC counted from the real report
+ * (`reportEpochMs` carries it along a chain), and never below known ground.
+ * The result is a motion fix only, never a report for the trail.
+ *
  * @param {{time: Cesium.JulianDate, epochMs?: number, position: Cesium.Cartesian3,
- *   velocity?: number, track?: number}} newest Existing immutable history fix.
- * @param {{epochMs: number, velocity: number, track: number, turnRateDps?: number}} next
+ *   velocity?: number, track?: number, staleHolds?: boolean, reportEpochMs?: number}} newest
+ *   Existing immutable history fix.
+ * @param {{epochMs: number, velocity: number, track: number, turnRateDps?: number,
+ *   cachedGroundFloor?: Function}} next
  *   New kinematics and the wall/source epoch at which they become authoritative.
  * @returns {{time: Cesium.JulianDate, epochMs: number, position: Cesium.Cartesian3,
- *   velocity: number, track: number}|null} Forward synthetic fix, or null.
+ *   velocity: number, track: number, staleHolds?: boolean, reportEpochMs: number}|null}
+ *   Forward synthetic fix, or null.
  */
 export function synthesizeForwardKinematicsFix(newest, next = {}) {
   if (!newest?.position || !newest?.time || !Number.isFinite(next.epochMs))
@@ -461,12 +621,18 @@ export function synthesizeForwardKinematicsFix(newest, next = {}) {
     : Cesium.JulianDate.toDate(newest.time).getTime();
   if (!Number.isFinite(startEpochMs) || next.epochMs <= startEpochMs)
     return null;
+  if (newest.staleHolds === true) return null;
 
   const previousVelocity = Number.isFinite(newest.velocity)
     ? Math.max(0, newest.velocity)
     : 0;
   const previousTrack = Number.isFinite(newest.track) ? newest.track : 0;
-  const dtSec = (next.epochMs - startEpochMs) / 1000;
+  const dtSec = staleCoastSecondsFrom({
+    newest: { epochMs: startEpochMs, reportEpochMs: newest.reportEpochMs },
+    elapsedSec: (next.epochMs - startEpochMs) / 1000,
+    coastLimitSec: Infinity,
+    holds: false,
+  });
   arcOffsetEnu(
     previousVelocity,
     previousTrack,
@@ -485,14 +651,21 @@ export function synthesizeForwardKinematicsFix(newest, next = {}) {
     Cesium.Ellipsoid.WGS84,
     _forwardFixEnu,
   );
-  const position = Cesium.Matrix4.multiplyByPoint(
-    enu,
-    _forwardFixOffset,
-    new Cesium.Cartesian3(),
+  const position = floorCoastedPosition(
+    Cesium.Matrix4.multiplyByPoint(
+      enu,
+      _forwardFixOffset,
+      new Cesium.Cartesian3(),
+    ),
+    next.cachedGroundFloor,
   );
   return {
     time: Cesium.JulianDate.fromDate(new Date(next.epochMs)),
     epochMs: next.epochMs,
+    reportEpochMs: Number.isFinite(newest.reportEpochMs)
+      ? newest.reportEpochMs
+      : startEpochMs,
+    staleHolds: newest.staleHolds,
     position,
     velocity: Number.isFinite(next.velocity)
       ? Math.max(0, next.velocity)

src/data/routeSegment.test.mjs

@@ -140,7 +140,10 @@ test('the actual native tracked label hides the wrong leg and keeps a healthy ro
     ].map((name) => [name, {}]),
   );
   services.aircraftPresentation = { isTr3b: () => false };
-  const parts = { queries: { _contactLabel: (_, info) => info.callsign } };
+  const parts = {
+    queries: { _contactLabel: (_, info) => info.callsign },
+    motion: { _signalLost: () => null },
+  };
   const tracking = createTracking({ flightState, services, parts });
   for (const [position, expected] of [
     [beyond, false],

src/data/trailContinuity.js

@@ -0,0 +1,607 @@
+/**
+ * @module trailContinuity
+ * @description Where a tracked aircraft's trail lifts the pen.
+ *
+ * A trail is a chronological list of fixes `{position, timeMs, nullAltitude,
+ * breakBefore, drawn}`: reports, and (`drawn`) points where the tracked icon
+ * was drawn between them. Between two consecutive fixes that are not one path, a new
+ * segment starts and nothing is drawn between them: no dashes, no arcs.
+ * Nothing is dropped except the oldest fixes past the layer's point limit.
+ *
+ * Two points are not one path when time does not advance, when the gap is
+ * longer than 48 minutes (a coverage hole or a separate flight), when the
+ * implied great-circle speed is over 1,500 kt, or when either point has no
+ * reported altitude. A parked aircraft is the exception: a pair under three
+ * hours apart within 0.1 nm stays one path.
+ *
+ * Civil and military flight tracking share these rules.
+ */
+import * as Cesium from 'cesium';
+
+/** @constant {number} A gap longer than this is a coverage hole or a separate flight. */
+export const TRAIL_CONTINUITY_MAX_GAP_MS = 48 * 60_000;
+
+/** @constant {number} A pair implying more than this great-circle speed is not one path. */
+export const TRAIL_CONTINUITY_MAX_SPEED_KT = 1_500;
+
+/** @constant {number} A parked aircraft may stop reporting this long and stay one path. */
+export const TRAIL_STATIONARY_HOLD_MAX_GAP_MS = 3 * 60 * 60_000;
+
+/** @constant {number} How close (nautical miles) two points must be to count as parked. */
+export const TRAIL_STATIONARY_HOLD_MAX_DISTANCE_NM = 0.1;
+
+/** @constant {number} The head never joins a body point farther than this from the aircraft. */
+export const TRAIL_HEAD_JOIN_MAX_M = 150_000;
+
+const METRES_PER_NM = 1852;
+
+/** @constant {number} Mean Earth radius (m) for great-circle trail distances. */
+const EARTH_MEAN_RADIUS_M = 6_371_008.8;
+
+/**
+ * Whether two consecutive trail points belong to different paths. Time that
+ * does not advance, a gap over TRAIL_CONTINUITY_MAX_GAP_MS or an implied
+ * speed over TRAIL_CONTINUITY_MAX_SPEED_KT breaks the line, except for a
+ * parked aircraft: a pair under three hours apart within 0.1 nm.
+ * @param {number} deltaMs - Later point's time minus the earlier one's.
+ * @param {number} distanceM - Great-circle distance between the points.
+ * @returns {boolean}
+ */
+export function trailPairBreaks(deltaMs, distanceM) {
+  if (!(deltaMs > 0) || !Number.isFinite(distanceM)) return true;
+  const distanceNm = distanceM / METRES_PER_NM;
+  if (
+    deltaMs <= TRAIL_STATIONARY_HOLD_MAX_GAP_MS &&
+    distanceNm <= TRAIL_STATIONARY_HOLD_MAX_DISTANCE_NM
+  )
+    return false;
+  return (
+    deltaMs > TRAIL_CONTINUITY_MAX_GAP_MS ||
+    distanceNm / (deltaMs / 3_600_000) > TRAIL_CONTINUITY_MAX_SPEED_KT
+  );
+}
+
+/**
+ * Whether the head segment may join the last body point to the aircraft:
+ * within TRAIL_HEAD_JOIN_MAX_M and not broken under `trailPairBreaks`.
+ * @param {number} deltaMs - Current time minus the body point's time.
+ * @param {number} distanceM - Distance from the body point to the aircraft.
+ * @returns {boolean}
+ */
+export function trailHeadJoins(deltaMs, distanceM) {
+  return (
+    distanceM <= TRAIL_HEAD_JOIN_MAX_M && !trailPairBreaks(deltaMs, distanceM)
+  );
+}
+
+/**
+ * Great-circle distance between two trail positions, in metres.
+ * @param {Cesium.Cartesian3} a
+ * @param {Cesium.Cartesian3} b
+ * @returns {number}
+ */
+export function trailDistanceM(a, b) {
+  return Cesium.Cartesian3.angleBetween(a, b) * EARTH_MEAN_RADIUS_M;
+}
+
+/**
+ * Whether the pen lifts between two consecutive fixes.
+ * @param {object|undefined} prev - Earlier fix, if any.
+ * @param {object} next - Later fix.
+ * @returns {boolean}
+ */
+export function trailFixBreaks(prev, next) {
+  if (!prev || prev.nullAltitude || next.nullAltitude) return true;
+  // A recorded point is where the icon was drawn; the speed between it and a
+  // report it was eased onto is not a flight speed, so only the gap counts.
+  if (prev.drawn || next.drawn)
+    return Math.abs(next.timeMs - prev.timeMs) > TRAIL_CONTINUITY_MAX_GAP_MS;
+  return trailPairBreaks(
+    next.timeMs - prev.timeMs,
+    trailDistanceM(prev.position, next.position),
+  );
+}
+
+/**
+ * Build backfill fixes from chronological waypoints. Every waypoint with a
+ * position is kept. A waypoint without one (no altitude and no known ground)
+ * is skipped, and the line still breaks there.
+ * @param {{position: Cesium.Cartesian3|null, timeMs: number, nullAltitude: boolean}[]} waypoints
+ * @returns {{fixes: object[], breakAfter: boolean}} The fixes, and whether the
+ *   line breaks after the last one because trailing waypoints were skipped.
+ */
+export function trailBackfillFixes(waypoints) {
+  const fixes = [];
+  let skipped = false;
+  for (const { position, timeMs, nullAltitude } of waypoints) {
+    if (!position) {
+      skipped = true;
+      continue;
+    }
+    const fix = { position, timeMs, nullAltitude: !!nullAltitude };
+    fix.breakBefore = skipped || trailFixBreaks(fixes[fixes.length - 1], fix);
+    skipped = false;
+    fixes.push(fix);
+  }
+  return { fixes, breakAfter: skipped };
+}
+
+/**
+ * Put backfill fixes ahead of the live trail. The join follows the same
+ * rule, so a distant history tail never connects to the first live fix.
+ * Past `maxPoints` the oldest recorded points go first, then the oldest
+ * reports.
+ * @param {{fixes: object[], breakAfter: boolean}} backfill
+ * @param {object[]} live - Live fixes; the first one's break is recomputed.
+ * @param {number} maxPoints - Point limit across all segments.
+ * @returns {object[]} The joined trail.
+ */
+export function joinTrailBackfill({ fixes, breakAfter }, live, maxPoints) {
+  if (!fixes.length) return live;
+  if (live.length)
+    live[0].breakBefore =
+      breakAfter || trailFixBreaks(fixes[fixes.length - 1], live[0]);
+  return keepTrailLimit(fixes.concat(live), maxPoints);
+}
+
+/**
+ * How many fixes the drawn body holds: every fix at or before the display
+ * time, where the aircraft is drawn. Fixes ahead of it (reports newer than
+ * the delayed display) are reached by the head segment instead, so the trail
+ * never runs past the aircraft, and once the display passes the newest
+ * report the body ends there.
+ * @param {object[]} fixes
+ * @param {number} displayMs - Time the aircraft is drawn at.
+ * @returns {number}
+ */
+export function trailBodyCount(fixes, displayMs) {
+  let count = fixes.length;
+  while (count > 0 && fixes[count - 1].timeMs > displayMs) count -= 1;
+  return count;
+}
+
+/**
+ * Leave out the newest recorded points that lie inside the aircraft's 3D
+ * model, so the body never runs through the fuselage; the head then runs
+ * from the newest point outside the model to the plane. Reports are kept.
+ * @param {object[]} fixes
+ * @param {number} count - From `trailBodyCount`.
+ * @param {Cesium.Cartesian3|null} modelCenter - The model's centre, or null
+ *   when no model is the visual.
+ * @param {number|null} envelopeM - The model's radius (m).
+ * @returns {number}
+ */
+export function trailBodyCountOutsideModel(
+  fixes,
+  count,
+  modelCenter,
+  envelopeM,
+) {
+  if (!modelCenter || !(envelopeM > 0)) return count;
+  while (
+    count > 0 &&
+    fixes[count - 1].drawn &&
+    Cesium.Cartesian3.distance(fixes[count - 1].position, modelCenter) <
+      envelopeM
+  )
+    count -= 1;
+  return count;
+}
+
+/**
+ * The drawn trail body, split at breaks: the first `count` fixes.
+ * @param {object[]} fixes
+ * @param {number} count - From `trailBodyCount`.
+ * @returns {Cesium.Cartesian3[][]}
+ */
+export function trailBodySegments(fixes, count) {
+  const segments = [];
+  for (let i = 0; i < count; i++) {
+    const fix = fixes[i];
+    // A live report with no altitude keeps no position; it only breaks the line.
+    if (!fix.position) continue;
+    if (fix.breakBefore || !segments.length) segments.push([]);
+    segments[segments.length - 1].push(fix.position);
+  }
+  return segments;
+}
+
+/**
+ * Draw body segments. A trail without breaks support draws only the newest
+ * segment, never a line across a break.
+ * @param {{setSegments?: Function, setPositions: Function}} trail
+ * @param {Cesium.Cartesian3[][]} segments
+ */
+export function drawTrailSegments(trail, segments) {
+  if (typeof trail.setSegments === 'function') trail.setSegments(segments);
+  else trail.setPositions(segments[segments.length - 1] ?? []);
+}
+
+/** @constant {number} A head shorter than this (m) is not drawn: the aircraft is holding at its last report. */
+const TRAIL_HEAD_MIN_M = 0.5;
+
+/**
+ * The body fix the head segment starts from, or null when the head is not
+ * drawn: no body yet, the aircraft holding at that fix, a body point more
+ * than 150 km from the aircraft, or one too long ago or too far away to be
+ * one path.
+ * @param {object[]} fixes
+ * @param {Cesium.Cartesian3} head - Displayed aircraft position.
+ * @param {number} nowMs - Current time.
+ * @param {number} count - Body fixes, from `trailBodyCount`.
+ * @returns {object|null}
+ */
+export function trailHeadStartFix(fixes, head, nowMs, count) {
+  if (count < 1 || !head) return null;
+  const last = fixes[count - 1];
+  if (!last.position) return null;
+  if (Cesium.Cartesian3.distance(last.position, head) < TRAIL_HEAD_MIN_M)
+    return null;
+  return trailHeadJoins(
+    nowMs - last.timeMs,
+    trailDistanceM(last.position, head),
+  )
+    ? last
+    : null;
+}
+
+/**
+ * Epoch ms of a position-history fix (`{epochMs?, time}`).
+ * @param {object|undefined} fix
+ * @returns {number} NaN when unknown.
+ */
+export function historyFixEpochMs(fix) {
+  if (Number.isFinite(fix?.epochMs)) return fix.epochMs;
+  return fix?.time ? Cesium.JulianDate.toDate(fix.time).getTime() : NaN;
+}
+
+/** @constant {number} The tracked icon's drawn position is recorded at least this often (ms of display time)... */
+export const TRAIL_RECORD_STEP_MS = 1_000;
+
+/** @constant {number} ...or whenever it has moved this far (m), whichever comes first. */
+export const TRAIL_RECORD_STEP_M = 150;
+
+/** @constant {number} A recorded point is kept only where dropping it would move the line by more than this (m) across the track or in height. */
+export const TRAIL_THIN_TOLERANCE_M = 30;
+
+/** @constant {number} A recorded point is kept where dropping it would leave a straight piece longer than this (m), so a long leg still has vertices. */
+export const TRAIL_THIN_MAX_SPAN_M = 10_000;
+
+/** @constant {number} A kept point stands in for at most this many dropped positions; past it the point is kept. */
+const TRAIL_THIN_MAX_ABSORBED = 512;
+
+/** @constant {number} A recorded point is dropped where the line turns back sharper than this (degrees)... */
+export const TRAIL_SPUR_TURN_DEG = 120;
+
+/** @constant {number} ...over a step shorter than this (m). */
+export const TRAIL_SPUR_MAX_STEP_M = 500;
+
+/** @constant {number} A recorded point closer than this (m) to the previous one is not kept: the aircraft is holding. */
+const TRAIL_RECORD_MIN_STEP_M = 1;
+
+const _u = new Cesium.Cartesian3();
+const _v = new Cesium.Cartesian3();
+
+/** Cosine of the turn at `b` on the way a → b → c, or NaN for a zero step. */
+function turnCos(a, b, c) {
+  Cesium.Cartesian3.subtract(b, a, _u);
+  Cesium.Cartesian3.subtract(c, b, _v);
+  const lu = Cesium.Cartesian3.magnitude(_u);
+  const lv = Cesium.Cartesian3.magnitude(_v);
+  if (!(lu > 0 && lv > 0)) return NaN;
+  return Cesium.Cartesian3.dot(_u, _v) / (lu * lv);
+}
+
+const SPUR_COS = Math.cos(Cesium.Math.toRadians(TRAIL_SPUR_TURN_DEG));
+
+/** Whether entry `i` and its neighbours are one drawn piece of line. */
+function joinedAt(fixes, i) {
+  const a = fixes[i - 1];
+  const b = fixes[i];
+  const c = fixes[i + 1];
+  return (
+    a?.position &&
+    b?.position &&
+    c?.position &&
+    !b.breakBefore &&
+    !c.breakBefore
+  );
+}
+
+/** Whether the line turns back on itself at entry `i` over a short step. */
+function spurAt(fixes, i) {
+  if (!joinedAt(fixes, i)) return false;
+  const a = fixes[i - 1].position;
+  const b = fixes[i].position;
+  const c = fixes[i + 1].position;
+  if (
+    Math.min(
+      Cesium.Cartesian3.distance(a, b),
+      Cesium.Cartesian3.distance(b, c),
+    ) > TRAIL_SPUR_MAX_STEP_M
+  )
+    return false;
+  return turnCos(a, b, c) < SPUR_COS;
+}
+
+/** Remove entry `i` and re-judge the break before the entry that takes its place. */
+function removeAt(fixes, i) {
+  fixes.splice(i, 1);
+  if (i < fixes.length)
+    fixes[i].breakBefore = trailFixBreaks(fixes[i - 1], fixes[i]);
+}
+
+/**
+ * Keep the point limit: drop the oldest recorded point first, and the oldest
+ * report only when no recorded point is left.
+ * @param {object[]} fixes - The trail, mutated.
+ * @param {number} maxPoints
+ * @returns {object[]} `fixes`.
+ */
+function keepTrailLimit(fixes, maxPoints) {
+  let excess = fixes.length - maxPoints;
+  if (excess <= 0) return fixes;
+  for (let i = 0; i < fixes.length && excess > 0;) {
+    if (fixes[i].drawn) {
+      removeAt(fixes, i);
+      excess -= 1;
+    } else i += 1;
+  }
+  if (excess > 0) fixes.splice(0, excess);
+  return fixes;
+}
+
+const _ab = new Cesium.Cartesian3();
+const _ap = new Cesium.Cartesian3();
+const _off = new Cesium.Cartesian3();
+const _up = new Cesium.Cartesian3();
+
+/** Whether `p` lies within the thinning tolerance of segment a → b, across the track and in height. */
+function nearSegment(p, a, b) {
+  Cesium.Cartesian3.subtract(b, a, _ab);
+  Cesium.Cartesian3.subtract(p, a, _ap);
+  const len2 = Cesium.Cartesian3.magnitudeSquared(_ab);
+  const t =
+    len2 > 0
+      ? Math.max(0, Math.min(1, Cesium.Cartesian3.dot(_ap, _ab) / len2))
+      : 0;
+  Cesium.Cartesian3.multiplyByScalar(_ab, t, _off);
+  Cesium.Cartesian3.subtract(_ap, _off, _off);
+  Cesium.Cartesian3.normalize(p, _up);
+  const vertical = Cesium.Cartesian3.dot(_off, _up);
+  const across = Math.sqrt(
+    Math.max(0, Cesium.Cartesian3.magnitudeSquared(_off) - vertical * vertical),
+  );
+  return (
+    Math.abs(vertical) <= TRAIL_THIN_TOLERANCE_M &&
+    across <= TRAIL_THIN_TOLERANCE_M
+  );
+}
+
+/**
+ * Drop recorded point `i` when the line without it stays within the
+ * tolerance of every position it stood for, and the straight piece left is
+ * no longer than TRAIL_THIN_MAX_SPAN_M. The positions it stood for go on to
+ * the next entry. Reports are never dropped.
+ * @returns {boolean} Whether it was dropped.
+ */
+function thinAt(fixes, i) {
+  const entry = fixes[i];
+  if (!entry?.drawn || entry.sealed || !joinedAt(fixes, i)) return false;
+  const a = fixes[i - 1].position;
+  const next = fixes[i + 1];
+  if (next.sealed) return false;
+  const b = next.position;
+  if (Cesium.Cartesian3.distance(a, b) > TRAIL_THIN_MAX_SPAN_M) return false;
+  const stood = entry.absorbed ?? [];
+  const nextStood = next.absorbed ?? [];
+  if (stood.length + nextStood.length + 1 > TRAIL_THIN_MAX_ABSORBED)
+    return false;
+  if (!nearSegment(entry.position, a, b)) return false;
+  for (const p of stood) if (!nearSegment(p.position, a, b)) return false;
+  for (const p of nextStood) if (!nearSegment(p.position, a, b)) return false;
+  next.absorbed = stood.concat(
+    [{ position: entry.position, timeMs: entry.timeMs }],
+    nextStood,
+  );
+  removeAt(fixes, i);
+  return true;
+}
+
+/**
+ * Thin the recorded points either side of entry `i`, reaching past a
+ * neighbouring report: the point recorded just before a report is only
+ * judged once the line goes on beyond it.
+ */
+function thinAround(fixes, i) {
+  if (!thinAt(fixes, i + 1) && fixes[i + 1] && !fixes[i + 1].drawn)
+    thinAt(fixes, i + 2);
+  if (thinAt(fixes, i - 1)) i -= 1;
+  else if (fixes[i - 1] && !fixes[i - 1].drawn) thinAt(fixes, i - 2);
+}
+
+/**
+ * Drop recorded points that make a spur next to entry `i`. Reports are never
+ * dropped; at a report the recorded neighbour is dropped instead.
+ * @returns {number} The index entry `i` ends up at.
+ */
+function removeSpursAround(fixes, i) {
+  for (let guard = 0; guard < 64; guard++) {
+    let changed = false;
+    for (const j of [i - 1, i, i + 1]) {
+      if (!spurAt(fixes, j)) continue;
+      let drop = -1;
+      if (fixes[j].drawn) drop = j;
+      else if (fixes[j + 1]?.drawn) drop = j + 1;
+      else if (fixes[j - 1]?.drawn) drop = j - 1;
+      if (drop < 0) continue;
+      removeAt(fixes, drop);
+      if (drop < i) i -= 1;
+      changed = true;
+      break;
+    }
+    if (!changed) break;
+  }
+  return i;
+}
+
+/** Index a fix at `timeMs` goes in at: after every entry at or before it. */
+function insertionIndex(fixes, timeMs) {
+  let i = fixes.length;
+  while (i > 0 && fixes[i - 1].timeMs > timeMs) i -= 1;
+  return i;
+}
+
+/**
+ * Insert an entry at `i`, judge both breaks, remove spurs, thin the recorded
+ * points either side and keep the limit.
+ */
+function insertAt(fixes, i, entry, maxPoints) {
+  fixes.splice(i, 0, entry);
+  entry.breakBefore = trailFixBreaks(fixes[i - 1], entry);
+  const after = fixes[i + 1];
+  if (after) {
+    after.breakBefore = trailFixBreaks(entry, after);
+    // Positions the next entry stood for that come before this one are now
+    // on this entry's side.
+    if (after.absorbed?.length) {
+      const mine = after.absorbed.filter((p) => p.timeMs < entry.timeMs);
+      if (mine.length) {
+        entry.absorbed = mine;
+        after.absorbed = after.absorbed.filter((p) => p.timeMs >= entry.timeMs);
+      }
+    }
+  }
+  i = removeSpursAround(fixes, i);
+  thinAround(fixes, i);
+  keepTrailLimit(fixes, maxPoints);
+}
+
+/** Index of the newest report before entry `i`, or -1. */
+function reportBefore(fixes, i) {
+  let j = i - 1;
+  while (j >= 0 && fixes[j].drawn) j -= 1;
+  return j;
+}
+
+/** Index of the oldest report at or after `i`, or `fixes.length`. */
+function reportFrom(fixes, i) {
+  let j = i;
+  while (j < fixes.length && fixes[j].drawn) j += 1;
+  return j;
+}
+
+/**
+ * Put a report into the trail in time order. Reports are the record: they
+ * are kept as anchors among the recorded points.
+ *
+ * - A report the display has already passed replaces the guess drawn after
+ *   it: the recorded points timed after it go, and nothing is recorded until
+ *   the icon has eased onto the correction (`displayMs` + `settleMs`), so the
+ *   line reads arc, report, corrected path.
+ * - Recorded points never join two reports that break under
+ *   `trailPairBreaks` (too long or too fast apart to be one flight): the
+ *   points between them go and the line breaks.
+ * - A live report with no altitude keeps no position and only breaks the
+ *   line.
+ * @param {object[]} fixes - The trail, mutated.
+ * @param {Cesium.Cartesian3} position - Owned position.
+ * @param {number} timeMs - The report's epoch ms.
+ * @param {number} maxPoints - Point limit across all segments.
+ * @param {{nullAltitude?: boolean, displayMs?: number, settleMs?: number}} [options]
+ *   `displayMs`: the time the aircraft is drawn at as the report arrives;
+ *   `settleMs`: how long the icon takes to ease onto a correction.
+ * @returns {object[]} `fixes`.
+ */
+export function insertTrailFix(
+  fixes,
+  position,
+  timeMs,
+  maxPoints,
+  { nullAltitude = false, displayMs = NaN, settleMs = 0 } = {},
+) {
+  const entry = {
+    position: nullAltitude ? null : position,
+    timeMs,
+    nullAltitude,
+    drawn: false,
+    breakBefore: false,
+  };
+  let i = insertionIndex(fixes, timeMs);
+  // The guess drawn after a report the display already passed.
+  fixes.splice(i, reportFrom(fixes, i) - i);
+  if (displayMs > timeMs) entry.settleUntilMs = displayMs + settleMs;
+  // Recorded points across a pair of reports that are not one flight.
+  const before = reportBefore(fixes, i);
+  if (before >= 0 && trailFixBreaks(fixes[before], entry)) {
+    fixes.splice(before + 1, i - before - 1);
+    i = before + 1;
+  }
+  insertAt(fixes, i, entry, maxPoints);
+  return fixes;
+}
+
+/**
+ * Record where the tracked aircraft was drawn at a display time. Between
+ * reports this recorded path is the trail, so the line follows every turn
+ * the icon made instead of a straight chord between two reports. Points are
+ * thinned by shape: one stays only where dropping it would move the line by
+ * more than TRAIL_THIN_TOLERANCE_M across the track or in height, or leave a
+ * straight piece longer than TRAIL_THIN_MAX_SPAN_M. Nothing is recorded
+ * while the aircraft holds.
+ * @param {object[]} fixes - The trail, mutated.
+ * @param {Cesium.Cartesian3} position - Owned drawn position.
+ * @param {number} timeMs - Display time it was drawn for.
+ * @param {number} maxPoints - Point limit across all segments.
+ * @returns {boolean} Whether the trail changed.
+ */
+export function recordTrailPoint(fixes, position, timeMs, maxPoints) {
+  const i = insertionIndex(fixes, timeMs);
+  const prev = fixes[i - 1];
+  if (
+    prev?.position &&
+    Cesium.Cartesian3.distance(prev.position, position) <
+      TRAIL_RECORD_MIN_STEP_M
+  )
+    return false;
+  const entry = {
+    position,
+    timeMs,
+    nullAltitude: false,
+    drawn: true,
+    breakBefore: false,
+  };
+  insertAt(fixes, i, entry, maxPoints);
+  // Entries before the second report behind this point can never be thinned
+  // again, so they let go of the positions they stood for.
+  const passed = reportBefore(fixes, fixes.lastIndexOf(entry));
+  if (passed >= 0) sealThrough(fixes, reportBefore(fixes, passed));
+  return true;
+}
+
+/** Release the absorbed positions of entry `i` and everything before it. */
+function sealThrough(fixes, i) {
+  for (let j = i; j >= 0 && !fixes[j].sealed; j--) {
+    fixes[j].sealed = true;
+    delete fixes[j].absorbed;
+  }
+}
+
+/**
+ * Whether the icon's drawn position must not be recorded at a display time:
+ * a report on either side of it has no altitude (the icon is drawn at an
+ * invented height), the reports on either side are not one flight
+ * (`trailPairBreaks`), or the icon is still easing onto a report that
+ * arrived after the display had passed it.
+ * @param {object[]} fixes
+ * @param {number} displayMs
+ * @returns {boolean}
+ */
+export function trailRecordingBlocked(fixes, displayMs) {
+  const i = insertionIndex(fixes, displayMs);
+  const before = fixes[reportBefore(fixes, i)];
+  const after = fixes[reportFrom(fixes, i)];
+  if (before?.nullAltitude || after?.nullAltitude) return true;
+  if (before?.settleUntilMs > displayMs) return true;
+  return !!(before && after && trailFixBreaks(before, after));
+}

src/data/trailRenderer.js

@@ -14,6 +14,18 @@
  *    the whole history renders at one readable alpha (dimmer where it passes
  *    behind/below geometry).
  *
+ * A trail with breaks draws each segment separately and nothing between
+ * them. Every segment is cut into chunks of up to CHUNK_POINTS points;
+ * adjacent chunks share their boundary point, so the line has no seam. A
+ * chunk that can no longer change (every chunk except the newest one of the
+ * newest segment) is FROZEN: an entity with constant positions, built once as
+ * static geometry, so it costs nothing per frame. Only the newest chunk stays
+ * dynamic. A freshly frozen chunk is also drawn dynamically until its static
+ * geometry is in the scene, so freezing never shows a blank frame. A chunk is
+ * identified by its position objects, so a chunk survives every update that
+ * keeps those points: appends, a backfill prepended ahead of it, or the
+ * oldest points dropping off the front (which rebuilds only the first chunk).
+ *
  * Trails update at poll cadence (~15-60 s) plus once on history backfill, so
  * assigning a fresh positions array per update is cheap. `allowPicking` has
  * no entity equivalent; the polyline entity is excluded from clicks by never
@@ -42,6 +54,13 @@ const TRAIL_ALPHA = 0.85;
 const TRAIL_OCCLUDED_ALPHA = 0.4;
 /** @constant {number} Squared distance (m^2) below which consecutive points are merged. */
 const MIN_SEGMENT_DISTANCE_SQ = 0.01;
+/** @constant {Cesium.Cartesian3[]} What a spare dynamic polyline draws. */
+const NO_POSITIONS = Object.freeze([]);
+/** @constant {number} Points per chunk; adjacent chunks share one point. */
+export const CHUNK_POINTS = 128;
+/** @constant {number} Visible frames a frozen chunk's stand-in waits for its
+ *  static geometry before it is dropped anyway. */
+const STAND_IN_MAX_FRAMES = 600;
 
 /**
  * Create an always-visible polyline trail bound to a viewer.
@@ -49,93 +68,285 @@ const MIN_SEGMENT_DISTANCE_SQ = 0.01;
  * @param {object} options - Trail options.
  * @param {string} options.color - CSS color string for the trail hue.
  * @param {number} [options.width=2.5] - Polyline width in pixels.
- * @returns {{setPositions: function(Cesium.Cartesian3[]): void, setVisible: function(boolean): void, clear: function(): void, destroy: function(): void}}
- *   Trail handle: setPositions replaces the geometry, setVisible temporarily
- *   hides it without discarding history, clear empties it, and destroy removes
- *   the entity permanently.
+ * @returns {{setPositions: function(Cesium.Cartesian3[]): void, setSegments: function(Cesium.Cartesian3[][]): void, setVisible: function(boolean): void, clear: function(): void, destroy: function(): void}}
+ *   Trail handle: setPositions replaces the geometry with one line,
+ *   setSegments with one line per segment, setVisible temporarily hides it
+ *   without discarding history, clear empties it, and destroy removes the
+ *   entities permanently.
  */
 export function createTrail(viewer, { color, width = 2.5 }) {
   const baseColor = Cesium.Color.fromCssColorString(color);
-  /** @type {Cesium.Cartesian3[]} Current deduped positions (owned copy). */
-  let current = [];
+  const material = baseColor.withAlpha(TRAIL_ALPHA);
+  const depthFailMaterial = baseColor.withAlpha(TRAIL_OCCLUDED_ALPHA);
   let destroyed = false;
   let visible = true;
-  /** @type {Cesium.Entity|null} */
-  let entity = null;
+  /** @type {{positions: Cesium.Cartesian3[], entity: Cesium.Entity, standIn: boolean, frames: number}[]} */
+  let frozen = [];
+  /** Newest chunk of the newest segment: the only part that changes. */
+  let tail = NO_POSITIONS;
+  /** What the dynamic polylines draw: the tail, then frozen chunks whose
+   *  static geometry is not in the scene yet. */
+  let dynamicRuns = [];
+  /** @type {Cesium.Entity[]} Pooled dynamic polylines; spares draw nothing. */
+  const dynamicEntities = [];
+  let removeReadyListener = null;
+  const scratchSphere = new Cesium.BoundingSphere();
 
-  function ensureEntity() {
-    if (entity || destroyed || !viewer || viewer.isDestroyed()) return;
-    entity = viewer.entities.add({
+  const usable = () => !destroyed && viewer && !viewer.isDestroyed();
+
+  function polyline(positions) {
+    return {
+      positions,
+      width,
+      material,
+      // The owner-locked rule (round 6): a segment below the photoreal
+      // mesh renders dimmed — it must never disappear into the ground.
+      depthFailMaterial,
+      // Round 8 (owner: UAL1104's Pacific trail "cutting through the
+      // globe"): NONE draws straight 3D chords between waypoints — over a
+      // sparse trans-oceanic trace a single segment spans hundreds of km
+      // and tunnels through the planet. GEODESIC subdivides each segment
+      // along the curved surface (heights interpolated), so long legs hug
+      // the globe instead of chording through it.
+      arcType: Cesium.ArcType.GEODESIC,
+    };
+  }
+
+  function addEntity(positions) {
+    return viewer.entities.add({
       id: `gev-trail:${++_trailSeq}`,
       show: visible,
-      polyline: {
-        // CallbackProperty so a positions swap never rebuilds the entity —
-        // Cesium re-reads on change; `false` marks it non-constant.
-        positions: new Cesium.CallbackProperty(() => current, false),
-        width,
-        material: baseColor.withAlpha(TRAIL_ALPHA),
-        // The owner-locked rule (round 6): a segment below the photoreal
-        // mesh renders dimmed — it must never disappear into the ground.
-        depthFailMaterial: baseColor.withAlpha(TRAIL_OCCLUDED_ALPHA),
-        // Round 8 (owner: UAL1104's Pacific trail "cutting through the
-        // globe"): NONE draws straight 3D chords between waypoints — over a
-        // sparse trans-oceanic trace a single segment spans hundreds of km
-        // and tunnels through the planet. GEODESIC subdivides each segment
-        // along the curved surface (heights interpolated), so long legs hug
-        // the globe instead of chording through it.
-        arcType: Cesium.ArcType.GEODESIC,
-      },
+      polyline: polyline(positions),
     });
   }
 
+  function removeEntity(entity) {
+    try {
+      viewer.entities.remove(entity);
+    } catch {
+      /* torn down */
+    }
+  }
+
+  function syncDynamic() {
+    dynamicRuns = [];
+    if (tail.length >= 2) dynamicRuns.push(tail);
+    for (const chunk of frozen)
+      if (chunk.standIn) dynamicRuns.push(chunk.positions);
+    while (dynamicEntities.length < Math.max(1, dynamicRuns.length)) {
+      const index = dynamicEntities.length;
+      dynamicEntities.push(
+        addEntity(
+          // CallbackProperty so a positions swap never rebuilds the entity —
+          // Cesium re-reads on change; `false` marks it non-constant.
+          new Cesium.CallbackProperty(
+            () => dynamicRuns[index] ?? NO_POSITIONS,
+            false,
+          ),
+        ),
+      );
+    }
+  }
+
+  /** Whether a frozen chunk's static geometry is drawn in the scene. */
+  function staticReady(chunk) {
+    const display = viewer.dataSourceDisplay;
+    if (typeof display?.getBoundingSphere !== 'function') return true;
+    if (!visible) return false;
+    chunk.frames += 1;
+    if (chunk.frames > STAND_IN_MAX_FRAMES) return true;
+    try {
+      return (
+        display.getBoundingSphere(chunk.entity, false, scratchSphere) ===
+        Cesium.BoundingSphereState.DONE
+      );
+    } catch {
+      return false;
+    }
+  }
+
+  /** Drop stand-ins whose static geometry is in; stop listening when none are left. */
+  function resolveStandIns() {
+    if (!usable()) return;
+    let changed = false;
+    for (const chunk of frozen) {
+      if (chunk.standIn && staticReady(chunk)) {
+        chunk.standIn = false;
+        changed = true;
+      }
+    }
+    if (changed) syncDynamic();
+    if (!frozen.some((chunk) => chunk.standIn)) stopListening();
+  }
+
+  function stopListening() {
+    if (removeReadyListener) removeReadyListener();
+    removeReadyListener = null;
+  }
+
+  function listenForStandIns() {
+    if (removeReadyListener) return;
+    const event = viewer.scene?.postRender;
+    if (typeof event?.addEventListener !== 'function') {
+      resolveStandIns();
+      return;
+    }
+    removeReadyListener = event.addEventListener(resolveStandIns);
+  }
+
+  /** Drop consecutive near-duplicates: zero-length segments add nothing. */
+  function dedupe(cartesians) {
+    const positions = [];
+    for (const position of Array.isArray(cartesians) ? cartesians : []) {
+      if (!position) continue;
+      const last = positions[positions.length - 1];
+      if (
+        last &&
+        Cesium.Cartesian3.distanceSquared(last, position) <
+          MIN_SEGMENT_DISTANCE_SQ
+      )
+        continue;
+      positions.push(position);
+    }
+    return positions;
+  }
+
+  /** Whether `chunk` holds exactly the same position objects as `seg` from `start`. */
+  function sameRun(chunk, seg, start) {
+    const positions = chunk.positions;
+    if (start + positions.length > seg.length) return false;
+    for (let k = 0; k < positions.length; k++)
+      if (positions[k] !== seg[start + k]) return false;
+    return true;
+  }
+
+  /**
+   * Cut segments into chunks, reusing every frozen chunk whose points are
+   * still drawn in the same order. New chunks end where a reusable one begins,
+   * so dropping the oldest point rebuilds only the first chunk.
+   */
+  function reconcile(segments) {
+    const byStart = new Map();
+    for (const chunk of frozen) byStart.set(chunk.positions[0], chunk);
+    const kept = new Set();
+    const reusable = (seg, j) => {
+      const chunk = byStart.get(seg[j]);
+      return chunk && !kept.has(chunk) && sameRun(chunk, seg, j) ? chunk : null;
+    };
+    const next = [];
+    let nextTail = NO_POSITIONS;
+    segments.forEach((seg, s) => {
+      const newest = s === segments.length - 1;
+      let i = 0;
+      while (i < seg.length - 1) {
+        const existing = reusable(seg, i);
+        if (existing) {
+          kept.add(existing);
+          next.push(existing);
+          i += existing.positions.length - 1;
+          continue;
+        }
+        if (newest && seg.length - i <= CHUNK_POINTS) {
+          nextTail = seg.slice(i);
+          break;
+        }
+        let end = Math.min(seg.length - 1, i + CHUNK_POINTS - 1);
+        for (let j = i + 1; j < end; j++) {
+          if (reusable(seg, j)) {
+            end = j;
+            break;
+          }
+        }
+        const positions = seg.slice(i, end + 1);
+        next.push({
+          positions,
+          entity: addEntity(positions),
+          standIn: true,
+          frames: 0,
+        });
+        i = end;
+      }
+    });
+    for (const chunk of frozen)
+      if (!kept.has(chunk)) removeEntity(chunk.entity);
+    frozen = next;
+    tail = nextTail;
+    syncDynamic();
+    if (frozen.some((chunk) => chunk.standIn)) listenForStandIns();
+    else stopListening();
+  }
+
+  function release() {
+    stopListening();
+    for (const chunk of frozen) removeEntity(chunk.entity);
+    frozen = [];
+    tail = NO_POSITIONS;
+    dynamicRuns = [];
+  }
+
   return {
     /**
-     * Replace the trail geometry with a chronological position list
+     * Replace the trail geometry with one chronological position list
      * (oldest first). Fewer than 2 distinct positions clears the trail.
      * @param {Cesium.Cartesian3[]} cartesians - Positions, oldest -> newest.
      */
     setPositions(cartesians) {
-      if (destroyed || !viewer || viewer.isDestroyed()) return;
-      // Drop consecutive near-duplicates: zero-length segments add nothing.
-      const positions = [];
-      for (const position of Array.isArray(cartesians) ? cartesians : []) {
-        if (!position) continue;
-        const last = positions[positions.length - 1];
-        if (
-          last &&
-          Cesium.Cartesian3.distanceSquared(last, position) <
-            MIN_SEGMENT_DISTANCE_SQ
-        )
-          continue;
-        positions.push(position);
-      }
-      current = positions.length >= 2 ? positions : [];
-      ensureEntity();
+      this.setSegments([cartesians]);
+    },
+
+    /**
+     * Replace the trail geometry with chronological segments (oldest first).
+     * Nothing is drawn between segments. A segment with fewer than 2 distinct
+     * positions draws nothing. Pass the same position objects on every
+     * update: a chunk whose points are unchanged is never rebuilt.
+     * @param {Cesium.Cartesian3[][]} list - Segments, each oldest -> newest.
+     */
+    setSegments(list) {
+      if (!usable()) return;
+      reconcile(
+        (Array.isArray(list) ? list : [])
+          .map(dedupe)
+          .filter((positions) => positions.length >= 2),
+      );
     },
 
     /** Temporarily hide/show the trail without discarding accumulated history. */
     setVisible(nextVisible) {
       visible = nextVisible !== false;
-      if (entity) entity.show = visible;
+      for (const entity of dynamicEntities) entity.show = visible;
+      for (const chunk of frozen) chunk.entity.show = visible;
     },
 
-    /** Empty the trail without removing the entity (cheap re-arm). */
+    /** Empty the trail and release its frozen geometry; dynamic polylines stay for re-arm. */
     clear() {
-      current = [];
+      if (!usable()) return;
+      release();
     },
 
-    /** Remove the trail entity permanently (layer disable/teardown). */
+    /** Remove every trail entity permanently (layer disable/teardown). */
     destroy() {
-      destroyed = true;
-      current = [];
-      if (entity && viewer && !viewer.isDestroyed()) {
-        try {
-          viewer.entities.remove(entity);
-        } catch {
-          /* torn down */
-        }
+      if (usable()) {
+        release();
+        for (const entity of dynamicEntities) removeEntity(entity);
       }
-      entity = null;
+      destroyed = true;
+      frozen = [];
+      tail = NO_POSITIONS;
+      dynamicRuns = [];
+      dynamicEntities.length = 0;
+      stopListening();
+    },
+
+    /** Test seam: frozen chunks (positions, stand-in state) and the dynamic runs. */
+    _chunksForTest() {
+      return {
+        frozen: frozen.map((chunk) => ({
+          positions: chunk.positions,
+          standIn: chunk.standIn,
+          entity: chunk.entity,
+        })),
+        dynamic: dynamicRuns.slice(),
+      };
     },
   };
 }

src/data/trailRenderer.test.mjs

@@ -1,7 +1,7 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import * as Cesium from 'cesium';
-import { createTrail } from './trailRenderer.js';
+import { CHUNK_POINTS, createTrail } from './trailRenderer.js';
 
 test('trail visibility can change without discarding its accumulated geometry', () => {
   const added = [];
@@ -37,3 +37,253 @@ test('trail visibility can change without discarding its accumulated geometry',
   trail.destroy();
   assert.deepEqual(removed, [added[0]]);
 });
+
+/**
+ * A fake viewer. `ready` holds the frozen-chunk entities whose static
+ * geometry is in the scene; `frame()` runs one post-render pass.
+ */
+function fakeViewer({ scene = true } = {}) {
+  const added = [];
+  const removed = [];
+  const ready = new Set();
+  let listeners = [];
+  const viewer = {
+    added,
+    removed,
+    ready,
+    isDestroyed: () => false,
+    entities: {
+      add(definition) {
+        added.push(definition);
+        return definition;
+      },
+      remove(entity) {
+        removed.push(entity);
+      },
+    },
+    listenerCount: () => listeners.length,
+    frame() {
+      for (const listener of listeners.slice()) listener();
+    },
+  };
+  if (scene) {
+    viewer.scene = {
+      postRender: {
+        addEventListener(listener) {
+          listeners.push(listener);
+          return () => {
+            listeners = listeners.filter((l) => l !== listener);
+          };
+        },
+      },
+    };
+    viewer.dataSourceDisplay = {
+      getBoundingSphere: (entity) =>
+        ready.has(entity)
+          ? Cesium.BoundingSphereState.DONE
+          : Cesium.BoundingSphereState.PENDING,
+    };
+  }
+  return viewer;
+}
+
+const valueOf = (positions) =>
+  typeof positions?.getValue === 'function' ? positions.getValue() : positions;
+
+/** Every line the scene can draw now: live entities with two or more points. */
+function drawnRuns(viewer) {
+  return viewer.added
+    .filter((entity) => !viewer.removed.includes(entity))
+    .map((entity) => valueOf(entity.polyline.positions))
+    .filter((positions) => positions.length >= 2);
+}
+
+/** Distinct position objects across all drawn lines. */
+function drawnPoints(viewer) {
+  return new Set(drawnRuns(viewer).flat());
+}
+
+function line(count, offset = 0) {
+  return Array.from(
+    { length: count },
+    (_, i) => new Cesium.Cartesian3(6_378_137 + offset, i * 1000, 0),
+  );
+}
+
+test('a trail with breaks draws each segment and nothing between them', () => {
+  const viewer = fakeViewer({ scene: false });
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  const first = line(2, 0);
+  const lone = line(1, 50_000);
+  const second = line(3, 100_000);
+
+  trail.setSegments([first, lone, second]);
+  assert.deepEqual(
+    drawnRuns(viewer).sort((a, b) => a[0].x - b[0].x),
+    [first, second],
+  );
+  for (const entity of viewer.added) {
+    assert.match(String(entity.id), /^gev-trail:/);
+    assert.equal(entity.polyline.width, 2.5);
+  }
+
+  // The first segment is gone later: its geometry is released.
+  trail.setSegments([second]);
+  assert.deepEqual(drawnRuns(viewer), [second]);
+
+  trail.setVisible(false);
+  assert.ok(
+    viewer.added
+      .filter((entity) => !viewer.removed.includes(entity))
+      .every((entity) => entity.show === false),
+  );
+  trail.destroy();
+  assert.equal(new Set(viewer.removed).size, viewer.added.length);
+});
+
+test('chunks share their boundary points and only the newest chunk is dynamic', () => {
+  const viewer = fakeViewer({ scene: false });
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  const points = line(300);
+  trail.setPositions(points);
+  const { frozen, dynamic } = trail._chunksForTest();
+  assert.deepEqual(
+    frozen.map((chunk) => chunk.positions.length),
+    [CHUNK_POINTS, CHUNK_POINTS],
+  );
+  assert.equal(frozen[0].positions.at(-1), frozen[1].positions[0]);
+  assert.equal(dynamic.length, 1);
+  assert.equal(frozen[1].positions.at(-1), dynamic[0][0]);
+  assert.equal(dynamic[0].at(-1), points.at(-1));
+  assert.equal(drawnPoints(viewer).size, points.length);
+  // A frozen chunk is constant geometry, not a per-frame callback.
+  for (const chunk of frozen)
+    assert.ok(Array.isArray(chunk.entity.polyline.positions));
+});
+
+test('freezing a chunk keeps every point drawn, and the dynamic copy goes only once the static geometry is in', () => {
+  const viewer = fakeViewer();
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  const points = line(CHUNK_POINTS + 20);
+  for (let n = 2; n <= points.length; n++) {
+    trail.setPositions(points.slice(0, n));
+    assert.equal(drawnPoints(viewer).size, n, `points drawn at ${n}`);
+  }
+  const { frozen } = trail._chunksForTest();
+  assert.equal(frozen.length, 1);
+  // Not in the scene yet: the chunk is still drawn by a dynamic polyline.
+  viewer.frame();
+  assert.equal(trail._chunksForTest().frozen[0].standIn, true);
+  assert.ok(
+    trail._chunksForTest().dynamic.includes(frozen[0].positions),
+    'the dynamic copy stays until the static geometry is ready',
+  );
+  viewer.ready.add(frozen[0].entity);
+  viewer.frame();
+  const after = trail._chunksForTest();
+  assert.equal(after.frozen[0].standIn, false);
+  assert.ok(!after.dynamic.includes(frozen[0].positions));
+  assert.equal(after.dynamic.length, 1);
+  assert.equal(viewer.listenerCount(), 0, 'no listener once nothing waits');
+  // The frozen entity is the same object: freezing built it once.
+  assert.equal(after.frozen[0].entity, frozen[0].entity);
+});
+
+test('a hidden trail keeps its dynamic copy until it is shown and the static geometry is in', () => {
+  const viewer = fakeViewer();
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  trail.setVisible(false);
+  trail.setPositions(line(CHUNK_POINTS + 5));
+  const [chunk] = trail._chunksForTest().frozen;
+  viewer.ready.add(chunk.entity);
+  viewer.frame();
+  assert.equal(trail._chunksForTest().frozen[0].standIn, true);
+  trail.setVisible(true);
+  viewer.frame();
+  assert.equal(trail._chunksForTest().frozen[0].standIn, false);
+});
+
+test('dropping the oldest point rebuilds only the first chunk', () => {
+  const viewer = fakeViewer({ scene: false });
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  const points = line(400);
+  trail.setPositions(points);
+  const before = trail._chunksForTest().frozen;
+  trail.setPositions(points.slice(1));
+  const after = trail._chunksForTest().frozen;
+  assert.equal(after.length, before.length);
+  assert.notEqual(after[0].entity, before[0].entity);
+  assert.ok(viewer.removed.includes(before[0].entity));
+  for (let k = 1; k < before.length; k++)
+    assert.equal(after[k].entity, before[k].entity);
+  assert.equal(after[0].positions.at(-1), after[1].positions[0]);
+  assert.equal(drawnPoints(viewer).size, 399);
+});
+
+test('a backfill prepended ahead of frozen chunks keeps them and joins at a shared point', () => {
+  const viewer = fakeViewer({ scene: false });
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  const live = line(300, 0);
+  trail.setPositions(live);
+  const before = trail._chunksForTest().frozen;
+  // History older than every live point, joined as one path.
+  const history = Array.from(
+    { length: 200 },
+    (_, i) => new Cesium.Cartesian3(6_378_137, -(200 - i) * 1000, 0),
+  );
+  trail.setPositions(history.concat(live));
+  const after = trail._chunksForTest().frozen;
+  for (const chunk of before)
+    assert.ok(after.some((c) => c.entity === chunk.entity));
+  for (let k = 0; k + 1 < after.length; k++)
+    assert.equal(after[k].positions.at(-1), after[k + 1].positions[0]);
+  assert.equal(drawnPoints(viewer).size, 500);
+
+  // A backfill that breaks from the live trail is its own segment.
+  trail.setSegments([history, live]);
+  assert.equal(drawnPoints(viewer).size, 500);
+  const runs = drawnRuns(viewer);
+  assert.ok(
+    !runs.some((run) => run.includes(history.at(-1)) && run.includes(live[0])),
+    'no line across the break',
+  );
+});
+
+test('clearing and re-tracking release every frozen chunk', () => {
+  const viewer = fakeViewer();
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  trail.setPositions(line(600));
+  const frozen = trail._chunksForTest().frozen.map((chunk) => chunk.entity);
+  assert.ok(frozen.length >= 4);
+  assert.equal(viewer.listenerCount(), 1);
+  trail.clear();
+  for (const entity of frozen) assert.ok(viewer.removed.includes(entity));
+  assert.equal(viewer.listenerCount(), 0);
+  assert.equal(drawnRuns(viewer).length, 0);
+
+  // Another aircraft: nothing of the first one survives.
+  const other = line(200, 900_000);
+  trail.setPositions(other);
+  assert.equal(drawnPoints(viewer).size, 200);
+  trail.destroy();
+  assert.equal(new Set(viewer.removed).size, viewer.added.length);
+  assert.equal(viewer.listenerCount(), 0);
+});
+
+test('replacing the newest point and removing points inside the trail draws exactly the new trail', () => {
+  const viewer = fakeViewer({ scene: false });
+  const trail = createTrail(viewer, { color: '#ffffff' });
+  const points = line(400);
+  trail.setPositions(points.slice());
+  // A recorded point replaces the newest one; a spur point inside a frozen
+  // chunk and one at a chunk boundary go.
+  points[points.length - 1] = new Cesium.Cartesian3(6_378_137, 399_500, 10);
+  points.splice(200, 1);
+  points.splice(CHUNK_POINTS - 1, 1);
+  trail.setPositions(points.slice());
+  assert.deepEqual(drawnPoints(viewer), new Set(points));
+  // Every drawn line runs through consecutive trail points only.
+  const next = new Map(points.slice(0, -1).map((p, i) => [p, points[i + 1]]));
+  for (const run of drawnRuns(viewer))
+    for (let i = 1; i < run.length; i++) assert.equal(next.get(run[i - 1]), run[i]);
+});

src/data/trailReports.test.mjs

@@ -0,0 +1,165 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import {
+  insertTrailFix,
+  recordTrailPoint,
+  trailBodySegments,
+  trailPairBreaks,
+  trailDistanceM,
+  trailRecordingBlocked,
+} from './trailContinuity.js';
+
+// Reports decide where the line may run: recorded points never join two
+// reports too far apart to be one flight, a report the display has already
+// passed replaces the guess drawn after it, and old entries let go of the
+// positions they stood for.
+
+const SEC = 1000;
+const LIMIT = 2_000;
+const enu = Cesium.Transforms.eastNorthUpToFixedFrame(
+  Cesium.Cartesian3.fromDegrees(-100, 40, 0),
+);
+const at = (e, n, u = 10) =>
+  Cesium.Matrix4.multiplyByPoint(
+    enu,
+    new Cesium.Cartesian3(e, n, u),
+    new Cesium.Cartesian3(),
+  );
+
+/** No two neighbours in one drawn segment are too far apart to be one flight. */
+function assertNoBrokenJoin(fixes) {
+  const reports = fixes.filter((fix) => !fix.drawn && fix.position);
+  for (const segment of trailBodySegments(fixes, fixes.length)) {
+    const inSegment = reports.filter((r) =>
+      segment.some((p) => Cesium.Cartesian3.distance(p, r.position) < 0.01),
+    );
+    for (let i = 1; i < inSegment.length; i++)
+      assert.equal(
+        trailPairBreaks(
+          inSegment[i].timeMs - inSegment[i - 1].timeMs,
+          trailDistanceM(inSegment[i - 1].position, inSegment[i].position),
+        ),
+        false,
+        `one segment joins reports at ${inSegment[i - 1].timeMs / SEC} s and ${
+          inSegment[i].timeMs / SEC
+        } s`,
+      );
+  }
+}
+
+test('two reports 60 s and 51 km apart are not joined, even by recorded points', () => {
+  const A = Cesium.Cartesian3.fromDegrees(-100, 40, 9_000);
+  const B = Cesium.Cartesian3.fromDegrees(-99.4, 40, 9_000);
+  const fixes = [];
+  insertTrailFix(fixes, A, 0, LIMIT);
+  insertTrailFix(fixes, B, 60 * SEC, LIMIT); // ahead of the display
+  for (let t = 1; t < 60; t++) {
+    // Nothing is recorded across the pair.
+    assert.equal(trailRecordingBlocked(fixes, t * SEC), true);
+  }
+  assert.equal(trailBodySegments(fixes, fixes.length).length, 2);
+  // Points recorded before the far report was known are dropped.
+  const late = [];
+  insertTrailFix(late, A, 0, LIMIT);
+  for (let t = 1; t < 60; t++)
+    recordTrailPoint(
+      late,
+      Cesium.Cartesian3.lerp(A, B, t / 60, new Cesium.Cartesian3()),
+      t * SEC,
+      LIMIT,
+    );
+  insertTrailFix(late, B, 60 * SEC, LIMIT);
+  assert.equal(late.filter((fix) => fix.drawn).length, 0);
+  assert.equal(trailBodySegments(late, late.length).length, 2);
+  assertNoBrokenJoin(late);
+});
+
+test('a coast, a hold and then a far report: nothing is drawn across', () => {
+  const A = at(0, 0, 9_000);
+  const fixes = [];
+  insertTrailFix(fixes, A, 0, LIMIT);
+  // The icon coasts east for 90 s at 230 m/s, then holds.
+  for (let t = 1; t <= 200; t++)
+    recordTrailPoint(
+      fixes,
+      at(230 * Math.min(t, 90), 0, 9_000),
+      t * SEC,
+      LIMIT,
+    );
+  assert.ok(fixes.filter((fix) => fix.drawn).length > 0);
+  // The next report is 150 km away two minutes later: over 1,500 kt.
+  const B = at(0, 150_000, 9_000);
+  insertTrailFix(fixes, B, 120 * SEC, LIMIT);
+  assertNoBrokenJoin(fixes);
+  const segments = trailBodySegments(fixes, fixes.length);
+  const segmentOf = (p) =>
+    segments.findIndex((s) =>
+      s.some((q) => Cesium.Cartesian3.distance(p, q) < 0.01),
+    );
+  assert.notEqual(segmentOf(A), segmentOf(B));
+  // No recorded point between the two reports remains.
+  assert.equal(
+    fixes.filter((fix) => fix.drawn && fix.timeMs < 120 * SEC).length,
+    0,
+  );
+});
+
+for (const off of [200, 400, 600, 1_000]) {
+  test(`a report the display already passed, ${off} m off a 2 km turn, leaves no spike`, () => {
+    const R = 2_000;
+    const w = 100 / R;
+    const pos = (t, r = R) =>
+      at(r * Math.sin(w * t), R - r * Math.cos(w * t), 3_000);
+    const fixes = [];
+    insertTrailFix(fixes, pos(0), 0, LIMIT);
+    for (let t = 1; t <= 60; t++)
+      recordTrailPoint(fixes, pos(t), t * SEC, LIMIT);
+    const report = pos(50, R - off);
+    // It arrives with the display at 60 s; the ease takes 900 ms.
+    insertTrailFix(fixes, report, 50 * SEC, LIMIT, {
+      displayMs: 60 * SEC,
+      settleMs: 900,
+    });
+    // The guess drawn after the report is gone...
+    assert.deepEqual(
+      fixes.filter((fix) => fix.drawn && fix.timeMs > 50 * SEC),
+      [],
+    );
+    // ...nothing is recorded while the icon eases onto the correction...
+    assert.equal(trailRecordingBlocked(fixes, 60.5 * SEC), true);
+    assert.equal(trailRecordingBlocked(fixes, 61 * SEC), false);
+    // ...and the line reads arc, report, corrected path.
+    for (let t = 61; t <= 70; t++)
+      recordTrailPoint(fixes, pos(t, R - off), t * SEC, LIMIT);
+    const line = trailBodySegments(fixes, fixes.length).flat();
+    const r = line.findIndex(
+      (p) => Cesium.Cartesian3.distance(p, report) < 0.01,
+    );
+    assert.ok(r > 0);
+    for (const p of line.slice(0, r))
+      assert.ok(
+        Math.abs(Cesium.Cartesian3.distance(p, at(0, R, 3_000)) - R) < 31,
+        'a point before the report is off the arc',
+      );
+    for (const p of line.slice(r + 1))
+      assert.ok(
+        Math.abs(Cesium.Cartesian3.distance(p, at(0, R, 3_000)) - (R - off)) <
+          31,
+        'a point after the report is off the corrected path',
+      );
+  });
+}
+
+test('entries behind the report before the display let go of the positions they stood for', () => {
+  const fixes = [];
+  for (let s = 0; s <= 2 * 3_600; s++) {
+    const p = at(230 * s, 0, 10_000);
+    if (s % 15 === 0) insertTrailFix(fixes, p, s * SEC, LIMIT);
+    else recordTrailPoint(fixes, p, s * SEC, LIMIT);
+  }
+  let held = 0;
+  for (const fix of fixes) held += fix.absorbed?.length ?? 0;
+  // Only the newest stretch, up to two reports back, keeps what it stood for.
+  assert.ok(held <= 30, `${held} positions still held`);
+});

src/data/trailThinning.test.mjs

@@ -0,0 +1,180 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import {
+  insertTrailFix,
+  joinTrailBackfill,
+  recordTrailPoint,
+  TRAIL_THIN_MAX_SPAN_M,
+  TRAIL_THIN_TOLERANCE_M,
+} from './trailContinuity.js';
+
+// Recorded points are thinned by shape: a point stays only where the line
+// would move without it. Reports always stay.
+
+const SEC = 1000;
+const LIMIT = 2_000;
+const LAT0 = 40;
+const LON0 = -100;
+const M_PER_DEG_LAT = 111_320;
+const mPerDegLon = (lat) => M_PER_DEG_LAT * Math.cos((lat * Math.PI) / 180);
+
+/** Straight and level east at `speed` m/s, `tSec` after the start. */
+const eastbound = (speed, tSec, heightM = 10_000) =>
+  Cesium.Cartesian3.fromDegrees(
+    LON0 + (speed * tSec) / mPerDegLon(LAT0),
+    LAT0,
+    heightM,
+  );
+
+/** A standard-rate (3°/s) left turn from north, radius v / ω, `tSec` in. */
+function turning(speed, tSec, heightM = 10_000) {
+  const omega = (3 * Math.PI) / 180;
+  const radius = speed / omega;
+  // Centre west of the start; heading north, turning left (towards west).
+  const angle = omega * tSec;
+  const east = -radius + radius * Math.cos(angle);
+  const north = radius * Math.sin(angle);
+  return Cesium.Cartesian3.fromDegrees(
+    LON0 + east / mPerDegLon(LAT0),
+    LAT0 + north / M_PER_DEG_LAT,
+    heightM,
+  );
+}
+
+/** Distance from p to the nearest point of a polyline. */
+function distanceToPath(p, path) {
+  let best = Infinity;
+  const ab = new Cesium.Cartesian3();
+  const ap = new Cesium.Cartesian3();
+  const closest = new Cesium.Cartesian3();
+  for (let i = 1; i < path.length; i++) {
+    Cesium.Cartesian3.subtract(path[i], path[i - 1], ab);
+    Cesium.Cartesian3.subtract(p, path[i - 1], ap);
+    const len2 = Cesium.Cartesian3.magnitudeSquared(ab);
+    const t =
+      len2 > 0
+        ? Math.max(0, Math.min(1, Cesium.Cartesian3.dot(ap, ab) / len2))
+        : 0;
+    Cesium.Cartesian3.add(
+      path[i - 1],
+      Cesium.Cartesian3.multiplyByScalar(ab, t, closest),
+      closest,
+    );
+    best = Math.min(best, Cesium.Cartesian3.distance(p, closest));
+  }
+  return best;
+}
+
+const drawnCount = (fixes) => fixes.filter((fix) => fix.drawn).length;
+
+test('one hour straight and level with a report every 15 s adds under 60 recorded points', () => {
+  const speed = 230;
+  const fixes = [];
+  for (const t of [0, 15, 30])
+    insertTrailFix(fixes, eastbound(speed, t), t * SEC, LIMIT);
+  // Live order: a report arrives 30 s before the display reaches it, and the
+  // drawn icon is recorded once a second.
+  for (let t = 1; t <= 3_600; t++) {
+    const reportAt = t + 30;
+    if (reportAt % 15 === 0 && reportAt <= 3_600)
+      insertTrailFix(fixes, eastbound(speed, reportAt), reportAt * SEC, LIMIT);
+    recordTrailPoint(fixes, eastbound(speed, t), t * SEC, LIMIT);
+  }
+  assert.ok(
+    drawnCount(fixes) < 60,
+    `${drawnCount(fixes)} recorded points kept`,
+  );
+  assert.equal(fixes.filter((fix) => !fix.drawn).length, 241);
+});
+
+test('with no report at all, a straight leg keeps one point per 10 km and no more', () => {
+  const speed = 230;
+  const fixes = [];
+  insertTrailFix(fixes, eastbound(speed, 0), 0, LIMIT);
+  for (let t = 1; t <= 3_600; t++)
+    recordTrailPoint(fixes, eastbound(speed, t), t * SEC, LIMIT);
+  const legKm = (speed * 3_600) / 1_000;
+  assert.equal(TRAIL_THIN_MAX_SPAN_M, 10_000);
+  assert.ok(
+    drawnCount(fixes) <= Math.ceil(legKm / 10) + 1,
+    `${drawnCount(fixes)} points over ${legKm} km`,
+  );
+  for (let i = 1; i < fixes.length; i++)
+    assert.ok(
+      Cesium.Cartesian3.distance(fixes[i - 1].position, fixes[i].position) <=
+        TRAIL_THIN_MAX_SPAN_M + speed,
+    );
+});
+
+test('a standard-rate 180° turn keeps its curve: every recorded position stays within 30 m of the kept line', () => {
+  const speed = 230;
+  const fixes = [];
+  const recorded = [];
+  insertTrailFix(fixes, turning(speed, 0), 0, LIMIT);
+  for (let t = 1; t <= 60; t++) {
+    const position = turning(speed, t);
+    recorded.push(position);
+    recordTrailPoint(fixes, position, t * SEC, LIMIT);
+  }
+  const line = fixes.map((fix) => fix.position);
+  assert.equal(TRAIL_THIN_TOLERANCE_M, 30);
+  for (const position of recorded) {
+    const off = distanceToPath(position, line);
+    assert.ok(off <= TRAIL_THIN_TOLERANCE_M + 1, `${off.toFixed(1)} m off`);
+  }
+  assert.ok(drawnCount(fixes) < recorded.length, 'nothing was thinned');
+  assert.ok(drawnCount(fixes) >= 6, 'the curve was flattened');
+});
+
+test('a climb off the straight line keeps its vertices', () => {
+  const fixes = [];
+  insertTrailFix(fixes, eastbound(200, 0, 1_000), 0, LIMIT);
+  // 30 s level, then a 60 s climb of 20 m/s, then level again.
+  const height = (t) => 1_000 + 20 * Math.min(60, Math.max(0, t - 30));
+  const recorded = [];
+  for (let t = 1; t <= 150; t++) {
+    const position = eastbound(200, t, height(t));
+    recorded.push(position);
+    recordTrailPoint(fixes, position, t * SEC, LIMIT);
+  }
+  const line = fixes.map((fix) => fix.position);
+  for (const position of recorded)
+    assert.ok(distanceToPath(position, line) <= TRAIL_THIN_TOLERANCE_M + 1);
+});
+
+test('at the limit, old recorded points go before any report from the backfill', () => {
+  const max = 300;
+  const backfill = [];
+  for (let i = 0; i < 100; i++)
+    backfill.push({
+      position: eastbound(200, i * 60 - 20_000),
+      timeMs: (i * 60 - 20_000) * SEC,
+      nullAltitude: false,
+    });
+  let fixes = [];
+  insertTrailFix(fixes, turning(150, 0), 0, max);
+  // A long series of turns keeps many recorded points.
+  for (let t = 1; t <= 2_000; t++)
+    recordTrailPoint(fixes, turning(150, t), t * SEC, max);
+  assert.ok(drawnCount(fixes) > 150);
+  fixes = joinTrailBackfill(
+    {
+      fixes: backfill.map((b) => ({ ...b, breakBefore: false })),
+      breakAfter: false,
+    },
+    fixes,
+    max,
+  );
+  assert.ok(fixes.length <= max);
+  const reports = fixes.filter((fix) => !fix.drawn);
+  assert.equal(reports.length, 101, 'a report was dropped');
+  // More recording at the limit drops recorded points, never reports.
+  for (let t = 2_001; t <= 2_400; t++)
+    recordTrailPoint(fixes, turning(150, t), t * SEC, max);
+  assert.ok(fixes.length <= max);
+  assert.equal(fixes.filter((fix) => !fix.drawn).length, 101);
+  // The recorded points that remain are the newest.
+  const drawnTimes = fixes.filter((fix) => fix.drawn).map((fix) => fix.timeMs);
+  assert.ok(Math.min(...drawnTimes) > 1_000 * SEC);
+});

src/i18n/locales/ar.json

@@ -615,6 +615,7 @@
   "LOADING": "جارٍ التحميل",
   "DEGRADED": "متدهور",
   "STALE": "متقادم",
+  "SIGNAL LOST · {time}": "انقطعت الإشارة · {time}",
   "FALLBACK": "بديل",
   "PARTIAL": "جزئي",
   "UNCERTAIN": "غير مؤكد",

src/i18n/locales/de.json

@@ -72,6 +72,7 @@
   "LOADING": "LÄDT",
   "DEGRADED": "EINGESCHRÄNKT",
   "STALE": "VERALTET",
+  "SIGNAL LOST · {time}": "SIGNAL VERLOREN · {time}",
   "PARTIAL": "TEILWEISE",
   "FALLBACK": "FALLBACK",
   "SUMMARY": "ÜBERSICHT",

src/i18n/locales/es.json

@@ -500,6 +500,7 @@
   "LOADING": "CARGANDO",
   "DEGRADED": "DEGRADADO",
   "STALE": "DESACTUALIZADO",
+  "SIGNAL LOST · {time}": "SEÑAL PERDIDA · {time}",
   "FALLBACK": "RESPALDO",
   "PARTIAL": "PARCIAL",
   "UNCERTAIN": "INCIERTO",

src/i18n/locales/fr.json

@@ -329,6 +329,7 @@
   "LOADING": "CHARGEMENT",
   "DEGRADED": "DÉGRADÉ",
   "STALE": "OBSOLÈTE",
+  "SIGNAL LOST · {time}": "SIGNAL PERDU · {time}",
   "PARTIAL": "PARTIEL",
   "FALLBACK": "SECOURS",
   "view band|STREET": "RUE",

src/i18n/locales/he.json

@@ -328,6 +328,7 @@
   "LOADING": "טוען",
   "DEGRADED": "מופחת",
   "STALE": "מיושן",
+  "SIGNAL LOST · {time}": "האות אבד · {time}",
   "PARTIAL": "חלקי",
   "FALLBACK": "חלופה",
   "view band|STREET": "רחוב",

src/i18n/locales/it.json

@@ -329,6 +329,7 @@
   "LOADING": "CARICAMENTO",
   "DEGRADED": "DEGRADATO",
   "STALE": "OBSOLETO",
+  "SIGNAL LOST · {time}": "SEGNALE PERSO · {time}",
   "PARTIAL": "PARZIALE",
   "FALLBACK": "RISERVA",
   "view band|STREET": "STRADA",

src/i18n/locales/ja.json

@@ -323,6 +323,7 @@
   "LOADING": "読込中",
   "DEGRADED": "性能低下",
   "STALE": "更新遅延",
+  "SIGNAL LOST · {time}": "信号途絶 · {time}",
   "PARTIAL": "部分的",
   "FALLBACK": "フォールバック",
   "view band|STREET": "街路",

src/i18n/locales/pt.json

@@ -329,6 +329,7 @@
   "LOADING": "CARREGANDO",
   "DEGRADED": "DEGRADADO",
   "STALE": "DESATUALIZADO",
+  "SIGNAL LOST · {time}": "SINAL PERDIDO · {time}",
   "PARTIAL": "PARCIAL",
   "FALLBACK": "ALTERNATIVO",
   "view band|STREET": "RUA",

src/i18n/locales/ru.json

@@ -143,6 +143,7 @@
   "{count}h ago": "{count} ч назад",
   "STALE · cached {age}": "УСТАРЕЛ · кэш {age}",
   "STALE": "УСТАРЕЛ",
+  "SIGNAL LOST · {time}": "СИГНАЛ ПОТЕРЯН · {time}",
   "refreshing...": "обновление...",
   "loading...": "загрузка...",
   "KEY REQUIRED": "ТРЕБУЕТСЯ КЛЮЧ",

src/i18n/locales/uk.json

@@ -332,6 +332,7 @@
   "LOADING": "ЗАВАНТАЖЕННЯ",
   "DEGRADED": "ПЕРЕБОЇ",
   "STALE": "ЗАСТАРІЛО",
+  "SIGNAL LOST · {time}": "СИГНАЛ ВТРАЧЕНО · {time}",
   "PARTIAL": "ЧАСТКОВО",
   "FALLBACK": "РЕЗЕРВ",
   "view band|STREET": "ВУЛИЦЯ",

src/i18n/locales/zh-CN.json

@@ -323,6 +323,7 @@
   "LOADING": "加载中",
   "DEGRADED": "降级",
   "STALE": "过期",
+  "SIGNAL LOST · {time}": "信号丢失 · {time}",
   "PARTIAL": "部分",
   "FALLBACK": "备用",
   "view band|STREET": "街道",

src/i18n/locales/zh-Hant.json

@@ -323,6 +323,7 @@
   "LOADING": "加載中",
   "DEGRADED": "降級",
   "STALE": "過期",
+  "SIGNAL LOST · {time}": "訊號遺失 · {time}",
   "PARTIAL": "部分",
   "FALLBACK": "備用",
   "view band|STREET": "街道",

src/layers/flights/motion.js

@@ -5,11 +5,15 @@ import {
   lerpAngleDeg,
   speedRamp,
   courseBetweenCartesians,
+  floorCoastedPosition,
   staleCoastLimitSeconds,
+  staleCoastSecondsFrom,
+  staleHoldsForFix,
   arcOffsetEnu,
   courseSlewCapDps,
   limitCourseStep,
   corridorPathLatLon,
+  COAST_MAX_SPEED_MPS,
 } from '../../data/motionModel.js';
 import { CLASS_SCALE_2D } from '../../data/aircraftClass.js';
 import {
@@ -248,30 +252,88 @@ export function createMotion({
       );
     }
 
-    // Newest POSITION is older than renderTime. OpenSky can still be receiving
-    // fresh contact/kinematic messages for that aircraft; freezing at a hard
-    // 60 s-after-position boundary produced the visible stop → catch-up → stop
-    // cadence. Coast through the latest real contact plus a bounded grace
-    // window, with an absolute cap so a stale cached feed cannot drift forever.
+    // Newest POSITION is older than renderTime. Coast on the last speed and
+    // heading for five minutes past it, then stop, dimmed as SIGNAL LOST.
+    // A contact last heard on or near the ground holds there instead, and a
+    // coast never goes below the ground.
+    const { coastSec } = _coastPlan(newest, info, elapsedSec);
+    _extrapolateFix(
+      newest,
+      info,
+      coastSec,
+      out,
+      (info && info.turnRateDps) || 0,
+    );
+    return coastSec > 0 ? floorCoastedPosition(out, cachedGroundFloor) : out;
+  }
+
+  /**
+   * How far a contact coasts past its newest fix at a display time
+   * `elapsedSec` after it: up to five minutes, or not at all for a contact
+   * that holds at a report on or near the ground. The hold decision is made once
+   * per report and kept on it.
+   * @param {{epochMs?: number, time: Cesium.JulianDate}} newest
+   * @param {object|undefined} info - The contact's record.
+   * @param {number} elapsedSec
+   * @returns {{coastSec: number, holds: boolean}}
+   */
+  function _coastPlan(newest, info, elapsedSec) {
     const coastLimitSec = staleCoastLimitSeconds({
       // `epochMs` is captured once when the poll is normalized. Avoid allocating
-      // a Date per aircraft on every 12 Hz fleet tick.
+      // a Date per aircraft on every fleet tick.
       fixEpochMs: Number.isFinite(newest.epochMs)
         ? newest.epochMs
         : Cesium.JulianDate.toDate(newest.time).getTime(),
       lastContactEpochMs: info?.lastContactEpochMs,
-      // Permit one minute of contact grace but cap any cached-feed drift at
-      // five minutes. Source backoff is exposed separately as a STALE cue.
-      minimumSec: 60,
+      // A plane that stops reporting keeps flying for five minutes, whatever
+      // its contact time, then stops dimmed as SIGNAL LOST. A missed update
+      // shows STALE throughout.
+      minimumSec: 300,
       maximumSec: 300,
     });
-    return _extrapolateFix(
+    const holds = staleHoldsForFix(newest, {
+      onGround: info?.onGround,
+      cachedGroundFloor,
+    });
+    return {
+      coastSec: staleCoastSecondsFrom({
+        newest,
+        elapsedSec,
+        coastLimitSec,
+        holds,
+      }),
+      holds,
+    };
+  }
+
+  /**
+   * Whether a contact has stopped where its coast ended (or held at its last
+   * report) with no newer report, and since when it has had none.
+   * @param {string} icao24
+   * @returns {{reportEpochMs: number, holds: boolean}|null} Null while a
+   *   report is ahead of the display time or the contact is still coasting.
+   */
+  function _signalLost(icao24) {
+    const newest = flightState._positionHistory.get(icao24)?.at(-1);
+    if (!newest) return null;
+    const newestMs = Number.isFinite(newest.epochMs)
+      ? newest.epochMs
+      : Cesium.JulianDate.toDate(newest.time).getTime();
+    const elapsedSec =
+      (flightState.now() - RENDER_DELAY_SEC * 1000 - newestMs) / 1000;
+    if (!(elapsedSec > 0)) return null;
+    const { coastSec, holds } = _coastPlan(
       newest,
-      info,
-      Math.min(elapsedSec, coastLimitSec),
-      out,
-      (info && info.turnRateDps) || 0,
+      flightState.records.data.get(icao24),
+      elapsedSec,
     );
+    if (!(elapsedSec > coastSec)) return null;
+    return {
+      reportEpochMs: Number.isFinite(newest.reportEpochMs)
+        ? newest.reportEpochMs
+        : newestMs,
+      holds,
+    };
   }
 
   /**
@@ -286,9 +348,12 @@ export function createMotion({
    */
 
   function _extrapolateFix(fix, info, dt, out, turnRateDps = 0) {
-    const speed = Number.isFinite(fix.velocity)
-      ? fix.velocity
-      : (info && info.velocity) || 0;
+    const speed = Math.min(
+      Number.isFinite(fix.velocity)
+        ? fix.velocity
+        : (info && info.velocity) || 0,
+      COAST_MAX_SPEED_MPS,
+    );
     const heading = Number.isFinite(fix.track)
       ? fix.track
       : (info && info.true_track) || 0;
@@ -1131,6 +1196,7 @@ export function createMotion({
   return {
     _refloorStaleGroundedContacts,
     _deadReckon,
+    _signalLost,
     _extrapolateFix,
     _isTrackWarmingUp,
     _trackedFocusSizePx,

src/layers/flights/policy.js

@@ -132,7 +132,7 @@ export const TRAIL_COLOR = '#00d4ff';
 
 /** @constant {number} Combined cap on trail vertices (backfill + live accumulation). */
 
-export const TRAIL_MAX_POINTS = 400;
+export const TRAIL_MAX_POINTS = 2000;
 
 // ---------------------------------------------------------------------------
 // Render-behind smoothing (PRD WS-C C2 — approved product decision):
@@ -471,3 +471,6 @@ export const FLOOR_SEED_GRACE_MS = 90_000;
  *  reloads synchronously. */
 
 export const IR_RELOAD_BATCH = 40;
+
+/** Alpha of a contact that has missed an update or lost its signal: the stale fade, for the fleet and the tracked aircraft alike. */
+export const STALE_ALPHA = 0.45;

src/layers/flights/recordPolicy.js

@@ -9,6 +9,10 @@ export const LANDED_MISSING_POLL_LIMIT = 1;
 
 export const MISSING_POLL_LIMIT = 3;
 
+/** The tracked contact is kept this long after its newest fix: five minutes
+ *  coasting, then five stopped as SIGNAL LOST. */
+export const TRACKED_RETENTION_MS = 10 * 60_000;
+
 /** Cooldown in milliseconds after a transient source error. */
 export const ERROR_BACKOFF_INTERVAL = 20000;
 

src/layers/flights/records.js

@@ -11,6 +11,7 @@ import {
   GROUND_FLOOR_WARM_MAX_ALT_M,
   LANDED_MISSING_POLL_LIMIT,
   MISSING_POLL_LIMIT,
+  TRACKED_RETENTION_MS,
 } from './recordPolicy.js';
 
 /** Source-independent aircraft metadata and bounded missing-poll retention. */
@@ -23,10 +24,45 @@ export class FlightRecords {
     };
     this.data = new Map();
     this.missingPolls = new Map();
+    // Contacts whose last poll re-sent a fix no newer than the one before.
+    this.fixRepeated = new Set();
+    this.lastSnapshotMs = null;
+    this.snapshotAdvanced = true;
     this.geoidNCache = new Map();
     this.geoidReady = false;
   }
 
+  /**
+   * Start reading a snapshot taken at `observedAtMs`. A fix re-sent in a
+   * newer snapshot marks its contact as not updated; the same snapshot read
+   * again does not.
+   * @param {number} observedAtMs
+   */
+  beginSnapshot(observedAtMs) {
+    this.snapshotAdvanced =
+      !Number.isFinite(observedAtMs) ||
+      !Number.isFinite(this.lastSnapshotMs) ||
+      observedAtMs > this.lastSnapshotMs;
+    if (Number.isFinite(observedAtMs))
+      this.lastSnapshotMs = Math.max(
+        this.lastSnapshotMs ?? -Infinity,
+        observedAtMs,
+      );
+  }
+
+  /** Note a received fix: the newest fix time, and whether it advanced. */
+  _noteFix(id, prevMeta, fixEpochMs) {
+    const prevFixMs = prevMeta?.newestFixMs;
+    // A fix counts as new only when it moves on by a second: feeds that
+    // derive fix time from receipt (readsb `now − seen_pos`, 0.1 s) jitter.
+    if (!Number.isFinite(prevFixMs) || fixEpochMs >= prevFixMs + 1000) {
+      this.fixRepeated.delete(id);
+      return fixEpochMs;
+    }
+    if (this.snapshotAdvanced) this.fixRepeated.add(id);
+    return prevFixMs;
+  }
+
   receive(
     observation,
     { viewerLatDeg, viewerLonDeg, trackedId, floorWarmPoints },
@@ -158,6 +194,12 @@ export class FlightRecords {
     //   (2) otherwise the SAME default policy `alt` already uses, so the two
     //       never disagree on the genuine "no data yet" case (a never-reported
     //       aircraft has no prior render height, so it lands here unchanged).
+    // Neither altitude this poll nor a held render height: the height below
+    // is the airborne default, invented. The tracked trail keeps no point there.
+    const renderAltitudeInvented =
+      pickedAltM == null &&
+      !onGround &&
+      !Number.isFinite(prevMeta?.renderAltitudeM);
     let renderAltitudeM;
     if (pickedAltM != null) {
       renderAltitudeM = pickedAltM;
@@ -204,9 +246,15 @@ export class FlightRecords {
     const routeHeld =
       Boolean(prevMeta?.routeCallsign) &&
       prevMeta.routeCallsign === nextCallsign.toUpperCase();
+    const fixEpochMs =
+      Number.isFinite(observation.positionTimeMs) &&
+      observation.positionTimeMs > 0
+        ? observation.positionTimeMs
+        : Date.now();
     const meta = {
       sourceReference: observation.reference,
       observedReceiptMs: Date.now(),
+      newestFixMs: this._noteFix(icao24, prevMeta, fixEpochMs),
       callsign: nextCallsign,
       altitude: alt,
       // geoAltitudeM/renderAltitudeM are ADDITIVE fields alongside the
@@ -215,6 +263,7 @@ export class FlightRecords {
       // still read `altitude`/baro).
       geoAltitudeM,
       renderAltitudeM,
+      renderAltitudeInvented,
       onGround,
       // Round 7: sticky airborne history — the landed fast-cull only
       // applies to contacts that actually flew this session.
@@ -276,15 +325,25 @@ export class FlightRecords {
     };
     this.data.set(icao24, meta);
 
-    const fixEpochMs =
-      Number.isFinite(observation.positionTimeMs) &&
-      observation.positionTimeMs > 0
-        ? observation.positionTimeMs
-        : Date.now();
     return { icao24, prevMeta, meta, groundFlipped, fixEpochMs };
   }
 
-  absence(id, { complete, retainIds, likelyLanded }) {
+  absence(id, { complete, retainIds, likelyLanded, tracked = false }) {
+    const misses = (this.missingPolls.get(id) || 0) + 1;
+    // The tracked contact has missed an update: it is stale from the first
+    // one, and kept (so the camera keeps its target) until ten minutes after
+    // its newest fix, whatever the snapshot says. With no fix time known it
+    // takes the ordinary limits below.
+    const meta = this.data.get(id);
+    const fixMs = meta?.newestFixMs ?? meta?.observedReceiptMs;
+    if (tracked && Number.isFinite(fixMs)) {
+      if (Date.now() - fixMs < TRACKED_RETENTION_MS) {
+        this.missingPolls.set(id, misses);
+        return 'stale';
+      }
+      this.missingPolls.delete(id);
+      return 'remove';
+    }
     // Partial admissions and position-less rows do not prove absence, but
     // retention remains bounded; a likely-landed position-less contact
     // still takes the landed limit.
@@ -293,7 +352,6 @@ export class FlightRecords {
       Date.now() - (this.data.get(id)?.observedReceiptMs ?? 0) < 300000
     )
       return 'retain';
-    const misses = (this.missingPolls.get(id) || 0) + 1;
     const limit = likelyLanded ? LANDED_MISSING_POLL_LIMIT : MISSING_POLL_LIMIT;
     if (misses < limit) {
       this.missingPolls.set(id, misses);
@@ -306,6 +364,7 @@ export class FlightRecords {
   forget(id) {
     this.data.delete(id);
     this.missingPolls.delete(id);
+    this.fixRepeated.delete(id);
     this.geoidNCache.delete(id);
   }
 }

src/layers/flights/records.test.mjs

@@ -164,3 +164,99 @@ test('a route belongs to its callsign: a new callsign drops it, a blank one keep
   assert.equal(next.meta.airline, null);
   assert.equal(next.meta.routeCallsign, null);
 });
+
+test('a contact with no altitude yet is marked as drawn at an invented height; a held one is not', () => {
+  const store = records();
+  const none = store.receive(
+    observation({ baroAltitudeM: null, ellipsoidAltitudeM: null }),
+    view(),
+  );
+  assert.equal(none.meta.renderAltitudeInvented, true);
+  const known = store.receive(
+    observation({ positionTimeMs: 1700000005000 }),
+    view(),
+  );
+  assert.equal(known.meta.renderAltitudeInvented, false);
+  const held = store.receive(
+    observation({
+      baroAltitudeM: null,
+      ellipsoidAltitudeM: null,
+      positionTimeMs: 1700000010000,
+    }),
+    view(),
+  );
+  assert.equal(held.meta.renderAltitudeInvented, false);
+  const parked = records().receive(
+    observation({ id: 'abc124', baroAltitudeM: null, onGround: true }),
+    view(),
+  );
+  assert.equal(parked.meta.renderAltitudeInvented, false);
+});
+
+test('the tracked contact is stale from its first missed update and kept until ten minutes after its newest fix', () => {
+  const realNow = Date.now;
+  const fixMs = Date.UTC(2026, 10, 9, 20, 0);
+  let clock = fixMs + 5_000;
+  Date.now = () => clock;
+  try {
+    for (const snapshot of [
+      { complete: true },
+      { complete: true, retainIds: new Set(['abc123']) },
+      { complete: false },
+    ]) {
+      const store = records();
+      store.receive(observation({ positionTimeMs: fixMs }), view());
+      const absent = (tracked) =>
+        store.absence('abc123', { ...snapshot, likelyLanded: false, tracked });
+      clock = fixMs + 15_000;
+      assert.equal(absent(true), 'stale');
+      assert.equal(store.missingPolls.get('abc123'), 1);
+      // Kept long after the untracked limits, measured from the fix time.
+      clock = fixMs + 9 * 60_000;
+      assert.equal(absent(true), 'stale');
+      clock = fixMs + 10 * 60_000 - 1;
+      assert.equal(absent(true), 'stale');
+      clock = fixMs + 10 * 60_000;
+      assert.equal(absent(true), 'remove');
+      assert.equal(store.missingPolls.has('abc123'), false);
+    }
+    // Untracked contacts keep their own limits.
+    const store = records();
+    store.receive(observation({ positionTimeMs: fixMs }), view());
+    clock = fixMs + 15_000;
+    const untracked = (snapshot) =>
+      store.absence('abc123', { ...snapshot, likelyLanded: false });
+    assert.equal(untracked({ complete: false }), 'retain');
+    assert.deepEqual(
+      [1, 2, 3].map(() => untracked({ complete: true })),
+      ['stale', 'stale', 'remove'],
+    );
+  } finally {
+    Date.now = realNow;
+  }
+});
+
+test('a repeated fix time marks the contact as not updated; removal is unchanged', () => {
+  const store = records();
+  const fixMs = Date.UTC(2026, 10, 9, 20, 0);
+  store.beginSnapshot(fixMs + 2_000);
+  store.receive(observation({ positionTimeMs: fixMs }), view());
+  assert.equal(store.fixRepeated.has('abc123'), false);
+  // The next poll re-sends the same position.
+  store.beginSnapshot(fixMs + 12_000);
+  store.receive(observation({ positionTimeMs: fixMs }), view());
+  assert.equal(store.fixRepeated.has('abc123'), true);
+  assert.equal(store.missingPolls.has('abc123'), false);
+  // The same snapshot read again is not a missed update.
+  store.beginSnapshot(fixMs + 12_000);
+  store.receive(observation({ positionTimeMs: fixMs }), view());
+  assert.equal(store.fixRepeated.has('abc123'), true);
+  // A newer fix clears it.
+  store.beginSnapshot(fixMs + 22_000);
+  store.receive(observation({ positionTimeMs: fixMs + 10_000 }), view());
+  assert.equal(store.fixRepeated.has('abc123'), false);
+  store.beginSnapshot(fixMs + 32_000);
+  store.receive(observation({ positionTimeMs: fixMs + 10_000 }), view());
+  store.forget('abc123');
+  assert.equal(store.fixRepeated.has('abc123'), false);
+});

src/layers/flights/rendering.js

@@ -39,6 +39,7 @@ import {
   COURSE_MAX_DPS,
   HIDDEN_CONTACT_DR_TICKS,
   hiddenContactDrPhase,
+  STALE_ALPHA,
 } from './policy.js';
 import { AIRCRAFT_ENVIRONMENT_MAP_OPTIONS } from '../../data/aircraftModelLighting.js';
 
@@ -680,6 +681,19 @@ export function createRendering({
    *  follow-camera's bounding sphere is ALWAYS ready — toggling 3D, or tracking while already zoomed
    *  in, can never stall or freeze the centering (the old model-graphic-on-entity failure mode). */
 
+  /** Fade the tracked 3D model like the billboard while the plane is stale. */
+
+  function _applyTrackedModelFade(model) {
+    const alpha = parts.tracking._trackedStaleCue?.(flightState._trackedIcao)
+      ?.dimmed
+      ? STALE_ALPHA
+      : 1;
+    if (model.color?.alpha === alpha) return;
+    model.color = (
+      flightState._irBoost ? Cesium.Color.WHITE : Cesium.Color.CYAN
+    ).withAlpha(alpha);
+  }
+
   function _updateTrackedModel() {
     const active =
       flightState._trackedIcao &&
@@ -823,6 +837,7 @@ export function createRendering({
         parts.motion._trackedDisplayCourse(),
         flightState._trackedModel.modelMatrix,
       );
+      _applyTrackedModelFade(flightState._trackedModel);
       if (!flightState._trackedModel.ready) return;
       const spec = _modelSpec(
         flightState.records.data.get(flightState._trackedIcao)?.klass,
@@ -849,6 +864,13 @@ export function createRendering({
   }
 
   function _fleetTick() {
+    // Every frame, shown or not: the tracked trail records where its icon is
+    // drawn and moves its body on (the head callback only reads the result),
+    // and the tracked label's stale cue follows the clock.
+    if (flightState._viewer) {
+      parts.tracking._trailTick?.();
+      parts.tracking._staleCueTick?.();
+    }
     if (
       !flightState._viewer ||
       !flightState._billboardCollection ||
@@ -1081,7 +1103,9 @@ export function createRendering({
           flightState._cockpitContactMode && !isCockpitNear
             ? 1
             : _fleetBillboardScale(icao24, info?.klass),
-        baseAlpha: flightState.records.missingPolls.get(icao24) ? 0.45 : 1,
+        baseAlpha: flightState.records.missingPolls.get(icao24)
+          ? STALE_ALPHA
+          : 1,
         baseColor,
         focusFactor: focus.factor,
         cameraDistanceM,

src/layers/flights/snapshotRenderer.js

@@ -3,6 +3,7 @@ import { cyberSonarBaseAlpha } from '../../cyberSonar.js';
 import {
   turnRateFromFixHistory,
   liftRepeatedGroundFix,
+  staleHoldsForFix,
   synthesizeForwardKinematicsFix,
 } from '../../data/motionModel.js';
 import { aircraftIcon } from '../../data/aircraftIcons.js';
@@ -22,7 +23,7 @@ export function createFlightSnapshotRenderer({
   queries,
 }) {
   const { refreshMilitaryRegistryIfStale, isMilitaryIcao } = militaryRegistry;
-  const { warmGroundFloor } = groundFloor;
+  const { warmGroundFloor, cachedGroundFloor } = groundFloor;
   const { sampleMeshFloorCells } = meshFloor;
   return function applySnapshot(snapshot, viewer) {
     const currentIcaos = new Set();
@@ -39,6 +40,7 @@ export function createFlightSnapshotRenderer({
       ? Cesium.Math.toDegrees(viewerCarto.longitude)
       : null;
     const floorWarmPoints = [];
+    records.beginSnapshot(snapshot.observedAtMs);
 
     // Classification and display policy consume source-independent observations.
     refreshMilitaryRegistryIfStale();
@@ -112,6 +114,9 @@ export function createFlightSnapshotRenderer({
           position: position.clone(),
           velocity: meta.velocity,
           track: meta.true_track,
+          // Drawn at an invented height: a trail seeded from history keeps no
+          // position for it.
+          inventedHeight: meta.renderAltitudeInvented === true,
         });
         if (history.length > POSITION_HISTORY_LIMIT) {
           history.shift();
@@ -125,7 +130,10 @@ export function createFlightSnapshotRenderer({
         // Round 2 (owner): ground traffic appends too — taxi history stays
         // live after touchdown (grounded flights positions are already
         // surface-clamped via the surfaceM chain, so the ground leg drapes).
-        if (isTracked) tracking._appendTrailFix(position.clone());
+        if (isTracked)
+          tracking._appendTrailFix(position.clone(), fixEpochMs, {
+            nullAltitude: meta.renderAltitudeInvented === true,
+          });
       } else {
         const modelOwnsGroundVisual = rendering._modelOwnsVisual(icao24);
         if (!modelOwnsGroundVisual) {
@@ -133,21 +141,28 @@ export function createFlightSnapshotRenderer({
         }
         // Apply fresh kinematics only from a forward synthetic fix. Mutating
         // the historical fix reprojects the entire stale interval and snaps
-        // the rendered aircraft when course or speed changes late.
+        // the rendered aircraft when course or speed changes late. The stale
+        // rules apply: none for a contact that holds at its report, a coast
+        // capped from the real report, never below the ground. It is a motion
+        // aid, not a report, so it never joins the trail.
         const kinematicsChanged =
           newest.velocity !== meta.velocity || newest.track !== meta.true_track;
-        if (kinematicsChanged) {
+        const holds = staleHoldsForFix(newest, {
+          onGround: meta.onGround,
+          cachedGroundFloor,
+        });
+        if (kinematicsChanged && !holds) {
           const synthetic = synthesizeForwardKinematicsFix(newest, {
             epochMs: Date.now(),
             velocity: meta.velocity,
             track: meta.true_track,
             turnRateDps: meta.turnRateDps,
+            cachedGroundFloor,
           });
           if (synthetic) {
             history.push(synthetic);
             if (history.length > POSITION_HISTORY_LIMIT) history.shift();
             meta.turnRateDps = turnRateFromFixHistory(history);
-            if (isTracked) tracking._appendTrailFix(synthetic.position.clone());
           }
         }
       }
@@ -252,6 +267,7 @@ export function createFlightSnapshotRenderer({
         complete: snapshot.complete,
         retainIds: snapshot.retainIds,
         likelyLanded: queries._likelyLanded(icao24),
+        tracked: icao24 === flightState._trackedIcao,
       });
       if (absence === 'retain') continue;
       if (absence === 'stale') {

src/layers/flights/snapshotStale.test.mjs

@@ -0,0 +1,153 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createFlightSnapshotRenderer } from './snapshotRenderer.js';
+
+// A poll that repeats a contact's last position with new speed or course
+// must not move it past where the stale rules allow, and never adds a
+// report to the tracked trail.
+
+const ICAO = '00a002';
+const SEC = 1000;
+
+/** Any property is a no-op function unless given. */
+function stub(overrides = {}) {
+  return new Proxy(overrides, {
+    get: (target, key) => (key in target ? target[key] : () => undefined),
+  });
+}
+
+function harness({ ground = () => null }) {
+  const appended = [];
+  const history = [];
+  const flightState = {
+    _trackedIcao: ICAO,
+    _trackedEntity: null,
+    _positionHistory: new Map([[ICAO, history]]),
+    _billboards: new Map([[ICAO, { position: new Cesium.Cartesian3() }]]),
+    _billboardCollection: stub(),
+    _cullPositions: new Map(),
+    _displayCourse: new Map(),
+    _groundSnap: stub(),
+    _displayFloorState: new Map(),
+    _models: new Map(),
+    _viewer: null,
+    _cockpitContactMode: false,
+    records: { data: new Map() },
+  };
+  let meta = null;
+  let fixEpochMs = 0;
+  const records = stub({
+    receive: () => {
+      flightState.records.data.set(ICAO, meta);
+      return { prevMeta: meta, meta, groundFlipped: false, fixEpochMs };
+    },
+    absence: () => 'retain',
+  });
+  const apply = createFlightSnapshotRenderer({
+    flightState,
+    records,
+    militaryRegistry: stub({ isMilitaryIcao: () => false }),
+    groundFloor: stub({ cachedGroundFloor: ground }),
+    meshFloor: stub(),
+    rendering: stub({
+      _modelOwnsVisual: () => false,
+      _fleetBillboardColor: () => Cesium.Color.WHITE,
+    }),
+    tracking: stub({
+      _militaryLayerSuppresses: () => false,
+      _appendTrailFix: (position, epochMs) =>
+        appended.push({ position, epochMs }),
+    }),
+    motion: stub(),
+    enrichment: stub(),
+    queries: stub({ _likelyLanded: () => false }),
+  });
+  return {
+    history,
+    appended,
+    poll(nextMeta, epochMs) {
+      meta = nextMeta;
+      fixEpochMs = epochMs;
+      apply({ records: [{ id: ICAO }], complete: true }, null);
+    },
+  };
+}
+
+const lightAircraft = (velocity, track) => ({
+  rawLat: 38.1436,
+  rawLon: -122.5561,
+  renderAltitudeM: 10,
+  altitude: 40,
+  velocity,
+  true_track: track,
+  onGround: false,
+  wasAirborne: true,
+  klass: 'light',
+});
+
+test('a low contact whose position repeats while its speed changes is not moved and adds nothing to the trail', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({ ground: () => -30 });
+  h.poll(lightAircraft(36.4, 63), t0);
+  const first = h.history[0].position;
+  for (const [i, velocity] of [30, 22, 14, 9].entries()) {
+    t.mock.timers.tick(10 * SEC);
+    h.poll(lightAircraft(velocity, 63 + i * 20), t0);
+  }
+  assert.equal(h.history.length, 1, 'a synthetic fix moved a holding contact');
+  assert.equal(h.history[0].position, first);
+  assert.equal(h.appended.length, 1, 'only the real report joins the trail');
+});
+
+test('a cruising contact whose position repeats coasts no further than the stale cap, and the synthetic fix is not a report', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({});
+  const cruise = (velocity, track) => ({
+    rawLat: 36,
+    rawLon: -115,
+    renderAltitudeM: 9_000,
+    altitude: 9_000,
+    velocity,
+    true_track: track,
+    onGround: false,
+    wasAirborne: true,
+    klass: 'jet',
+  });
+  h.poll(cruise(220, 90), t0);
+  const report = h.history[0].position;
+  t.mock.timers.tick(300 * SEC);
+  h.poll(cruise(200, 100), t0);
+  assert.equal(h.history.length, 2);
+  const moved = Cesium.Cartesian3.distance(report, h.history[1].position);
+  assert.ok(
+    moved <= 220 * 300 + 1,
+    `the synthetic fix coasted ${moved.toFixed(0)} m`,
+  );
+  assert.equal(h.appended.length, 1, 'the synthetic fix joined the trail');
+});
+
+test('a report drawn at an invented height says so on its history fix', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({});
+  h.poll({ ...lightAircraft(60, 90), renderAltitudeInvented: true }, t0);
+  h.poll({ ...lightAircraft(60, 90), rawLon: -122.55 }, t0 + 10 * SEC);
+  assert.equal(h.history[0].inventedHeight, true);
+  assert.equal(h.history[1].inventedHeight, false);
+});
+
+test('with the ground unknown, a low barometric altitude alone does not hold a contact', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({});
+  h.poll(lightAircraft(36.4, 63), t0);
+  t.mock.timers.tick(10 * SEC);
+  h.poll(lightAircraft(30, 80), t0);
+  // It coasts under the stale cap instead.
+  assert.equal(h.history.length, 2);
+  assert.equal(h.history[0].staleHolds, false);
+  assert.equal(h.appended.length, 1);
+});

src/layers/flights/staleHold.test.mjs

@@ -0,0 +1,421 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createMotion } from './motion.js';
+import { createFlightState } from './state.js';
+import { createTracking } from './tracking.js';
+import {
+  staleCoastSeconds,
+  staleHoldsAtLastReport,
+} from '../../data/motionModel.js';
+import { TRAIL_CONTINUITY_MAX_SPEED_KT } from '../../data/trailContinuity.js';
+import { RENDER_DELAY_SEC } from './policy.js';
+
+const SEC = 1000;
+// Ellipsoidal ground at a sea-level airfield (geoid about -32 m).
+const FIELD_GROUND_M = -30;
+
+/** A motion module over one contact's history, with a fixed clock. */
+function motionFixture({ info, fixes, nowMs, ground = () => null }) {
+  const flightState = {
+    records: { data: new Map([['00a001', info]]) },
+    _positionHistory: new Map([['00a001', fixes]]),
+    _focusEvidenceIds: new Set(),
+    _scratchArc: {},
+    _scratchOffset: new Cesium.Cartesian3(),
+    _scratchRenderTime: new Cesium.JulianDate(),
+    _scratchCoastCarto: new Cesium.Cartographic(),
+    now: () => nowMs,
+  };
+  const motion = createMotion({
+    flightState,
+    services: {
+      groundFloor: {
+        cachedGroundFloor: ground,
+        floorAltitudeM: (alt, g) =>
+          Number.isFinite(g) ? Math.max(alt, g + 1.5) : alt,
+      },
+      camera: {},
+      focus: {},
+    },
+    parts: {},
+  });
+  return { flightState, motion };
+}
+
+function fix(timeMs, lat, lon, heightM, velocity, track) {
+  return {
+    time: Cesium.JulianDate.fromDate(new Date(timeMs)),
+    epochMs: timeMs,
+    position: Cesium.Cartesian3.fromDegrees(lon, lat, heightM),
+    velocity,
+    track,
+  };
+}
+
+const height = (position) => Cesium.Cartographic.fromCartesian(position).height;
+
+test('a stale contact holds at its last report when it reported on the ground or low', () => {
+  assert.equal(staleHoldsAtLastReport({ onGround: true }), true);
+  // A light aircraft on final: -30 m barometric, its render height 1.5 m above the known ground.
+  assert.equal(
+    staleHoldsAtLastReport({
+      onGround: false,
+      heightM: FIELD_GROUND_M + 1.5,
+      groundM: FIELD_GROUND_M,
+    }),
+    true,
+  );
+  // Ground unknown: a low barometric altitude alone does not hold it; it
+  // coasts under the cap.
+  assert.equal(
+    staleHoldsAtLastReport({ onGround: false, heightM: 120 }),
+    false,
+  );
+  // A high airfield: 1,600 m barometric but 60 m above its ground.
+  assert.equal(
+    staleHoldsAtLastReport({
+      onGround: false,
+      heightM: 1_660,
+      groundM: 1_600,
+    }),
+    true,
+  );
+  assert.equal(
+    staleHoldsAtLastReport({
+      onGround: false,
+      heightM: 10_000,
+      groundM: 20,
+    }),
+    false,
+  );
+});
+
+test('a stale contact coasts up to its source limit at any speed, and a holding one not at all', () => {
+  const coast = (over) =>
+    staleCoastSeconds({
+      elapsedSec: 400,
+      coastLimitSec: 300,
+      speedMps: 230,
+      holds: false,
+      ...over,
+    });
+  assert.equal(coast(), 300);
+  assert.equal(coast({ elapsedSec: 200 }), 200);
+  assert.equal(coast({ coastLimitSec: 60 }), 60);
+  assert.equal(coast({ speedMps: 1_000 }), 300);
+  assert.equal(coast({ holds: true }), 0);
+  assert.equal(coast({ elapsedSec: -5 }), 0);
+});
+
+test('a light aircraft on short final goes stale after a low, slow, descending report, the icon holds at that report, not below the ground', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 38.1279, -122.5425, 122, 40, 340),
+    fix(t0 + 30 * SEC, 38.133, -122.5494, 38, 37, 10),
+    fix(t0 + 60 * SEC, 38.1436, -122.5561, FIELD_GROUND_M + 1.5, 36.4, 63),
+  ];
+  const info = {
+    altitude: -30,
+    renderAltitudeM: FIELD_GROUND_M + 1.5,
+    rawLat: 38.1436,
+    rawLon: -122.5561,
+    velocity: 36.4,
+    true_track: 63,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: t0 + 60 * SEC,
+  };
+  for (const afterSec of [5, 45, 90, 240]) {
+    const nowMs = t0 + 60 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC;
+    const { motion } = motionFixture({
+      info,
+      fixes,
+      nowMs,
+      ground: () => FIELD_GROUND_M,
+    });
+    const drawn = motion._deadReckon('00a001', new Cesium.Cartesian3());
+    assert.ok(
+      Cesium.Cartesian3.distance(drawn, fixes[2].position) < 0.01,
+      `${afterSec} s after the report the icon is at the report`,
+    );
+    assert.ok(height(drawn) >= FIELD_GROUND_M);
+  }
+});
+
+function cruiser(lastContactAfterSec) {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 40, -100, 10_000, 230, 90),
+    fix(t0 + 15 * SEC, 40, -99.96, 10_000, 230, 90),
+  ];
+  const info = {
+    altitude: 10_000,
+    renderAltitudeM: 10_000,
+    rawLat: 40,
+    rawLon: -99.96,
+    velocity: 230,
+    true_track: 90,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: t0 + 15 * SEC + lastContactAfterSec * SEC,
+  };
+  const at = (afterSec) => {
+    const nowMs = t0 + 15 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC;
+    const { motion } = motionFixture({ info, fixes, nowMs });
+    return motion._deadReckon('00a001', new Cesium.Cartesian3());
+  };
+  const d = (p) => Cesium.Cartesian3.distance(p, fixes[1].position);
+  return { at, d };
+}
+
+test('a cruising contact still heard after its last position coasts past 90 s, for up to 5 minutes, then stops', () => {
+  // Contact kept arriving for four minutes after the last position.
+  const { at, d } = cruiser(240);
+  const near = (got, want) => Math.abs(got - want) < want * 0.005 + 50;
+  assert.ok(near(d(at(90)), 230 * 90));
+  assert.ok(near(d(at(200)), 230 * 200), 'still moving at 200 s');
+  assert.ok(near(d(at(300)), 230 * 300));
+  assert.ok(near(d(at(400)), 230 * 300), 'stopped at 300 s');
+  assert.ok(Cesium.Cartesian3.distance(at(400), at(600)) < 0.01);
+});
+
+test('a cruising contact that stops reporting coasts five minutes, then stops', () => {
+  const { at, d } = cruiser(0);
+  assert.ok(Math.abs(d(at(60)) - 230 * 60) < 120);
+  assert.ok(Math.abs(d(at(300)) - 230 * 300) < 600);
+  assert.ok(Cesium.Cartesian3.distance(at(301), at(420)) < 0.01);
+});
+
+test('a bad reported speed coasts no faster than the 1,500 kt continuity limit', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  // 2,000 m/s (about 3,900 kt): a corrupt velocity, not a real aircraft.
+  const fixes = [
+    fix(t0, 40, -100, 10_000, 2_000, 90),
+    fix(t0 + 15 * SEC, 40, -99.96, 10_000, 2_000, 90),
+  ];
+  const info = {
+    altitude: 10_000,
+    renderAltitudeM: 10_000,
+    rawLat: 40,
+    rawLon: -99.96,
+    velocity: 2_000,
+    true_track: 90,
+    onGround: false,
+    wasAirborne: true,
+  };
+  const nowMs = t0 + 15 * SEC + RENDER_DELAY_SEC * SEC + 300 * SEC;
+  const { motion } = motionFixture({ info, fixes, nowMs });
+  const drawn = motion._deadReckon('00a001', new Cesium.Cartesian3());
+  const limitMps = (TRAIL_CONTINUITY_MAX_SPEED_KT * 1852) / 3600;
+  const d = Cesium.Cartesian3.distance(drawn, fixes[1].position);
+  assert.ok(Math.abs(d - limitMps * 300) < 1_500, `${d.toFixed(0)} m`);
+});
+
+test('coasting never takes a contact below the ground', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 39.7, -105.5, 3_400, 80, 270),
+    fix(t0 + 15 * SEC, 39.7, -105.514, 3_400, 80, 270),
+  ];
+  const info = {
+    altitude: 3_400,
+    renderAltitudeM: 3_400,
+    rawLat: 39.7,
+    rawLon: -105.514,
+    velocity: 80,
+    true_track: 270,
+    onGround: false,
+    wasAirborne: true,
+  };
+  // 3,000 m under the report, rising to 3,600 m along the coast west.
+  const nowMs = t0 + 15 * SEC + RENDER_DELAY_SEC * SEC + 60 * SEC;
+  const { motion } = motionFixture({
+    info,
+    fixes,
+    nowMs,
+    ground: (lat, lon) => (lon < -105.52 ? 3_600 : 3_000),
+  });
+  const drawn = motion._deadReckon('00a001', new Cesium.Cartesian3());
+  assert.ok(height(drawn) >= 3_600, `height ${height(drawn)}`);
+});
+
+// ---------------------------------------------------------------------------
+// The trail and head across a stale gap.
+
+function services(trail) {
+  const names = [
+    'picking',
+    'sprites',
+    'aircraftPresentation',
+    'camera',
+    'militaryRegistry',
+    'labels',
+    'meshFloor',
+    'focus',
+    'readout',
+    'context',
+    'render',
+    'recession',
+  ];
+  return {
+    ...Object.fromEntries(names.map((name) => [name, {}])),
+    groundSnap: { createGroundSnap: () => ({ clear() {} }) },
+    trails: trail,
+    geoid: { ensureGeoidReady: async () => {}, geoidHeight: () => 0 },
+    groundFloor: {
+      resolveGroundFloorCellsBounded: async () => {},
+      cachedGroundFloor: () => null,
+      floorAltitudeM: (alt) => alt,
+    },
+  };
+}
+
+function trailHarness({ fixes, nowMs, head }) {
+  const calls = [];
+  const trail = {
+    createTrail: () => ({
+      setSegments(segments) {
+        calls.push(segments.map((segment) => segment.slice()));
+      },
+      setPositions(positions) {
+        calls.push([positions.slice()]);
+      },
+      setVisible() {},
+      clear() {},
+      destroy() {},
+    }),
+  };
+  const svc = services(trail);
+  const flightState = createFlightState({
+    services: svc,
+    source: { label: 'Fixture aircraft' },
+  });
+  const added = [];
+  flightState._viewer = {
+    isDestroyed: () => false,
+    entities: {
+      add(definition) {
+        added.push(definition);
+        return definition;
+      },
+      remove() {},
+    },
+  };
+  flightState._trackedIcao = '00a001';
+  let clock = nowMs;
+  flightState.now = () => clock;
+  flightState._positionHistory.set('00a001', fixes);
+  let headPosition = head;
+  const tracking = createTracking({
+    flightState,
+    services: svc,
+    parts: {
+      motion: {
+        _isTrackWarmingUp: () => false,
+        _trackedTrailCached: () => headPosition,
+        _trackedDisplayPosition: () => headPosition,
+        _trackedModelCenterWorld: () => null,
+        _trackedModelEnvelopeM: () => null,
+      },
+    },
+    layer: {},
+  });
+  const headEntity = () =>
+    added.find((definition) =>
+      String(definition.id).startsWith('gev-trail:fl-head-'),
+    );
+  return {
+    tracking,
+    flightState,
+    drawn: () => (calls.at(-1) ?? []).flat(),
+    head: () => headEntity().polyline.positions.getValue(),
+    setClock: (ms) => {
+      clock = ms;
+    },
+    setHead: (position) => {
+      headPosition = position;
+    },
+  };
+}
+
+test('a holding contact draws no head, and the trail ends at the last report, never past the icon', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 38.1678, -122.527, 303, 45, 200),
+    fix(t0 + 30 * SEC, 38.1279, -122.5425, 122, 40, 340),
+    fix(t0 + 60 * SEC, 38.133, -122.5494, 38, 37, 10),
+    fix(t0 + 90 * SEC, 38.1436, -122.5561, 0, 36.4, 63),
+  ];
+  const last = fixes.at(-1).position;
+  // The display time is long past the last report; the icon holds there.
+  const h = trailHarness({
+    fixes,
+    nowMs: t0 + 90 * SEC + RENDER_DELAY_SEC * SEC + 120 * SEC,
+    head: Cesium.Cartesian3.clone(last),
+  });
+  h.tracking._startTrail('00a001');
+  // Every report, including the last, is in the trail: the head no longer
+  // cuts from the one before it to the icon.
+  assert.equal(h.drawn().length, 4);
+  assert.equal(h.drawn().at(-1), h.flightState._trailFixes.at(-1).position);
+  assert.deepEqual(h.head(), []);
+});
+
+test('while the display is between reports, the trail stops at the report behind the icon', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  // Reports every 10 s; the icon is drawn 30 s behind, between the 2nd and 3rd.
+  const fixes = [0, 1, 2, 3, 4, 5].map((i) =>
+    fix(t0 + i * 10 * SEC, 40, -100 + i * 0.03, 10_000, 230, 90),
+  );
+  const nowMs = t0 + 25 * SEC + RENDER_DELAY_SEC * SEC;
+  const icon = Cesium.Cartesian3.fromDegrees(-100 + 0.075, 40, 10_000);
+  const h = trailHarness({ fixes, nowMs, head: icon });
+  h.tracking._startTrail('00a001');
+  // Reports at 0, 10 and 20 s are behind the icon (display time 25 s).
+  assert.equal(h.drawn().length, 3);
+  const head = h.head();
+  assert.equal(head.length, 2);
+  assert.ok(Cesium.Cartesian3.distance(head[0], fixes[2].position) < 0.01);
+});
+
+test('the hold-or-coast decision is made once per report: a floor that resolves mid-coast does not snap the icon back', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 39.7, -104.6, 1_900, 70, 270),
+    fix(t0 + 15 * SEC, 39.7, -104.6123, 1_900, 70, 270),
+  ];
+  const info = {
+    altitude: 1_950,
+    renderAltitudeM: 1_900,
+    rawLat: 39.7,
+    rawLon: -104.6123,
+    velocity: 70,
+    true_track: 270,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: t0 + 15 * SEC + 120 * SEC,
+  };
+  // No floor known when the contact goes stale: baro says well clear of the
+  // ground, so it coasts. Mid-coast the floor resolves 100 m under the report.
+  let ground = () => null;
+  const at = (afterSec) => {
+    const nowMs = t0 + 15 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC;
+    const { motion } = motionFixture({
+      info,
+      fixes,
+      nowMs,
+      ground: (lat, lon) => ground(lat, lon),
+    });
+    return motion._deadReckon('00a001', new Cesium.Cartesian3());
+  };
+  const d = (p) => Cesium.Cartesian3.distance(p, fixes[1].position);
+  const before = d(at(30));
+  assert.ok(Math.abs(before - 70 * 30) < 50, `coasted ${before.toFixed(0)} m`);
+  ground = () => 1_800;
+  const after = d(at(31));
+  assert.ok(
+    after > before,
+    `the icon went from ${before.toFixed(0)} m to ${after.toFixed(0)} m`,
+  );
+});

src/layers/flights/state.js

@@ -195,9 +195,32 @@ export function createFlightState({ source, services }) {
 
   flightState._trailHeadSeq = 0;
 
-  /** @type {Cesium.Cartesian3[]} Chronological tracked-aircraft fixes (oldest first) */
+  /** @type {{position: Cesium.Cartesian3|null, timeMs: number, nullAltitude: boolean, breakBefore: boolean, drawn?: boolean}[]}
+   *  Chronological tracked-aircraft trail (oldest first): reports, and
+   *  (`drawn`) where the icon was drawn between them; `breakBefore` starts a
+   *  new segment. */
 
-  flightState._trailPositions = [];
+  flightState._trailFixes = [];
+
+  /** @type {number} Body fixes drawn: those at or before the display time. */
+
+  flightState._trailBodyCount = 0;
+
+  /** @type {Cesium.Cartesian3|null} Where the tracked icon was last recorded into the trail. */
+
+  flightState._trailRecordPosition = null;
+
+  /** @type {number} Display time (epoch ms) of that recording. */
+
+  flightState._trailRecordMs = -Infinity;
+
+  /** @type {string|undefined} The stale cue last written into the tracked label (see `_staleCueTick`). */
+
+  flightState._trackedStaleCueText = undefined;
+
+  /** @type {{key: string, cue: {text: string, dimmed: boolean}}|null} This frame's tracked stale cue (see `_trackedStaleCue`). */
+
+  flightState._trackedStaleCueCache = null;
 
   /** @type {number} Monotonic token — invalidates in-flight backfill responses */
 

src/layers/flights/tracking.js

@@ -1,5 +1,4 @@
 import * as Cesium from 'cesium';
-import { julianAt } from '../../time/julian.js';
 import { formatNumber, t } from '../../i18n/index.js';
 import { unitLabel } from '../../ui/unitLabels.js';
 import { nextCockpitNearContacts } from '../../data/cockpitAirLod.js';
@@ -21,12 +20,31 @@ import {
   TRACKED_MODEL_MAX_LOAD_FAILS,
   TRACKED_MODEL_RETRY_BACKOFF_MS,
   TRAIL_MAX_POINTS,
+  DR_CORRECTION_MS,
   RENDER_DELAY_SEC,
   TRAIL_COLOR,
   CYAN_TRANSPARENT,
+  STALE_ALPHA,
 } from './policy.js';
+import {
+  drawTrailSegments,
+  historyFixEpochMs,
+  insertTrailFix,
+  joinTrailBackfill,
+  recordTrailPoint,
+  trailBackfillFixes,
+  trailBodyCount,
+  trailBodyCountOutsideModel,
+  trailBodySegments,
+  trailHeadStartFix,
+  trailRecordingBlocked,
+  TRAIL_RECORD_STEP_M,
+  TRAIL_RECORD_STEP_MS,
+} from '../../data/trailContinuity.js';
 import { roundHeadingDegrees } from '../../data/headingDegrees.js';
 
+const STALE_CYAN = Cesium.Color.CYAN.withAlpha(STALE_ALPHA);
+
 export function createTracking({
   flightState,
   services,
@@ -333,18 +351,81 @@ export function createTracking({
   }
 
   /**
-   * Append one fix to the tracked aircraft's trail accumulation and refresh
-   * the rendered trail. Caller passes an owned (cloned) Cartesian3.
-   * @param {Cesium.Cartesian3} position - New fix position, appended at the head.
+   * Put one report into the tracked aircraft's trail, in time order, and
+   * refresh the rendered trail. Caller passes an owned (cloned) Cartesian3. A
+   * report that breaks from its neighbour starts a new segment (see
+   * trailContinuity).
+   * @param {Cesium.Cartesian3} position - The report's position.
+   * @param {number} [epochMs] - The report's time; defaults to the tracked
+   *   aircraft's newest position-history fix, which every caller has just pushed.
+   * @param {{nullAltitude?: boolean}} [options] - `nullAltitude`: the report
+   *   had no altitude, so its position is not kept and the line breaks there.
+   *   A report for a time the display has already passed replaces what was
+   *   drawn after it (see `insertTrailFix`).
    */
 
-  function _appendTrailFix(position) {
-    flightState._trailPositions.push(position);
-    if (flightState._trailPositions.length > TRAIL_MAX_POINTS)
-      flightState._trailPositions.shift();
+  function _appendTrailFix(position, epochMs, { nullAltitude = false } = {}) {
+    const history = flightState._positionHistory.get(flightState._trackedIcao);
+    const timeMs = Number.isFinite(epochMs)
+      ? epochMs
+      : historyFixEpochMs(history?.[history.length - 1]);
+    insertTrailFix(
+      flightState._trailFixes,
+      position,
+      timeMs,
+      TRAIL_MAX_POINTS,
+      {
+        nullAltitude,
+        displayMs: flightState.now() - RENDER_DELAY_SEC * 1000,
+        settleMs: DR_CORRECTION_MS,
+      },
+    );
     _refreshTrailDisplay();
   }
 
+  /**
+   * Per-frame trail work, run from the layer's preRender pass whether or not
+   * the head is visible: record where the tracked icon is drawn (at most once
+   * a second or every 150 m) and move the body on as the display time passes
+   * reports. The head callback only reads the result.
+   */
+
+  function _trailTick() {
+    if (!flightState._trackedIcao || !flightState._trail) return;
+    const displayMs = flightState.now() - RENDER_DELAY_SEC * 1000;
+    let changed = false;
+    if (
+      flightState._trailFixes.length &&
+      !parts.motion._isTrackWarmingUp() &&
+      !trailRecordingBlocked(flightState._trailFixes, displayMs)
+    ) {
+      const drawn = parts.motion._trackedDisplayPosition(
+        flightState._trackedIcao,
+      );
+      const last = flightState._trailRecordPosition;
+      if (
+        drawn &&
+        (!last ||
+          displayMs - flightState._trailRecordMs >= TRAIL_RECORD_STEP_MS ||
+          Cesium.Cartesian3.distance(last, drawn) >= TRAIL_RECORD_STEP_M)
+      ) {
+        flightState._trailRecordMs = displayMs;
+        flightState._trailRecordPosition = Cesium.Cartesian3.clone(
+          drawn,
+          flightState._trailRecordPosition ?? undefined,
+        );
+        changed = recordTrailPoint(
+          flightState._trailFixes,
+          Cesium.Cartesian3.clone(drawn),
+          displayMs,
+          TRAIL_MAX_POINTS,
+        );
+      }
+    }
+    if (changed || _trailBodyCountNow() !== flightState._trailBodyCount)
+      _refreshTrailDisplay();
+  }
+
   /**
    * Renders the trail with its head clamped to the render-behind display
    * position. Raw newest fixes run up to RENDER_DELAY_SEC ahead of the
@@ -354,17 +435,33 @@ export function createTracking({
    */
 
   function _refreshTrailDisplay() {
-    // The trail BODY is the accumulated fixes EXCLUDING the newest raw one — that newest
-    // fix is at ~now, ~one poll interval AHEAD of the delayed icon (rendered at
-    // now − RENDER_DELAY_SEC), so drawing it would push the trail in front of the plane.
-    // The cheap per-frame _trailHeadEntity segment bridges the last body point to the
-    // delayed dead-reckoned head, so the body primitive only rebuilds on a real fix
-    // (poll cadence), never at motion cadence.
+    // The trail BODY is the accumulated fixes at or before the delayed display time
+    // (now − RENDER_DELAY_SEC); newer fixes are AHEAD of the icon, so drawing them
+    // would push the trail in front of the plane. The cheap per-frame
+    // _trailHeadEntity segment bridges the last body point to the delayed
+    // dead-reckoned head, so the body rebuilds only on a real fix or when the
+    // display time passes one, never at motion cadence.
+    // A break lifts the pen: each segment is its own line, nothing between them.
     if (!flightState._trail) return;
-    flightState._trail.setPositions(
-      flightState._trailPositions.length > 1
-        ? flightState._trailPositions.slice(0, -1)
-        : flightState._trailPositions,
+    flightState._trailBodyCount = _trailBodyCountNow();
+    drawTrailSegments(
+      flightState._trail,
+      trailBodySegments(flightState._trailFixes, flightState._trailBodyCount),
+    );
+  }
+
+  /** Fixes at or before the time the aircraft is drawn (now − RENDER_DELAY_SEC),
+   *  less the newest recorded points inside its 3D model. */
+
+  function _trailBodyCountNow() {
+    return trailBodyCountOutsideModel(
+      flightState._trailFixes,
+      trailBodyCount(
+        flightState._trailFixes,
+        flightState.now() - RENDER_DELAY_SEC * 1000,
+      ),
+      parts.motion._trackedModelCenterWorld(),
+      parts.motion._trackedModelEnvelopeM(),
     );
   }
 
@@ -377,21 +474,24 @@ export function createTracking({
 
   function _startTrail(icao24) {
     flightState._trailBackfillToken += 1;
-    flightState._trailPositions = [];
+    flightState._trailFixes = [];
+    flightState._trailRecordPosition = null;
+    flightState._trailRecordMs = -Infinity;
     const history = flightState._positionHistory.get(icao24) || [];
-    // Seed only fixes at/behind the DELAYED display time (now − RENDER_DELAY_SEC). The
-    // newest ~RENDER_DELAY_SEC of fixes are AHEAD of the displayed icon; including them
-    // would draw the trail in front of the plane. They join the trail via _appendTrailFix
-    // as they age past the delay.
-    const seedRenderTime = Cesium.JulianDate.addSeconds(
-      julianAt(flightState.now()),
-      -RENDER_DELAY_SEC,
-      flightState._scratchWarmupTime,
-    );
+    // Seed every report. The body only draws those at or before the delayed
+    // display time (now − RENDER_DELAY_SEC); newer ones join it as the display
+    // passes them. A synthetic forward fix (it carries `reportEpochMs`) is a
+    // motion aid, never a report. A report drawn at an invented height keeps
+    // no position and breaks the line.
     for (const fix of history) {
-      if (Cesium.JulianDate.lessThanOrEquals(fix.time, seedRenderTime)) {
-        flightState._trailPositions.push(Cesium.Cartesian3.clone(fix.position));
-      }
+      if (Number.isFinite(fix.reportEpochMs)) continue;
+      insertTrailFix(
+        flightState._trailFixes,
+        Cesium.Cartesian3.clone(fix.position),
+        historyFixEpochMs(fix),
+        TRAIL_MAX_POINTS,
+        { nullAltitude: fix.inventedHeight === true },
+      );
     }
     if (!flightState._trail && flightState._viewer) {
       flightState._trail = createTrail(flightState._viewer, {
@@ -412,14 +512,13 @@ export function createTracking({
         show: !flightState._cockpitContactMode,
         polyline: {
           positions: new Cesium.CallbackProperty(() => {
-            // Need ≥2 accumulated points: the body draws all-but-newest, so the head must
-            // start at the last DISPLAYED body point (index n−2). With a single fix that
-            // point would be the sole raw fix — which is ~now, AHEAD of the delayed icon —
-            // so the segment would draw IN FRONT of the plane. Likewise during warm-up the
-            // icon predates all real history, so there is no valid body point behind it.
+            // The head starts at the last DISPLAYED body point, the newest fix at or
+            // before the delayed display time; a fix ahead of the icon would draw the
+            // segment IN FRONT of the plane. During warm-up the icon predates all real
+            // history, so there is no valid body point behind it.
             if (
               !flightState._trackedIcao ||
-              flightState._trailPositions.length < 2 ||
+              !flightState._trailFixes.length ||
               parts.motion._isTrackWarmingUp()
             )
               return [];
@@ -427,12 +526,19 @@ export function createTracking({
               parts.motion._trackedTrailCached() ||
               parts.motion._trackedDisplayPosition(flightState._trackedIcao);
             if (!head) return [];
-            // body[n−2] (last displayed body point) → delayed head: runs FORWARD, never a
-            // backward/reversing segment.
-            const start =
-              flightState._trailPositions[
-                flightState._trailPositions.length - 2
-              ];
+            // The last body point (a report, or where the icon was drawn a
+            // moment ago) → the icon. The body is moved on by _trailTick, never
+            // here: a callback must not add or remove entities.
+            // No head across a break: a body point more than 150 km away, or
+            // too long ago or too far to be one path, stays detached; none while
+            // the aircraft holds at its last point.
+            const last = trailHeadStartFix(
+              flightState._trailFixes,
+              head,
+              flightState.now(),
+              flightState._trailBodyCount,
+            );
+            if (!last) return [];
             // On a contact that has not moved this segment runs from inside the
             // model out to its own anchor — a line through the fuselage. The END
             // never gives, so a moving trail still terminates on the tail; the
@@ -441,7 +547,7 @@ export function createTracking({
             // See trailHeadStart. Read `head` in place and clone only on the draw
             // path — a suppressed parked contact runs this every frame.
             const from = trailHeadStart(
-              start,
+              last.position,
               head,
               parts.motion._trackedModelCenterWorld(),
               parts.motion._trackedModelEnvelopeM(),
@@ -520,7 +626,13 @@ export function createTracking({
       const time = observedAtMs / 1000;
       if (!Number.isFinite(lat) || !Number.isFinite(lon)) continue;
       if (!Number.isFinite(time) || time >= oldestFixEpochSec) continue;
-      parsed.push({ lat, lon, baroAlt });
+      parsed.push({
+        lat,
+        lon,
+        baroAlt,
+        onGround: waypoint.onGround === true,
+        timeMs: observedAtMs,
+      });
     }
     if (!parsed.length) return;
 
@@ -543,27 +655,30 @@ export function createTracking({
     )
       return;
 
-    const older = [];
-    let lastAltM = null; // carry-forward for no-baro points whose cell isn't warm yet
-    for (const { lat, lon, baroAlt } of parsed) {
-      const baroM = Number.isFinite(baroAlt)
-        ? baroAlt + geoidHeight(lat, lon)
-        : null;
-      let altM = floorAltitudeM(baroM, cachedGroundFloor(lat, lon));
-      // No baro + unresolved floor: hold the previous waypoint's altitude
-      // (continuity — never a dive/spike to a made-up height). Leading points
-      // with nothing to carry keep the old 10 km airborne default.
-      if (altM == null) altM = lastAltM != null ? lastAltM : 10000;
-      lastAltM = altM;
-      older.push(Cesium.Cartesian3.fromDegrees(lon, lat, altM));
-    }
-
-    flightState._trailPositions = older.concat(flightState._trailPositions);
-    if (flightState._trailPositions.length > TRAIL_MAX_POINTS) {
-      flightState._trailPositions = flightState._trailPositions.slice(
-        flightState._trailPositions.length - TRAIL_MAX_POINTS,
-      );
-    }
+    // Every waypoint is kept, each leg its own segment. A ground waypoint is
+    // at altitude 0, floored at the ground. A waypoint with no altitude is a
+    // break on both sides: it sits on the ground where the floor is known and
+    // is skipped otherwise; a skipped point still breaks the line.
+    const backfill = trailBackfillFixes(
+      parsed.map(({ lat, lon, baroAlt, onGround, timeMs }) => {
+        const known = Number.isFinite(baroAlt) ? baroAlt : onGround ? 0 : null;
+        const altM = floorAltitudeM(
+          known == null ? null : known + geoidHeight(lat, lon),
+          cachedGroundFloor(lat, lon),
+        );
+        return {
+          position:
+            altM == null ? null : Cesium.Cartesian3.fromDegrees(lon, lat, altM),
+          timeMs,
+          nullAltitude: known == null,
+        };
+      }),
+    );
+    flightState._trailFixes = joinTrailBackfill(
+      backfill,
+      flightState._trailFixes,
+      TRAIL_MAX_POINTS,
+    );
     _refreshTrailDisplay();
   }
 
@@ -573,7 +688,9 @@ export function createTracking({
 
   function _clearTrail() {
     flightState._trailBackfillToken += 1;
-    flightState._trailPositions = [];
+    flightState._trailFixes = [];
+    flightState._trailRecordPosition = null;
+    flightState._trailRecordMs = -Infinity;
     if (flightState._trail) flightState._trail.clear();
     if (
       flightState._trailHeadEntity &&
@@ -737,14 +854,90 @@ export function createTracking({
     flightState._pendingTrackingRestore = null;
   }
 
+  /**
+   * The tracked plane's stale cue and whether it is drawn with the stale fade.
+   * - SIGNAL LOST · <time since its last report>, dimmed: its coast has ended
+   *   (or it holds at its last report after a missed update) and no newer
+   *   report has come.
+   * - STALE, dimmed: it has missed an update and still coasts.
+   * - STALE, not dimmed: the whole feed is backing off.
+   * @param {string} icao24
+   * @returns {{text: string, dimmed: boolean}}
+   */
+
+  function _trackedStaleCue(icao24) {
+    const records = flightState.records;
+    const misses = records.missingPolls.get(icao24) || 0;
+    const repeated = records.fixRepeated?.has(icao24) === true;
+    const backoff = flightState.feed._backoff === true;
+    // Computed once per frame for the label, the icon colour and the model
+    // fade; a poll between frames changes the key.
+    const frame = flightState._viewer?.scene?.frameState?.frameNumber;
+    const key = Number.isFinite(frame)
+      ? `${frame}|${icao24}|${misses}|${repeated}|${backoff}`
+      : null;
+    const cached = flightState._trackedStaleCueCache;
+    if (key !== null && cached?.key === key) return cached.cue;
+    const cue = _computeTrackedStaleCue(
+      icao24,
+      misses > 0 || repeated,
+      backoff,
+    );
+    flightState._trackedStaleCueCache = key === null ? null : { key, cue };
+    return cue;
+  }
+
+  /** `_trackedStaleCue` without the per-frame cache. A repeated fix counts as missed. */
+
+  function _computeTrackedStaleCue(icao24, missed, backoff) {
+    const lost = parts.motion._signalLost(icao24);
+    if (lost && (missed || !lost.holds)) {
+      const sec = Math.max(
+        0,
+        Math.floor((flightState.now() - lost.reportEpochMs) / 1000),
+      );
+      const time =
+        sec < 60
+          ? `${formatNumber(sec, { useGrouping: false })} ${unitLabel('s')}`
+          : `${formatNumber(Math.floor(sec / 60), { useGrouping: false })} ${unitLabel('min')}`;
+      return { text: t('SIGNAL LOST · {time}', { time }), dimmed: true };
+    }
+    if (missed) return { text: t('STALE'), dimmed: true };
+    if (backoff) return { text: t('STALE'), dimmed: false };
+    return { text: '', dimmed: false };
+  }
+
+  /** The tracked billboard's colour: cyan, faded while stale, clear while the 3D model is the visual. */
+
+  function _trackedIconColor() {
+    if (parts.rendering._modelOwnsVisual(flightState._trackedIcao))
+      return CYAN_TRANSPARENT;
+    return _trackedStaleCue(flightState._trackedIcao).dimmed
+      ? STALE_CYAN
+      : Cesium.Color.CYAN;
+  }
+
+  /**
+   * Re-word the tracked label when its stale cue changes (STALE, SIGNAL LOST,
+   * each new second or minute of it). Run from the layer's per-frame pass.
+   */
+
+  function _staleCueTick() {
+    const icao24 = flightState._trackedIcao;
+    if (!icao24 || !flightState._trackedEntity) return;
+    const text = _trackedStaleCue(icao24).text;
+    if (text === flightState._trackedStaleCueText) return;
+    flightState._trackedStaleCueText = text;
+    _updateTrackedLabelModel(icao24);
+  }
+
   /** Multi-line tracked presentation text: "CS · FL · kts" + "Airline · Type" +
    *  "ORIG → DEST". The route line is gated by
    *  routePlausible so a wrong-leg adsbdb route is hidden, not displayed.
-   *  While the plane is in its missed-poll grace (coasting on dead reckoning
-   *  with sticky metadata), the first line carries a "· STALE" cue — the fleet's
-   *  45%-alpha billboard fade doesn't apply to the tracked plane (its entity
-   *  owns the visual), so without this the readout would present last-known
-   *  velocity/altitude as live. */
+   *  The first line carries the stale cue (`_trackedStaleCue`): STALE while
+   *  the plane coasts on dead reckoning after a missed update, SIGNAL LOST
+   *  once the coast has ended, so last-known velocity/altitude are never
+   *  presented as live. */
 
   function _trackedLabelText(icao24) {
     const info = flightState.records.data.get(icao24);
@@ -765,10 +958,7 @@ export function createTracking({
     const spd = info.velocity
       ? quantity(Math.round(info.velocity * 1.944), 'kts')
       : '';
-    const stale =
-      flightState.records.missingPolls.get(icao24) || flightState.feed._backoff
-        ? t('STALE')
-        : '';
+    const stale = _trackedStaleCue(icao24).text;
     const lines = [[cs, fl, spd, stale].filter(Boolean).join(' · ')];
     // Converted contacts report their class as TR-3B and nothing else — the
     // operator/type identity is exactly what the Easter egg is replacing.
@@ -967,15 +1157,9 @@ export function createTracking({
           CLASS_SCALE_2D[
             flightState.records.data.get(flightState._trackedIcao)?.klass
           ] || 1,
-        // Solid cyan when the billboard is the visual (zoomed out, 3D off, or model still loading);
-        // transparent once the STANDALONE tracked model is actually up (ready + shown).
-        color: new Cesium.CallbackProperty(
-          () =>
-            parts.rendering._modelOwnsVisual(flightState._trackedIcao)
-              ? CYAN_TRANSPARENT
-              : Cesium.Color.CYAN,
-          false,
-        ),
+        // Solid cyan (faded while stale) when the billboard is the visual (zoomed out, 3D off,
+        // or model still loading); transparent once the STANDALONE tracked model is actually up.
+        color: new Cesium.CallbackProperty(_trackedIconColor, false),
         sizeInMeters: false,
         scaleByDistance: new Cesium.NearFarScalar(1000, 3.0, 8000000, 0.5),
         alignedAxis: Cesium.Cartesian3.ZERO,
@@ -1257,6 +1441,7 @@ export function createTracking({
     _trackedModelRegimeActive,
     _syncTracked2dRotation,
     _appendTrailFix,
+    _trailTick,
     _refreshTrailDisplay,
     _startTrail,
     _backfillTrail,
@@ -1267,6 +1452,9 @@ export function createTracking({
     _applyPendingTrackingRestore,
     _cancelPendingTrackingRestore,
     _trackedLabelText,
+    _trackedStaleCue,
+    _trackedIconColor,
+    _staleCueTick,
     _updateTrackedLabelModel,
     _syncTrackedBillboardImage,
     _refreshTr3bContact,

src/layers/flights/trail.test.mjs

@@ -0,0 +1,414 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createFlightState } from './state.js';
+import { createTracking } from './tracking.js';
+import { TRAIL_MAX_POINTS } from './policy.js';
+import { trailHeadJoins, trailPairBreaks } from '../../data/trailContinuity.js';
+
+const MINUTE = 60_000;
+const HOUR = 60 * MINUTE;
+const NM = 1852;
+
+function services({ trails, floor = () => null } = {}) {
+  const names = [
+    'picking',
+    'sprites',
+    'aircraftPresentation',
+    'camera',
+    'militaryRegistry',
+    'labels',
+    'meshFloor',
+    'focus',
+    'readout',
+    'context',
+    'render',
+    'recession',
+  ];
+  return {
+    ...Object.fromEntries(names.map((name) => [name, {}])),
+    groundSnap: { createGroundSnap: () => ({ clear() {} }) },
+    trails,
+    geoid: { ensureGeoidReady: async () => {}, geoidHeight: () => 0 },
+    groundFloor: {
+      resolveGroundFloorCellsBounded: async () => {},
+      cachedGroundFloor: floor,
+      floorAltitudeM: (alt, ground) =>
+        Number.isFinite(alt)
+          ? alt
+          : Number.isFinite(ground)
+            ? ground + 1
+            : null,
+    },
+  };
+}
+
+/** A fake trail that records every segment list it is given. */
+function recordingTrail() {
+  const calls = [];
+  return {
+    calls,
+    // What draws: a lone point is a segment with no line.
+    last: () =>
+      (calls[calls.length - 1] ?? []).filter((segment) => segment.length >= 2),
+    createTrail: () => ({
+      setSegments(segments) {
+        calls.push(segments.map((segment) => segment.slice()));
+      },
+      setPositions(positions) {
+        calls.push([positions.slice()]);
+      },
+      setVisible() {},
+      clear() {
+        calls.push([]);
+      },
+      destroy() {},
+    }),
+  };
+}
+
+function viewer() {
+  const added = [];
+  return {
+    added,
+    isDestroyed: () => false,
+    entities: {
+      add(definition) {
+        added.push(definition);
+        return definition;
+      },
+      remove() {},
+    },
+  };
+}
+
+/**
+ * A tracked aircraft whose live history holds `liveFixes` ({timeMs, lat, lon}),
+ * with a track endpoint answering `waypoints`.
+ */
+function trackedHarness({
+  liveFixes = [],
+  waypoints = null,
+  nowMs,
+  floor,
+  head = null,
+} = {}) {
+  const trail = recordingTrail();
+  const svc = services({ trails: trail, floor });
+  const flightState = createFlightState({
+    services: svc,
+    source: {
+      label: 'Fixture aircraft',
+      async getTrack() {
+        if (!waypoints) throw new Error('404');
+        return { records: waypoints };
+      },
+    },
+  });
+  const v = viewer();
+  flightState._viewer = v;
+  flightState._trackedIcao = 'abc123';
+  flightState.now = () => nowMs;
+  flightState._positionHistory.set(
+    'abc123',
+    liveFixes.map((fix) => ({
+      time: Cesium.JulianDate.fromDate(new Date(fix.timeMs)),
+      epochMs: fix.timeMs,
+      position: Cesium.Cartesian3.fromDegrees(fix.lon, fix.lat, 10_000),
+    })),
+  );
+  const tracking = createTracking({
+    flightState,
+    services: svc,
+    parts: {
+      motion: {
+        _isTrackWarmingUp: () => false,
+        _trackedTrailCached: () => head,
+        _trackedDisplayPosition: () => head,
+        _trackedModelCenterWorld: () => null,
+        _trackedModelEnvelopeM: () => null,
+      },
+    },
+    layer: {},
+  });
+  return { trail, tracking, flightState, viewer: v };
+}
+
+const settle = () => new Promise((resolve) => setTimeout(resolve, 0));
+
+function waypoint(timeMs, lat, lon, baroAltitudeM = 10_000) {
+  return { observedAtMs: timeMs, latitude: lat, longitude: lon, baroAltitudeM };
+}
+
+/** `count` waypoints one minute apart flying east at ~420 kt from (lat, lon). */
+function leg(startMs, lat, lon, count) {
+  return Array.from({ length: count }, (_, i) =>
+    waypoint(startMs + i * MINUTE, lat, lon + i * 0.12),
+  );
+}
+
+function headPositions(v) {
+  const entity = v.added.find((definition) =>
+    String(definition.id).startsWith('gev-trail:fl-head-'),
+  );
+  return entity.polyline.positions.getValue();
+}
+
+test('a pair 40 minutes and 300 km apart stays joined', () => {
+  assert.equal(trailPairBreaks(40 * MINUTE, 300_000), false);
+});
+
+test('a pair 1 minute and 200 km apart breaks: faster than 1,500 kt', () => {
+  assert.equal(trailPairBreaks(MINUTE, 200_000), true);
+});
+
+test('a gap longer than 48 minutes breaks, and time that does not advance breaks', () => {
+  assert.equal(trailPairBreaks(49 * MINUTE, 10_000), true);
+  assert.equal(trailPairBreaks(48 * MINUTE, 10_000), false);
+  assert.equal(trailPairBreaks(0, 0), true);
+  assert.equal(trailPairBreaks(-MINUTE, 100), true);
+});
+
+test('a parked aircraft two hours apart within 0.1 nm stays joined', () => {
+  assert.equal(trailPairBreaks(2 * HOUR, 0.09 * NM), false);
+  assert.equal(trailPairBreaks(3 * HOUR + MINUTE, 0.09 * NM), true);
+  assert.equal(trailPairBreaks(2 * HOUR, 0.2 * NM), true);
+});
+
+test('the head joins within 150 km under the time and speed test', () => {
+  assert.equal(trailHeadJoins(30_000, 5_000), true);
+  assert.equal(trailHeadJoins(30 * MINUTE, 151_000), false);
+  assert.equal(trailHeadJoins(HOUR, 5_000), false);
+});
+
+test('a backfill with gaps of 10 h, 2 h, 1.2 h and 6.5 h draws five segments and no line across a gap', async () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const legs = [];
+  let start = t0;
+  const gaps = [10 * HOUR, 2 * HOUR, 1.2 * HOUR, 6.5 * HOUR];
+  const anchors = [
+    [30, 31],
+    [12, 0],
+    [14, -15],
+    [20, -40],
+    [35, -75],
+  ];
+  for (let i = 0; i < anchors.length; i++) {
+    const points = leg(start, anchors[i][0], anchors[i][1], 6);
+    legs.push(points);
+    start = points[points.length - 1].observedAtMs + (gaps[i] ?? 0);
+  }
+  const waypoints = legs.flat();
+  const lastLeg = legs[legs.length - 1];
+  const liveStart = lastLeg[lastLeg.length - 1].observedAtMs + MINUTE;
+  const liveFixes = [0, 1, 2].map((i) => ({
+    timeMs: liveStart + i * MINUTE,
+    lat: 35,
+    lon: -75 + 0.72 + i * 0.12,
+  }));
+  const nowMs = liveStart + 3 * MINUTE;
+  const { trail, tracking, flightState } = trackedHarness({
+    liveFixes,
+    waypoints,
+    nowMs,
+  });
+  tracking._startTrail('abc123');
+  await settle();
+
+  assert.equal(flightState._trailFixes.length, waypoints.length + 3);
+  const segments = trail.last();
+  assert.equal(segments.length, 5);
+  const bodyCount = segments.reduce((sum, segment) => sum + segment.length, 0);
+  // The body draws every fix at or before the display time: all of them here.
+  assert.equal(bodyCount, waypoints.length + 3);
+  for (const [index, segment] of segments.entries()) {
+    const expected = legs[index].map((point) =>
+      Cesium.Cartesian3.fromDegrees(point.longitude, point.latitude, 10_000),
+    );
+    for (const position of segment.slice(0, expected.length)) {
+      assert.ok(
+        expected.some((e) => Cesium.Cartesian3.distance(e, position) < 1),
+        `segment ${index} holds a point from another leg`,
+      );
+    }
+  }
+});
+
+test('a null-altitude waypoint breaks the line, and nothing is drawn at 10 km', async () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const waypoints = leg(t0, 40, 10, 6);
+  waypoints[3] = waypoint(waypoints[3].observedAtMs, 40, 10.36, null);
+  const nowMs = t0 + 8 * MINUTE;
+  const liveFixes = [
+    { timeMs: t0 + 6 * MINUTE, lat: 40, lon: 10.72 },
+    { timeMs: t0 + 7 * MINUTE, lat: 40, lon: 10.84 },
+  ];
+
+  // Unknown floor: the null point is skipped, and the line still breaks there.
+  const skipped = trackedHarness({ liveFixes, waypoints, nowMs });
+  skipped.tracking._startTrail('abc123');
+  await settle();
+  assert.equal(
+    skipped.flightState._trailFixes.length,
+    waypoints.length - 1 + 2,
+  );
+  const segments = skipped.trail.last();
+  assert.deepEqual(
+    segments.map((segment) => segment.length),
+    [3, 4],
+  );
+  for (const segment of segments) {
+    for (const position of segment) {
+      const height = Cesium.Cartographic.fromCartesian(position).height;
+      assert.ok(Math.abs(height - 10_000) < 1, 'only real altitudes drawn');
+    }
+  }
+
+  // Known floor: the point is kept on the ground, alone, joined to nothing.
+  const grounded = trackedHarness({
+    liveFixes,
+    waypoints,
+    nowMs,
+    floor: () => 120,
+  });
+  grounded.tracking._startTrail('abc123');
+  await settle();
+  assert.equal(grounded.flightState._trailFixes.length, waypoints.length + 2);
+  assert.deepEqual(
+    grounded.trail.last().map((segment) => segment.length),
+    [3, 4],
+  );
+  const kept = grounded.flightState._trailFixes[3];
+  assert.ok(
+    Math.abs(Cesium.Cartographic.fromCartesian(kept.position).height - 121) < 1,
+  );
+});
+
+test('a history tail far from the first live fix does not connect to it', async () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  // History ends 20 minutes before the first live fix, 1,400 km away.
+  const waypoints = leg(t0, 50, 0, 5);
+  const liveStart = t0 + 24 * MINUTE;
+  const liveFixes = [0, 1, 2].map((i) => ({
+    timeMs: liveStart + i * MINUTE,
+    lat: 50,
+    lon: 20 + i * 0.12,
+  }));
+  const { trail, tracking } = trackedHarness({
+    liveFixes,
+    waypoints,
+    nowMs: liveStart + 3 * MINUTE,
+  });
+  tracking._startTrail('abc123');
+  await settle();
+  assert.deepEqual(
+    trail.last().map((segment) => segment.length),
+    [5, 3],
+  );
+});
+
+test('the head is drawn to a nearby aircraft and not to one more than 150 km away', () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const liveFixes = [0, 1, 2].map((i) => ({
+    timeMs: t0 + i * MINUTE,
+    lat: 50,
+    lon: 1 + i * 0.12,
+  }));
+  const near = Cesium.Cartesian3.fromDegrees(1.3, 50, 10_000);
+  const joined = trackedHarness({
+    liveFixes,
+    nowMs: t0 + 3 * MINUTE,
+    head: near,
+  });
+  joined.tracking._startTrail('abc123');
+  const drawn = headPositions(joined.viewer);
+  assert.equal(drawn.length, 2);
+  assert.ok(Cesium.Cartesian3.distance(drawn[1], near) < 1e-6);
+
+  const far = Cesium.Cartesian3.fromDegrees(4, 50, 10_000);
+  const detached = trackedHarness({
+    liveFixes,
+    nowMs: t0 + 3 * MINUTE,
+    head: far,
+  });
+  detached.tracking._startTrail('abc123');
+  assert.deepEqual(headPositions(detached.viewer), []);
+
+  // An hour without a fix: the head no longer joins the last body point.
+  const stale = trackedHarness({ liveFixes, nowMs: t0 + HOUR, head: near });
+  stale.tracking._startTrail('abc123');
+  assert.deepEqual(headPositions(stale.viewer), []);
+});
+
+test('a live fix after a one-hour feed gap starts a new segment', () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const liveFixes = [0, 1, 2].map((i) => ({
+    timeMs: t0 + i * MINUTE,
+    lat: 50,
+    lon: 1 + i * 0.12,
+  }));
+  const { trail, tracking, flightState } = trackedHarness({
+    liveFixes,
+    nowMs: t0 + 3 * MINUTE,
+  });
+  tracking._startTrail('abc123');
+  assert.deepEqual(
+    trail.last().map((segment) => segment.length),
+    [3],
+  );
+  const later = t0 + 2 * MINUTE + HOUR;
+  // The display time moves past the new fixes.
+  flightState.now = () => later + 3 * MINUTE;
+  for (const [i, lon] of [3, 3.12, 3.24].entries()) {
+    tracking._appendTrailFix(
+      Cesium.Cartesian3.fromDegrees(lon, 50, 10_000),
+      later + i * MINUTE,
+    );
+  }
+  assert.equal(flightState._trailFixes.length, 6);
+  assert.deepEqual(
+    trail.last().map((segment) => segment.length),
+    [3, 3],
+  );
+});
+
+test('every waypoint of a 24 h history survives, and the trail stays within TRAIL_MAX_POINTS', async () => {
+  // A full day at one fix per minute.
+  const day = 24 * 60;
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const full = leg(t0, 10, -100, day);
+  const liveStart = t0 + day * MINUTE;
+  const liveFixes = [{ timeMs: liveStart, lat: 10, lon: -100 + day * 0.12 }];
+  const kept = trackedHarness({
+    liveFixes,
+    waypoints: full,
+    nowMs: liveStart + MINUTE,
+  });
+  kept.tracking._startTrail('abc123');
+  await settle();
+  assert.ok(TRAIL_MAX_POINTS >= day + 500);
+  assert.equal(kept.flightState._trailFixes.length, full.length + 1);
+  assert.deepEqual(
+    kept.trail.last().map((segment) => segment.length),
+    [day + 1],
+  );
+
+  const count = TRAIL_MAX_POINTS + 120;
+  const large = leg(
+    liveStart - count * MINUTE,
+    10,
+    -100 + (day - count) * 0.12,
+    count,
+  );
+  const capped = trackedHarness({
+    liveFixes,
+    waypoints: large,
+    nowMs: liveStart + MINUTE,
+  });
+  capped.tracking._startTrail('abc123');
+  await settle();
+  const fixes = capped.flightState._trailFixes;
+  assert.equal(fixes.length, TRAIL_MAX_POINTS);
+  // The oldest go first: the newest live fix is still the last point.
+  assert.equal(fixes[fixes.length - 1].timeMs, liveStart);
+});

src/layers/flights/trailRecord.test.mjs

@@ -0,0 +1,846 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createMotion } from './motion.js';
+import { createFlightState } from './state.js';
+import { createTracking } from './tracking.js';
+import { RENDER_DELAY_SEC, STALE_ALPHA } from './policy.js';
+
+// Between reports, a tracked trail is the path the icon was actually drawn
+// along: the real display position, recorded as the frames go by. These tests
+// drive the real motion and display path frame by frame.
+
+const SEC = 1000;
+const ICAO = '00a001';
+const T0 = Date.UTC(2026, 10, 9, 21, 0);
+
+function fix(timeMs, lat, lon, heightM, velocity, track) {
+  return {
+    time: Cesium.JulianDate.fromDate(new Date(timeMs)),
+    epochMs: timeMs,
+    position: Cesium.Cartesian3.fromDegrees(lon, lat, heightM),
+    velocity,
+    track,
+  };
+}
+
+function services(trail, ground) {
+  const names = [
+    'picking',
+    'sprites',
+    'aircraftPresentation',
+    'camera',
+    'militaryRegistry',
+    'labels',
+    'meshFloor',
+    'focus',
+    'readout',
+    'context',
+    'render',
+    'recession',
+  ];
+  return {
+    ...Object.fromEntries(names.map((name) => [name, {}])),
+    aircraftPresentation: {
+      isTr3b: () => false,
+      tr3bTypeLabel: (id, label) => label,
+    },
+    focus: {
+      nearFarScalarValueAtDistance: () => 1,
+      clearFocusTarget() {},
+      publishFocusTargetFromCachedPosition() {},
+    },
+    groundSnap: { createGroundSnap: () => ({ clear() {} }) },
+    trails: trail,
+    geoid: { ensureGeoidReady: async () => {}, geoidHeight: () => 0 },
+    groundFloor: {
+      resolveGroundFloorCellsBounded: async () => {},
+      cachedGroundFloor: ground,
+      stickyFloorCell: (lat, lon) => ({ lat, lon }),
+      neighborFloorM: () => null,
+      floorAltitudeM: (alt, g) =>
+        Number.isFinite(g) && Number.isFinite(alt)
+          ? Math.max(alt, g + 1.5)
+          : alt,
+    },
+  };
+}
+
+/**
+ * Real motion, real display position and real tracking over one contact.
+ * `frame()` runs one rendered frame: the layer's preRender work, then the
+ * head callback, as Cesium does.
+ */
+function harness({
+  fixes,
+  info,
+  nowMs,
+  ground = () => null,
+  envelopeM = null,
+}) {
+  const segmentsCalls = [];
+  let inHead = false;
+  const trail = {
+    createTrail: () => ({
+      setSegments(segments) {
+        assert.ok(
+          !inHead,
+          'the trail body was rebuilt inside the head callback',
+        );
+        segmentsCalls.push(segments.map((segment) => segment.slice()));
+      },
+      setPositions(positions) {
+        assert.ok(
+          !inHead,
+          'the trail body was rebuilt inside the head callback',
+        );
+        segmentsCalls.push([positions.slice()]);
+      },
+      setVisible() {},
+      clear() {},
+      destroy() {},
+    }),
+  };
+  const svc = services(trail, ground);
+  const flightState = createFlightState({
+    services: svc,
+    source: { label: 'Fixture aircraft' },
+  });
+  flightState.records.data.set(ICAO, info);
+  let frameNumber = 0;
+  const added = [];
+  let entityChanges = 0;
+  flightState._viewer = {
+    isDestroyed: () => false,
+    scene: {
+      frameState: {
+        get frameNumber() {
+          return frameNumber;
+        },
+      },
+    },
+    entities: {
+      add(definition) {
+        entityChanges += 1;
+        added.push(definition);
+        return definition;
+      },
+      remove() {
+        entityChanges += 1;
+      },
+    },
+  };
+  flightState._trackedIcao = ICAO;
+  let clock = nowMs;
+  flightState.now = () => clock;
+  flightState._positionHistory.set(ICAO, fixes);
+  const parts = {
+    rendering: { _modelOwnsVisual: () => false, _modelSpec: () => ({}) },
+    queries: { _contactLabel: (id, info) => info?.callsign || id },
+  };
+  parts.motion = createMotion({ flightState, services: svc, parts });
+  if (envelopeM != null) {
+    // A 3D model is the visual: its centre is the drawn position.
+    parts.motion._trackedModelCenterWorld = () =>
+      parts.motion._trackedDisplayCached();
+    parts.motion._trackedModelEnvelopeM = () => envelopeM;
+  }
+  const tracking = createTracking({
+    flightState,
+    services: svc,
+    parts,
+    layer: {},
+  });
+  parts.tracking = tracking;
+  const realDateNow = Date.now;
+  Date.now = () => clock;
+  const headEntity = () =>
+    added.find((definition) =>
+      String(definition.id).startsWith('gev-trail:fl-head-'),
+    );
+  const head = () => {
+    const before = entityChanges;
+    inHead = true;
+    try {
+      return headEntity().polyline.positions.getValue();
+    } finally {
+      inHead = false;
+      assert.equal(
+        entityChanges,
+        before,
+        'the head callback added or removed an entity',
+      );
+    }
+  };
+  /** Drawn icon positions, one per frame. */
+  const drawn = [];
+  const frame = ({ readHead = true } = {}) => {
+    frameNumber += 1;
+    tracking._trailTick();
+    const icon = parts.motion._trackedDisplayPosition(ICAO);
+    if (icon) drawn.push(Cesium.Cartesian3.clone(icon));
+    if (readHead) head();
+  };
+  const run = (toMs, { frameMs = 100, readHead = true } = {}) => {
+    while (clock < toMs) {
+      clock = Math.min(toMs, clock + frameMs);
+      frame({ readHead });
+    }
+  };
+  return {
+    flightState,
+    tracking,
+    motion: parts.motion,
+    head,
+    drawn,
+    run,
+    body: () => segmentsCalls.at(-1) ?? [],
+    bodyPoints: () => (segmentsCalls.at(-1) ?? []).flat(),
+    restore: () => {
+      Date.now = realDateNow;
+    },
+  };
+}
+
+/** Distance from p to the nearest point of a polyline. */
+function distanceToPath(p, path) {
+  let best = Infinity;
+  const ab = new Cesium.Cartesian3();
+  const ap = new Cesium.Cartesian3();
+  const closest = new Cesium.Cartesian3();
+  if (path.length === 1) return Cesium.Cartesian3.distance(p, path[0]);
+  for (let i = 1; i < path.length; i++) {
+    Cesium.Cartesian3.subtract(path[i], path[i - 1], ab);
+    Cesium.Cartesian3.subtract(p, path[i - 1], ap);
+    const len2 = Cesium.Cartesian3.magnitudeSquared(ab);
+    const t =
+      len2 > 0
+        ? Math.max(0, Math.min(1, Cesium.Cartesian3.dot(ap, ab) / len2))
+        : 0;
+    Cesium.Cartesian3.add(
+      path[i - 1],
+      Cesium.Cartesian3.multiplyByScalar(ab, t, closest),
+      closest,
+    );
+    best = Math.min(best, Cesium.Cartesian3.distance(p, closest));
+  }
+  return best;
+}
+
+/** No vertex turns the line back sharper than 120° over a short step. */
+function assertNoSpur(line, label) {
+  const u = new Cesium.Cartesian3();
+  const v = new Cesium.Cartesian3();
+  for (let i = 1; i < line.length - 1; i++) {
+    Cesium.Cartesian3.subtract(line[i], line[i - 1], u);
+    Cesium.Cartesian3.subtract(line[i + 1], line[i], v);
+    const lu = Cesium.Cartesian3.magnitude(u);
+    const lv = Cesium.Cartesian3.magnitude(v);
+    if (!(lu > 0 && lv > 0) || Math.min(lu, lv) > 500) continue;
+    const cos = Cesium.Cartesian3.dot(u, v) / (lu * lv);
+    assert.ok(
+      cos >= -0.5,
+      `${label}: spur at vertex ${i} of ${line.length} (turn ${(
+        (Math.acos(cos) * 180) /
+        Math.PI
+      ).toFixed(0)}°)`,
+    );
+  }
+}
+
+const indexOfPoint = (line, p) =>
+  line.findIndex((q) => Cesium.Cartesian3.distance(p, q) < 0.01);
+
+// A departing jet: four northbound climb reports, then a left turn through
+// west to south with no report while the icon flies it.
+function departure() {
+  const fixes = [
+    fix(T0, 37.6329, -122.3661, 213, 85, 8),
+    fix(T0 + 30 * SEC, 37.6542, -122.3616, 495, 95, 0),
+    fix(T0 + 60 * SEC, 37.6813, -122.3615, 808, 104, 350),
+    fix(T0 + 90 * SEC, 37.7066, -122.3755, 1257, 112, 340),
+  ];
+  const info = {
+    altitude: 1257,
+    renderAltitudeM: 1257,
+    rawLat: 37.7066,
+    rawLon: -122.3755,
+    velocity: 112,
+    true_track: 340,
+    turnRateDps: -2,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: T0 + 200 * SEC,
+  };
+  return { fixes, info };
+}
+
+test('a report newer than the display time arrives mid-turn: the trail between the two reports stays on the drawn arc', () => {
+  const { fixes, info } = departure();
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    // 40 s of turn with no new report: the icon flies the arc.
+    h.run(start + 40 * SEC);
+    const arc = h.drawn.slice();
+    // The next report is where the plane really was 60 s after the last one,
+    // still ahead of the display time (normal 30 s delay, slow feed). The icon
+    // now interpolates the straight line between the two reports and is eased
+    // onto it.
+    const at60 = h.motion._deadReckon(
+      ICAO,
+      new Cesium.Cartesian3(),
+      Cesium.JulianDate.fromDate(new Date(T0 + 150 * SEC)),
+    );
+    const c = Cesium.Cartographic.fromCartesian(at60);
+    const report = fix(
+      T0 + 150 * SEC,
+      Cesium.Math.toDegrees(c.latitude),
+      Cesium.Math.toDegrees(c.longitude),
+      c.height,
+      112,
+      220,
+    );
+    fixes.push(report);
+    info.true_track = 220;
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    // The display passes the new report.
+    h.run(start + 75 * SEC);
+    const line = h.bodyPoints();
+    const from = indexOfPoint(line, fixes[3].position);
+    const to = indexOfPoint(line, report.position);
+    assert.ok(from >= 0 && to > from, 'both reports are on the trail');
+    const between = line.slice(from, to + 1);
+    // Every vertex between the reports is where the icon was drawn...
+    const drawnPath = [fixes[3].position, ...h.drawn, report.position];
+    for (const vertex of between) {
+      const off = distanceToPath(vertex, drawnPath);
+      assert.ok(
+        off < 50,
+        `trail vertex ${off.toFixed(0)} m off the drawn path`,
+      );
+    }
+    // ...and the arc the icon flew is still there: no straight chord. (The
+    // arc strays far from the chord, so a chord would fail this.)
+    const chord = [fixes[3].position, report.position];
+    assert.ok(Math.max(...arc.map((p) => distanceToPath(p, chord))) > 300);
+    // The last 10 s of the arc are exempt: the correction pulled the icon back
+    // over them, and keeping them would fold the line back on itself.
+    for (const point of arc.slice(0, -100)) {
+      const off = distanceToPath(point, between);
+      assert.ok(off < 50, `drawn arc point ${off.toFixed(0)} m from the trail`);
+    }
+    assertNoSpur(line, 'trail');
+    assertNoSpur(h.head(), 'head');
+  } finally {
+    h.restore();
+  }
+});
+
+test('a fresh report after a hold resumes with no backward spur', () => {
+  const fixes = [0, 1, 2, 3].map((i) =>
+    fix(T0 + i * 15 * SEC, 40, -100 + i * 0.04, 10_000, 230, 90),
+  );
+  const info = {
+    altitude: 10_000,
+    renderAltitudeM: 10_000,
+    rawLat: 40,
+    rawLon: -100 + 0.12,
+    velocity: 230,
+    true_track: 90,
+    turnRateDps: 0,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: T0 + 45 * SEC,
+  };
+  const start = T0 + 45 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    // The contact coasts five minutes, then holds; recording stops while it
+    // holds.
+    h.run(start + 320 * SEC);
+    const held = h.bodyPoints().length;
+    h.run(start + 360 * SEC);
+    assert.equal(h.bodyPoints().length, held, 'the trail grew during a hold');
+    // A fresh report, a little north of the track, ahead of the display.
+    const display = start + 360 * SEC - RENDER_DELAY_SEC * SEC;
+    // Just past where it held: five minutes and ten seconds at 230 m/s.
+    const aheadDeg = (230 * 310) / (111_320 * Math.cos((40 * Math.PI) / 180));
+    const fresh = fix(
+      display + 10 * SEC,
+      40.01,
+      -100 + 0.12 + aheadDeg,
+      10_000,
+      230,
+      80,
+    );
+    fixes.push(fresh);
+    info.lastContactEpochMs = fresh.epochMs;
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(fresh.position),
+      fresh.epochMs,
+    );
+    h.run(start + 390 * SEC);
+    assertNoSpur(h.bodyPoints(), 'trail');
+    assertNoSpur(h.head(), 'head');
+    assert.ok(indexOfPoint(h.bodyPoints(), fresh.position) >= 0);
+  } finally {
+    h.restore();
+  }
+});
+
+test('with the head hidden the body keeps advancing, and untracking clears the recording', () => {
+  const { fixes, info } = departure();
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(start + 5 * SEC);
+    const before = h.bodyPoints().at(-1);
+    // Cockpit view: the head is hidden and its callback never runs.
+    h.run(start + 30 * SEC, { readHead: false });
+    // The body's end keeps following the icon; straight stretches are thinned.
+    const end = h.bodyPoints().at(-1);
+    assert.ok(
+      Cesium.Cartesian3.distance(end, before) > 1_000,
+      'the body stopped advancing',
+    );
+    assert.ok(Cesium.Cartesian3.distance(end, h.drawn.at(-1)) < 300);
+    h.tracking._clearTrail();
+    assert.equal(h.flightState._trailFixes.length, 0);
+    // A new track of the same plane starts from reports only.
+    h.tracking._startTrail(ICAO);
+    assert.equal(
+      h.flightState._trailFixes.filter((entry) => entry.drawn).length,
+      0,
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+test('a live fix with no altitude breaks the line and puts no point at an invented height', () => {
+  const { fixes, info } = departure();
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    const invented = Cesium.Cartesian3.fromDegrees(-122.38, 37.71, 10_000);
+    h.tracking._appendTrailFix(invented, T0 + 95 * SEC, { nullAltitude: true });
+    const later = Cesium.Cartesian3.fromDegrees(-122.39, 37.715, 1400);
+    h.tracking._appendTrailFix(later, T0 + 100 * SEC);
+    h.run(T0 + 100 * SEC + RENDER_DELAY_SEC * SEC + SEC, { frameMs: 500 });
+    for (const point of h.bodyPoints())
+      assert.ok(Cesium.Cartographic.fromCartesian(point).height < 5_000);
+    // The reports on either side of it are not joined.
+    const segments = h.body();
+    const holding = (p) =>
+      segments.findIndex((segment) =>
+        segment.some((q) => Cesium.Cartesian3.distance(p, q) < 0.01),
+      );
+    assert.notEqual(holding(fixes[3].position), holding(later));
+  } finally {
+    h.restore();
+  }
+});
+
+test('with a 3D model as the visual, the trail stops outside the model and the head runs from there to the plane', () => {
+  const { fixes, info } = departure();
+  info.turnRateDps = 0;
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const envelopeM = 300;
+  const h = harness({ fixes, info, nowMs: start, envelopeM });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(start + 30 * SEC);
+    const icon = h.drawn.at(-1);
+    const body = h.bodyPoints();
+    const head = h.head();
+    assert.ok(head.length >= 2, 'no head is drawn');
+    assert.ok(Cesium.Cartesian3.distance(head.at(-1), icon) < 0.01);
+    // No trail point inside the model, and the head starts where the body ends.
+    for (const point of body)
+      assert.ok(Cesium.Cartesian3.distance(point, icon) >= envelopeM - 1);
+    assert.ok(Cesium.Cartesian3.distance(head[0], body.at(-1)) < 0.01);
+    // Body and head together follow where the icon was drawn.
+    const line = body.concat(head);
+    for (const point of h.drawn) {
+      const off = distanceToPath(point, line);
+      assert.ok(off < 35, `drawn icon ${off.toFixed(0)} m off the trail`);
+    }
+  } finally {
+    h.restore();
+  }
+});
+
+test('a report the display already passed replaces the guess drawn after it, with no spike', () => {
+  const { fixes, info } = departure();
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(start + 40 * SEC);
+    const displayMs = T0 + 130 * SEC;
+    // A report for 10 s before the display time, 400 m inside the drawn arc.
+    const guess = h.motion._deadReckon(
+      ICAO,
+      new Cesium.Cartesian3(),
+      Cesium.JulianDate.fromDate(new Date(T0 + 120 * SEC)),
+    );
+    const c = Cesium.Cartographic.fromCartesian(guess);
+    const report = fix(
+      T0 + 120 * SEC,
+      Cesium.Math.toDegrees(c.latitude) - 400 / 111_320,
+      Cesium.Math.toDegrees(c.longitude),
+      c.height,
+      112,
+      250,
+    );
+    fixes.push(report);
+    info.true_track = 250;
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    h.run(start + 55 * SEC);
+    const recorded = h.flightState._trailFixes.filter((entry) => entry.drawn);
+    // Nothing recorded between the report and the end of the ease.
+    assert.deepEqual(
+      recorded
+        .filter(
+          (entry) =>
+            entry.timeMs > report.epochMs && entry.timeMs < displayMs + 900,
+        )
+        .map((entry) => entry.timeMs - T0),
+      [],
+    );
+    const line = h.bodyPoints();
+    assert.ok(indexOfPoint(line, report.position) >= 0);
+    assertNoSpur(line, 'trail');
+  } finally {
+    h.restore();
+  }
+});
+
+test('a history report at an invented height is seeded with no position, and nothing is recorded next to it', () => {
+  const { fixes, info } = departure();
+  fixes[3].inventedHeight = true;
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    const seeded = h.flightState._trailFixes.find(
+      (entry) => entry.timeMs === fixes[3].epochMs,
+    );
+    assert.equal(seeded.position, null);
+    assert.equal(seeded.nullAltitude, true);
+    h.run(start + 10 * SEC);
+    assert.equal(
+      h.flightState._trailFixes.filter((entry) => entry.drawn).length,
+      0,
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+// A turboprop at FL210 flying east out of receiver coverage: its last
+// position is at T0 + 30 s, and contact goes on for four more minutes.
+function outOfCoverage() {
+  const fixes = [
+    fix(T0, 43, -108.1, 6_400, 140, 90),
+    fix(T0 + 30 * SEC, 43, -108.05, 6_400, 140, 90),
+  ];
+  const info = {
+    altitude: 6_400,
+    renderAltitudeM: 6_400,
+    rawLat: 43,
+    rawLon: -108.05,
+    velocity: 140,
+    true_track: 90,
+    callsign: 'TEST7',
+    turnRateDps: 0,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: T0 + 30 * SEC + 240 * SEC,
+  };
+  return { fixes, info };
+}
+
+/** Display time (ms) `afterSec` past the last report. */
+const pastReport = (afterSec) =>
+  T0 + 30 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC;
+
+test('a plane that stops reporting keeps moving dimmed as STALE from its first missed update, then stops as SIGNAL LOST', () => {
+  const { fixes, info } = outOfCoverage();
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(30));
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: '',
+      dimmed: false,
+    });
+    assert.equal(h.tracking._trackedIconColor().alpha, 1);
+    // The first missed update.
+    h.flightState.records.missingPolls.set(ICAO, 1);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: 'STALE',
+      dimmed: true,
+    });
+    assert.match(h.tracking._trackedLabelText(ICAO), /STALE/);
+    // Drawn with the fleet's stale fade.
+    assert.equal(h.tracking._trackedIconColor().alpha, STALE_ALPHA);
+    // Still moving well past 90 s.
+    h.run(pastReport(200));
+    assert.ok(
+      Cesium.Cartesian3.distance(h.drawn.at(-1), h.drawn.at(-11)) > 100,
+    );
+    // The coast ends 300 s past the report; the plane stops there.
+    h.run(pastReport(310));
+    const recorded = h.flightState._trailFixes.filter((e) => e.drawn).length;
+    h.run(pastReport(330));
+    assert.ok(
+      Cesium.Cartesian3.distance(h.drawn.at(-1), h.drawn.at(-50)) < 0.01,
+    );
+    assert.equal(
+      h.flightState._trailFixes.filter((e) => e.drawn).length,
+      recorded,
+      'the trail kept recording a stopped plane',
+    );
+    // 330 s of display past the report is 330 s + the display delay of wall time.
+    const minutes = Math.floor((330 + RENDER_DELAY_SEC) / 60);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: `SIGNAL LOST · ${minutes} min`,
+      dimmed: true,
+    });
+    assert.match(
+      h.tracking._trackedLabelText(ICAO),
+      new RegExp(`SIGNAL LOST · ${minutes} min`),
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+test('a plane held at its last report shows SIGNAL LOST in seconds under a minute', () => {
+  // Its last report is 100 m above a known ground height: it holds there.
+  const { fixes, info } = outOfCoverage();
+  const h = harness({
+    fixes,
+    info,
+    nowMs: pastReport(-5),
+    ground: () => 6_300,
+  });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.flightState.records.missingPolls.set(ICAO, 1);
+    h.run(pastReport(10));
+    assert.ok(10 + RENDER_DELAY_SEC < 60);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: `SIGNAL LOST · ${10 + RENDER_DELAY_SEC} s`,
+      dimmed: true,
+    });
+  } finally {
+    h.restore();
+  }
+});
+
+test('a report arriving 200 s into a coast: the plane eases onto it and the trail drops what was drawn after it', () => {
+  const { fixes, info } = outOfCoverage();
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(200));
+    // Heard again: the report is for 10 s before the display time, 2 km north
+    // of where the coast put it.
+    const at = T0 + 30 * SEC + 190 * SEC;
+    const guess = h.motion._deadReckon(
+      ICAO,
+      new Cesium.Cartesian3(),
+      Cesium.JulianDate.fromDate(new Date(at)),
+    );
+    const c = Cesium.Cartographic.fromCartesian(guess);
+    const report = fix(
+      at,
+      Cesium.Math.toDegrees(c.latitude) + 2_000 / 111_320,
+      Cesium.Math.toDegrees(c.longitude),
+      c.height,
+      140,
+      90,
+    );
+    fixes.push(report);
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    assert.deepEqual(
+      h.flightState._trailFixes.filter(
+        (e) => e.drawn && e.timeMs > report.epochMs,
+      ),
+      [],
+    );
+    const from = h.drawn.length;
+    h.run(pastReport(203));
+    const after = h.drawn.slice(from - 1);
+    for (let i = 1; i < after.length; i++)
+      assert.ok(
+        Cesium.Cartesian3.distance(after[i], after[i - 1]) < 400,
+        'the plane jumped instead of easing',
+      );
+    // Eased onto the path from the report: 13 s on from it at 140 m/s.
+    const off = Cesium.Cartesian3.distance(after.at(-1), report.position);
+    assert.ok(
+      Math.abs(off - 140 * 13) < 150,
+      `${off.toFixed(0)} m from the report`,
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+test('a tracked plane that stops reporting: STALE from the first missed poll, SIGNAL LOST after the five-minute coast, kept at 9 min, removed at 10 min', () => {
+  const { fixes, info } = outOfCoverage();
+  const fixMs = fixes.at(-1).epochMs;
+  // Contact stops with the position: the plane left coverage.
+  info.lastContactEpochMs = fixMs;
+  info.newestFixMs = fixMs;
+  const h = harness({ fixes, info, nowMs: fixMs + 5 * SEC });
+  try {
+    h.tracking._startTrail(ICAO);
+    const records = h.flightState.records;
+    const polls = new Map();
+    let removedAtSec = null;
+    // A poll every 10 s that lists the plane without a position.
+    for (let afterSec = 10; afterSec <= 660; afterSec += 10) {
+      h.run(fixMs + afterSec * SEC, { frameMs: 500, readHead: false });
+      const result = records.absence(ICAO, {
+        complete: true,
+        retainIds: new Set([ICAO]),
+        likelyLanded: false,
+        tracked: true,
+      });
+      if (result === 'remove') {
+        removedAtSec = afterSec;
+        break;
+      }
+      polls.set(afterSec, {
+        cue: h.tracking._trackedStaleCue(ICAO),
+        drawn: Cesium.Cartesian3.clone(h.drawn.at(-1)),
+      });
+    }
+    // Wall time when the display reaches the end of the coast.
+    const coastEndSec = 300 + RENDER_DELAY_SEC;
+    for (const [afterSec, { cue }] of polls) {
+      if (afterSec <= coastEndSec) {
+        assert.deepEqual(cue, { text: 'STALE', dimmed: true }, `${afterSec} s`);
+      } else {
+        assert.deepEqual(
+          cue,
+          {
+            text: `SIGNAL LOST · ${Math.floor(afterSec / 60)} min`,
+            dimmed: true,
+          },
+          `${afterSec} s`,
+        );
+      }
+    }
+    const moved = (a, b) =>
+      Cesium.Cartesian3.distance(polls.get(a).drawn, polls.get(b).drawn);
+    // Moving through the coast, still at its last speed at four minutes.
+    assert.ok(Math.abs(moved(230, 240) - 140 * 10) < 20);
+    // Stopped once the coast has ended.
+    const after = Math.ceil(coastEndSec / 10) * 10;
+    assert.ok(moved(after, 540) < 0.01);
+    assert.equal(polls.get(300).cue.text, 'STALE');
+    assert.deepEqual(polls.get(540).cue, {
+      text: 'SIGNAL LOST · 9 min',
+      dimmed: true,
+    });
+    assert.equal(removedAtSec, 600);
+  } finally {
+    h.restore();
+  }
+});
+
+test('a repeated old fix counts as a missed update for the tracked plane', () => {
+  const { fixes, info } = outOfCoverage();
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(-2));
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: '',
+      dimmed: false,
+    });
+    // The feed re-sends the last position: its fix time did not advance.
+    h.flightState.records.fixRepeated.add(ICAO);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: 'STALE',
+      dimmed: true,
+    });
+    assert.equal(h.tracking._trackedIconColor().alpha, STALE_ALPHA);
+    assert.equal(h.flightState.records.missingPolls.has(ICAO), false);
+  } finally {
+    h.restore();
+  }
+});
+
+test('a report arriving after the plane has stopped: it eases in with no spur, and the trail keeps everything up to the stop', () => {
+  const { fixes, info } = outOfCoverage();
+  info.lastContactEpochMs = fixes.at(-1).epochMs;
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(360));
+    const stop = Cesium.Cartesian3.clone(h.drawn.at(-1));
+    assert.ok(Cesium.Cartesian3.distance(stop, h.drawn.at(-100)) < 0.01);
+    const kept = h.flightState._trailFixes
+      .filter((e) => e.drawn)
+      .map((e) => Cesium.Cartesian3.clone(e.position));
+    // The thinned straight coast, recorded up to where the plane stopped.
+    assert.ok(kept.length >= 2);
+    assert.ok(Cesium.Cartesian3.distance(kept.at(-1), stop) < 300);
+    // Heard again, for 20 s before the display time: the plane flew on, a
+    // little north of its old heading.
+    const at = T0 + 30 * SEC + 340 * SEC;
+    const lonDeg =
+      -108.05 + (140 * 340) / (111_320 * Math.cos((43 * Math.PI) / 180));
+    const report = fix(at, 43 + 1_000 / 111_320, lonDeg, 6_400, 140, 90);
+    fixes.push(report);
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    // Nothing was recorded after the stop, so nothing is dropped.
+    const after = h.flightState._trailFixes
+      .filter((e) => e.drawn)
+      .map((e) => e.position);
+    assert.equal(after.length, kept.length);
+    kept.forEach((p, i) =>
+      assert.ok(Cesium.Cartesian3.distance(p, after[i]) < 0.01),
+    );
+    const from = h.drawn.length;
+    h.run(pastReport(365));
+    const eased = h.drawn.slice(from - 1);
+    const jump = Cesium.Cartesian3.distance(eased[0], eased.at(-1));
+    for (let i = 1; i < eased.length; i++)
+      assert.ok(
+        Cesium.Cartesian3.distance(eased[i], eased[i - 1]) < jump / 2,
+        'the plane jumped instead of easing',
+      );
+    // On the path from the report: 25 s on from it at 140 m/s.
+    const off = Cesium.Cartesian3.distance(eased.at(-1), report.position);
+    assert.ok(Math.abs(off - 140 * 25) < 150, `${off.toFixed(0)} m`);
+    assertNoSpur(h.bodyPoints(), 'trail');
+    assertNoSpur(h.head(), 'head');
+    assert.ok(indexOfPoint(h.bodyPoints(), report.position) >= 0);
+  } finally {
+    h.restore();
+  }
+});

src/layers/military/motion.js

@@ -5,10 +5,14 @@ import {
   speedRamp,
   courseBetweenCartesians,
   COURSE_HOLD_SPEED_MPS,
+  floorCoastedPosition,
   staleCoastLimitSeconds,
+  staleCoastSecondsFrom,
+  staleHoldsForFix,
   arcOffsetEnu,
   courseSlewCapDps,
   limitCourseStep,
+  COAST_MAX_SPEED_MPS,
 } from '../../data/motionModel.js';
 import { CLASS_SCALE_2D } from '../../data/aircraftClass.js';
 import {
@@ -141,23 +145,86 @@ export function createMotion({
       );
     }
 
-    // Continue through fresh adsb.lol contacts even when the position epoch has
-    // not advanced, but keep an absolute stale-feed drift ceiling.
+    // Coast on the last speed and heading for five minutes past the newest
+    // fix, then stop, dimmed as SIGNAL LOST. A contact last heard on or near
+    // the ground holds there instead, and a coast never goes below the ground.
+    const { coastSec } = _coastPlan(newest, info, elapsedSec);
+    const cachedGroundFloor = services.groundFloor?.cachedGroundFloor;
+    _extrapolateFix(
+      newest,
+      info,
+      coastSec,
+      out,
+      (info && info.turnRateDps) || 0,
+    );
+    return coastSec > 0 ? floorCoastedPosition(out, cachedGroundFloor) : out;
+  }
+
+  /**
+   * How far a contact coasts past its newest fix at a display time
+   * `elapsedSec` after it: up to five minutes, or not at all for a contact
+   * that holds at a report on or near the ground. The hold decision is made once
+   * per report and kept on it.
+   * @param {{epochMs?: number, time: Cesium.JulianDate}} newest
+   * @param {object|undefined} info - The contact's record.
+   * @param {number} elapsedSec
+   * @returns {{coastSec: number, holds: boolean}}
+   */
+  function _coastPlan(newest, info, elapsedSec) {
     const coastLimitSec = staleCoastLimitSeconds({
-      // Captured at poll normalization so the fleet tick does not allocate a
-      // Date for every aircraft twelve times per second.
+      // `epochMs` is captured once when the poll is normalized. Avoid allocating
+      // a Date per aircraft on every fleet tick.
       fixEpochMs: Number.isFinite(newest.epochMs)
         ? newest.epochMs
         : Cesium.JulianDate.toDate(newest.time).getTime(),
       lastContactEpochMs: info?.lastContactEpochMs,
+      // Five minutes of flight after the last report, as flights.
+      minimumSec: 300,
+      maximumSec: 300,
     });
-    return _extrapolateFix(
+    const holds = staleHoldsForFix(newest, {
+      onGround: info?.onGround,
+      cachedGroundFloor: services.groundFloor?.cachedGroundFloor,
+    });
+    return {
+      coastSec: staleCoastSecondsFrom({
+        newest,
+        elapsedSec,
+        coastLimitSec,
+        holds,
+      }),
+      holds,
+    };
+  }
+
+  /**
+   * Whether a contact has stopped where its coast ended (or held at its last
+   * report) with no newer report, and since when it has had none.
+   * @param {string} icao24
+   * @returns {{reportEpochMs: number, holds: boolean}|null} Null while a
+   *   report is ahead of the display time or the contact is still coasting.
+   */
+  function _signalLost(icao24) {
+    const newest = flightState._positionHistory.get(icao24)?.at(-1);
+    if (!newest) return null;
+    const newestMs = Number.isFinite(newest.epochMs)
+      ? newest.epochMs
+      : Cesium.JulianDate.toDate(newest.time).getTime();
+    const elapsedSec =
+      (flightState.now() - RENDER_DELAY_SEC * 1000 - newestMs) / 1000;
+    if (!(elapsedSec > 0)) return null;
+    const { coastSec, holds } = _coastPlan(
       newest,
-      info,
-      Math.min(elapsedSec, coastLimitSec),
-      out,
-      (info && info.turnRateDps) || 0,
+      flightState.records.data.get(icao24),
+      elapsedSec,
     );
+    if (!(elapsedSec > coastSec)) return null;
+    return {
+      reportEpochMs: Number.isFinite(newest.reportEpochMs)
+        ? newest.reportEpochMs
+        : newestMs,
+      holds,
+    };
   }
 
   /**
@@ -172,9 +239,12 @@ export function createMotion({
    */
 
   function _extrapolateFix(fix, info, dt, out, turnRateDps = 0) {
-    const speed = Number.isFinite(fix.velocity)
-      ? fix.velocity
-      : (info && info.speedMps) || 0;
+    const speed = Math.min(
+      Number.isFinite(fix.velocity)
+        ? fix.velocity
+        : (info && info.speedMps) || 0,
+      COAST_MAX_SPEED_MPS,
+    );
     const heading = Number.isFinite(fix.track)
       ? fix.track
       : (info && info.track) || 0;
@@ -514,6 +584,7 @@ export function createMotion({
   }
   return {
     _deadReckon,
+    _signalLost,
     _extrapolateFix,
     _isTrackWarmingUp,
     _trackedFocusSizePx,

src/layers/military/policy.js

@@ -172,7 +172,7 @@ export const TRAIL_COLOR = '#FFB800';
 
 /** @constant {number} Combined cap on trail vertices (backfill + live accumulation). */
 
-export const TRAIL_MAX_POINTS = 400;
+export const TRAIL_MAX_POINTS = 2000;
 
 // ---------------------------------------------------------------------------
 // Render-behind smoothing (mirrors flights.js): the fleet renders at
@@ -271,3 +271,6 @@ export const TRACKED_MODEL_RETRY_BACKOFF_MS = 1500;
  *  synchronously (single primitive). */
 
 export const IR_RELOAD_BATCH = 40;
+
+/** Alpha of a contact that has missed an update or lost its signal: the stale fade, for the fleet and the tracked aircraft alike. */
+export const STALE_ALPHA = 0.45;

src/layers/military/recordPolicy.js

@@ -6,4 +6,8 @@ export const LANDED_MISSING_POLL_LIMIT = 1;
 
 export const MISSING_POLL_LIMIT = 3;
 
+/** The tracked contact is kept this long after its newest fix: five minutes
+ *  coasting, then five stopped as SIGNAL LOST. */
+export const TRACKED_RETENTION_MS = 10 * 60_000;
+
 export const ERROR_BACKOFF_INTERVAL = 20000;

src/layers/military/records.js

@@ -6,6 +6,7 @@ import {
   GROUND_FLOOR_WARM_MAX_ALT_M,
   LANDED_MISSING_POLL_LIMIT,
   MISSING_POLL_LIMIT,
+  TRACKED_RETENTION_MS,
 } from './recordPolicy.js';
 
 /** Portable military aircraft metadata and bounded missing-poll retention. */
@@ -14,9 +15,44 @@ export class MilitaryFlightRecords {
     this.services = { geoidHeight, cachedGroundFloor, floorAltitudeM };
     this.data = new Map();
     this.missingPolls = new Map();
+    // Contacts whose last poll re-sent a fix no newer than the one before.
+    this.fixRepeated = new Set();
+    this.lastSnapshotMs = null;
+    this.snapshotAdvanced = true;
     this.geoidNCache = new Map();
     this.geoidReady = false;
   }
+  /**
+   * Start reading a snapshot taken at `observedAtMs`. A fix re-sent in a
+   * newer snapshot marks its contact as not updated; the same snapshot read
+   * again does not.
+   * @param {number} observedAtMs
+   */
+  beginSnapshot(observedAtMs) {
+    this.snapshotAdvanced =
+      !Number.isFinite(observedAtMs) ||
+      !Number.isFinite(this.lastSnapshotMs) ||
+      observedAtMs > this.lastSnapshotMs;
+    if (Number.isFinite(observedAtMs))
+      this.lastSnapshotMs = Math.max(
+        this.lastSnapshotMs ?? -Infinity,
+        observedAtMs,
+      );
+  }
+
+  /** Note a received fix: the newest fix time, and whether it advanced. */
+  _noteFix(id, prevMeta, fixEpochMs) {
+    const prevFixMs = prevMeta?.newestFixMs;
+    // A fix counts as new only when it moves on by a second: feeds that
+    // derive fix time from receipt (readsb `now − seen_pos`, 0.1 s) jitter.
+    if (!Number.isFinite(prevFixMs) || fixEpochMs >= prevFixMs + 1000) {
+      this.fixRepeated.delete(id);
+      return fixEpochMs;
+    }
+    if (this.snapshotAdvanced) this.fixRepeated.add(id);
+    return prevFixMs;
+  }
+
   receive(
     aircraft,
     { observedAtMs, floorWarmPoints: _floorWarmPoints, modelOwnsVisual },
@@ -88,6 +124,10 @@ export class MilitaryFlightRecords {
     // already uses (which also carries the on-ground 0 m / last-known-baro
     // case), so the two never disagree on the "no data yet" case.
     let renderAltitudeM = pickedAltM != null ? pickedAltM : altitudeM;
+    // No altitude for an airborne contact: the height is the airborne
+    // default, invented. The tracked trail keeps no point there.
+    const renderAltitudeInvented =
+      pickedAltM == null && !onGround && aircraft.baroAltitudeM == null;
     // Field-test fix (RS46 heli-in-hillside, 2026-07-06): a baro-only
     // AIRBORNE contact near steep terrain can compute a render height
     // BELOW the local surface (no alt_geom; baro+N carries QNH error
@@ -127,9 +167,11 @@ export class MilitaryFlightRecords {
     // Sticky merge — adsb.lol intermittently drops flight/t/r/ownOp; hold
     // last-known-good (bounded by the layer's eviction, which deletes the entry).
     const stickyType = stickyText(type, prevMeta?.type);
+    const fixEpochMs = aircraft.positionTimeMs ?? observedAtMs;
     const meta = {
       sourceReference: aircraft.reference,
       observedReceiptMs: Date.now(),
+      newestFixMs: this._noteFix(icao24, prevMeta, fixEpochMs),
       callsign: stickyText(callsign, prevMeta?.callsign),
       type: stickyType,
       // Type outranks category automatically inside classifyAircraft.
@@ -146,6 +188,7 @@ export class MilitaryFlightRecords {
       // altitudeFt/baro).
       geoAltitudeM,
       renderAltitudeM,
+      renderAltitudeInvented,
       // A poll that omits track/speed holds the last values, like the civil
       // records; 0 only until one is known. A grounded row without a speed
       // reads 0, so a parked plane never coasts at its landing speed.
@@ -184,10 +227,25 @@ export class MilitaryFlightRecords {
       prevMeta,
       meta,
       groundFlipped,
-      fixEpochMs: aircraft.positionTimeMs ?? observedAtMs,
+      fixEpochMs,
     };
   }
-  absence(id, { complete, retainIds, likelyLanded }) {
+  absence(id, { complete, retainIds, likelyLanded, tracked = false }) {
+    const misses = (this.missingPolls.get(id) || 0) + 1;
+    // The tracked contact has missed an update: it is stale from the first
+    // one, and kept (so the camera keeps its target) until ten minutes after
+    // its newest fix, whatever the snapshot says. With no fix time known it
+    // takes the ordinary limits below.
+    const meta = this.data.get(id);
+    const fixMs = meta?.newestFixMs ?? meta?.observedReceiptMs;
+    if (tracked && Number.isFinite(fixMs)) {
+      if (Date.now() - fixMs < TRACKED_RETENTION_MS) {
+        this.missingPolls.set(id, misses);
+        return 'stale';
+      }
+      this.missingPolls.delete(id);
+      return 'remove';
+    }
     // Partial admissions and position-less rows do not prove absence, but
     // retention remains bounded; a likely-landed position-less contact
     // still takes the landed limit.
@@ -196,7 +254,6 @@ export class MilitaryFlightRecords {
       Date.now() - (this.data.get(id)?.observedReceiptMs ?? 0) < 300000
     )
       return 'retain';
-    const misses = (this.missingPolls.get(id) || 0) + 1;
     const limit = likelyLanded ? LANDED_MISSING_POLL_LIMIT : MISSING_POLL_LIMIT;
     if (misses < limit) {
       this.missingPolls.set(id, misses);
@@ -208,6 +265,7 @@ export class MilitaryFlightRecords {
   forget(id) {
     this.data.delete(id);
     this.missingPolls.delete(id);
+    this.fixRepeated.delete(id);
     this.geoidNCache.delete(id);
   }
 }

src/layers/military/records.test.mjs

@@ -174,3 +174,98 @@ test('military records hold the last track and speed when a poll omits them', ()
   assert.equal(parked.meta.speedMps, 0);
   assert.equal(parked.meta.track, 90);
 });
+
+test('an airborne military contact reported with no altitude is marked as drawn at an invented height', () => {
+  const records = store();
+  const none = records.receive(
+    observation({ baroAltitudeM: null, ellipsoidAltitudeM: null }),
+    context(),
+  );
+  assert.equal(none.meta.renderAltitudeInvented, true);
+  const known = records.receive(
+    observation({ positionTimeMs: 1700000005000 }),
+    context(),
+  );
+  assert.equal(known.meta.renderAltitudeInvented, false);
+  const parked = store().receive(
+    observation({ baroAltitudeM: null, onGround: true }),
+    context(),
+  );
+  assert.equal(parked.meta.renderAltitudeInvented, false);
+});
+
+test('the tracked military contact is stale from its first missed update and kept until ten minutes after its newest fix', () => {
+  const realNow = Date.now;
+  const fixMs = Date.UTC(2026, 10, 9, 20, 0);
+  let clock = fixMs + 5_000;
+  Date.now = () => clock;
+  try {
+    for (const snapshot of [
+      { complete: true },
+      { complete: true, retainIds: new Set(['aaa001']) },
+      { complete: false },
+    ]) {
+      const records = store();
+      records.receive(
+        observation({ positionTimeMs: fixMs }),
+        context({ observedAtMs: fixMs + 1_000 }),
+      );
+      const absent = (tracked) =>
+        records.absence('aaa001', {
+          ...snapshot,
+          likelyLanded: false,
+          tracked,
+        });
+      clock = fixMs + 15_000;
+      assert.equal(absent(true), 'stale');
+      assert.equal(records.missingPolls.get('aaa001'), 1);
+      clock = fixMs + 9 * 60_000;
+      assert.equal(absent(true), 'stale');
+      clock = fixMs + 10 * 60_000 - 1;
+      assert.equal(absent(true), 'stale');
+      clock = fixMs + 10 * 60_000;
+      assert.equal(absent(true), 'remove');
+      assert.equal(records.missingPolls.has('aaa001'), false);
+    }
+    const records = store();
+    records.receive(
+      observation({ positionTimeMs: fixMs }),
+      context({ observedAtMs: fixMs + 1_000 }),
+    );
+    clock = fixMs + 15_000;
+    const untracked = (snapshot) =>
+      records.absence('aaa001', { ...snapshot, likelyLanded: false });
+    assert.equal(untracked({ complete: false }), 'retain');
+    assert.deepEqual(
+      [1, 2, 3].map(() => untracked({ complete: true })),
+      ['stale', 'stale', 'remove'],
+    );
+  } finally {
+    Date.now = realNow;
+  }
+});
+
+test('a repeated military fix time marks the contact as not updated; removal is unchanged', () => {
+  const records = store();
+  const fixMs = Date.UTC(2026, 10, 9, 20, 0);
+  const poll = (snapshotMs, positionTimeMs) => {
+    records.beginSnapshot(snapshotMs);
+    records.receive(
+      observation({ positionTimeMs }),
+      context({ observedAtMs: snapshotMs }),
+    );
+  };
+  poll(fixMs + 2_000, fixMs);
+  assert.equal(records.fixRepeated.has('aaa001'), false);
+  // adsb.lol re-sends the last position for up to a minute.
+  poll(fixMs + 12_000, fixMs);
+  assert.equal(records.fixRepeated.has('aaa001'), true);
+  assert.equal(records.missingPolls.has('aaa001'), false);
+  poll(fixMs + 12_000, fixMs);
+  assert.equal(records.fixRepeated.has('aaa001'), true);
+  poll(fixMs + 22_000, fixMs + 10_000);
+  assert.equal(records.fixRepeated.has('aaa001'), false);
+  poll(fixMs + 32_000, fixMs + 10_000);
+  records.forget('aaa001');
+  assert.equal(records.fixRepeated.has('aaa001'), false);
+});

src/layers/military/rendering.js

@@ -52,6 +52,7 @@ import {
   COURSE_SLEW_DT_MAX_SEC,
   ROTATION_REFRESH_MS,
   COURSE_MAX_DPS,
+  STALE_ALPHA,
 } from './policy.js';
 import { AIRCRAFT_ENVIRONMENT_MAP_OPTIONS } from '../../data/aircraftModelLighting.js';
 
@@ -640,6 +641,19 @@ export function createRendering({
   /** Per-frame driver for the standalone tracked model (see flights.js for the rationale). The
    *  tracked entity stays a pure billboard, so the follow-camera never stalls/freezes on 3D-toggle. */
 
+  /** Fade the tracked 3D model like the billboard while the plane is stale. */
+
+  function _applyTrackedModelFade(model) {
+    const alpha = parts.tracking._trackedStaleCue?.(flightState._trackedIcao)
+      ?.dimmed
+      ? STALE_ALPHA
+      : 1;
+    if (model.color?.alpha === alpha) return;
+    model.color = (
+      flightState._irBoost ? Cesium.Color.WHITE : TRACKED_ICON_COLOR
+    ).withAlpha(alpha);
+  }
+
   function _updateTrackedModel() {
     const active =
       flightState._trackedIcao &&
@@ -787,6 +801,7 @@ export function createRendering({
         flightState._trackedModel.modelMatrix,
         spec.headingOffsetDeg,
       );
+      _applyTrackedModelFade(flightState._trackedModel);
       if (!flightState._trackedModel.ready) return;
       flightState._trackedModel.scale = trackedModelScaleForPixelCap({
         baseScale: spec.scale,
@@ -814,6 +829,13 @@ export function createRendering({
    */
 
   function _fleetTick() {
+    // Every frame, shown or not: the tracked trail records where its icon is
+    // drawn and moves its body on (the head callback only reads the result),
+    // and the tracked label's stale cue follows the clock.
+    if (flightState._viewer) {
+      parts.tracking._trailTick?.();
+      parts.tracking._staleCueTick?.();
+    }
     if (
       !flightState._viewer ||
       !flightState._billboardCollection ||
@@ -989,7 +1011,9 @@ export function createRendering({
           flightState._cockpitContactMode && !isCockpitNear
             ? 1
             : _militaryBillboardScale(icao24),
-        baseAlpha: flightState.records.missingPolls.get(icao24) ? 0.45 : 1,
+        baseAlpha: flightState.records.missingPolls.get(icao24)
+          ? STALE_ALPHA
+          : 1,
         baseColor,
         focusFactor: focus.factor,
         cameraDistanceM,

src/layers/military/snapshotRenderer.js

@@ -4,6 +4,7 @@ import { CLASS_SCALE_2D } from '../../data/aircraftClass.js';
 import {
   turnRateFromFixHistory,
   liftRepeatedGroundFix,
+  staleHoldsForFix,
   synthesizeForwardKinematicsFix,
 } from '../../data/motionModel.js';
 import { aircraftIcon } from '../../data/aircraftIcons.js';
@@ -29,6 +30,7 @@ export function createMilitarySnapshotRenderer({
   return function applySnapshot(snapshot, viewer) {
     const currentIcaos = new Set();
     const receiptNowMs = snapshot.observedAtMs;
+    records.beginSnapshot(snapshot.observedAtMs);
     // Field-test fix (RS46): coarse floor cells to warm for the below-ground
     // clamp — collected during the loop (low airborne contacts only),
     // batch-resolved once after it. Never a fetch inside the loop.
@@ -83,6 +85,9 @@ export function createMilitarySnapshotRenderer({
           position: position.clone(),
           velocity: meta.speedMps,
           track: meta.track,
+          // Drawn at an invented height: a trail seeded from history keeps no
+          // position for it.
+          inventedHeight: meta.renderAltitudeInvented === true,
         });
         if (history.length > POSITION_HISTORY_LIMIT) {
           history.shift();
@@ -95,26 +100,36 @@ export function createMilitarySnapshotRenderer({
         // (owner): ground traffic appends too — taxi history stays live
         // after touchdown; _appendTrailFix floors grounded positions at
         // the surface so the ground leg drapes instead of diving.
-        if (isTracked) tracking._appendTrailFix(position.clone());
+        if (isTracked)
+          tracking._appendTrailFix(position.clone(), fixEpochMs, {
+            nullAltitude: meta.renderAltitudeInvented === true,
+          });
       } else {
         const modelOwnsGroundVisual = rendering._modelOwnsVisual(icao24);
         if (!modelOwnsGroundVisual) {
           liftRepeatedGroundFix(newest, position, meta.onGround);
         }
+        // A forward synthetic fix follows the stale rules (none while the
+        // contact holds at its report, a coast capped from the real report,
+        // never below the ground) and is a motion aid, never a trail report.
         const kinematicsChanged =
           newest.velocity !== meta.speedMps || newest.track !== meta.track;
-        if (kinematicsChanged) {
+        const holds = staleHoldsForFix(newest, {
+          onGround: meta.onGround,
+          cachedGroundFloor,
+        });
+        if (kinematicsChanged && !holds) {
           const synthetic = synthesizeForwardKinematicsFix(newest, {
             epochMs: Date.now(),
             velocity: meta.speedMps,
             track: meta.track,
             turnRateDps: meta.turnRateDps,
+            cachedGroundFloor,
           });
           if (synthetic) {
             history.push(synthetic);
             if (history.length > POSITION_HISTORY_LIMIT) history.shift();
             meta.turnRateDps = turnRateFromFixHistory(history);
-            if (isTracked) tracking._appendTrailFix(synthetic.position.clone());
           }
         }
       }
@@ -261,6 +276,7 @@ export function createMilitarySnapshotRenderer({
         complete: snapshot.complete,
         retainIds: snapshot.retainIds,
         likelyLanded: queries._likelyLanded(icao24),
+        tracked: icao24 === flightState._trackedIcao,
       });
       if (absence === 'retain') continue;
       if (absence === 'stale') {

src/layers/military/snapshotStale.test.mjs

@@ -0,0 +1,152 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createMilitarySnapshotRenderer } from './snapshotRenderer.js';
+
+// A poll that repeats a contact's last position with new speed or course
+// must not move it past where the stale rules allow, and never adds a
+// report to the tracked trail.
+
+const ICAO = '00a005';
+const SEC = 1000;
+
+/** Any property is a no-op function unless given. */
+function stub(overrides = {}) {
+  return new Proxy(overrides, {
+    get: (target, key) => (key in target ? target[key] : () => undefined),
+  });
+}
+
+function harness({ ground = () => null }) {
+  const appended = [];
+  const history = [];
+  const flightState = {
+    _trackedIcao: ICAO,
+    _trackedEntity: null,
+    _positionHistory: new Map([[ICAO, history]]),
+    _billboards: new Map([[ICAO, { position: new Cesium.Cartesian3() }]]),
+    _billboardCollection: stub(),
+    _cullPositions: new Map(),
+    _displayCourse: new Map(),
+    _groundSnap: stub(),
+    _displayFloorState: new Map(),
+    _models: new Map(),
+    _viewer: null,
+    _cockpitContactMode: false,
+    records: { data: new Map() },
+  };
+  let meta = null;
+  let fixEpochMs = 0;
+  const records = stub({
+    receive: () => {
+      flightState.records.data.set(ICAO, meta);
+      return { prevMeta: meta, meta, groundFlipped: false, fixEpochMs };
+    },
+    absence: () => 'retain',
+  });
+  const apply = createMilitarySnapshotRenderer({
+    flightState,
+    records,
+    militaryRegistry: stub(),
+    groundFloor: stub({ cachedGroundFloor: ground }),
+    meshFloor: stub(),
+    rendering: stub({
+      _modelOwnsVisual: () => false,
+      _fleetBillboardColor: () => Cesium.Color.WHITE,
+    }),
+    tracking: stub({
+      _militaryLayerSuppresses: () => false,
+      _appendTrailFix: (position, epochMs) =>
+        appended.push({ position, epochMs }),
+    }),
+    motion: stub(),
+    enrichment: stub(),
+    queries: stub({ _likelyLanded: () => false }),
+  });
+  return {
+    history,
+    appended,
+    poll(nextMeta, epochMs) {
+      meta = nextMeta;
+      fixEpochMs = epochMs;
+      apply({ records: [{ id: ICAO }], complete: true }, null);
+    },
+  };
+}
+
+const lightAircraft = (velocity, track) => ({
+  rawLat: 38.1436,
+  rawLon: -122.5561,
+  renderAltitudeM: 10,
+  altitudeFt: 130,
+  speedMps: velocity,
+  track,
+  onGround: false,
+  wasAirborne: true,
+  klass: 'light',
+});
+
+test('military: a low contact whose position repeats while its speed changes is not moved and adds nothing to the trail', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({ ground: () => -30 });
+  h.poll(lightAircraft(36.4, 63), t0);
+  const first = h.history[0].position;
+  for (const [i, velocity] of [30, 22, 14, 9].entries()) {
+    t.mock.timers.tick(10 * SEC);
+    h.poll(lightAircraft(velocity, 63 + i * 20), t0);
+  }
+  assert.equal(h.history.length, 1, 'a synthetic fix moved a holding contact');
+  assert.equal(h.history[0].position, first);
+  assert.equal(h.appended.length, 1, 'only the real report joins the trail');
+});
+
+test('military: a cruising contact whose position repeats coasts no further than the stale cap, and the synthetic fix is not a report', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({});
+  const cruise = (velocity, track) => ({
+    rawLat: 36,
+    rawLon: -115,
+    renderAltitudeM: 9_000,
+    altitudeFt: 29_500,
+    speedMps: velocity,
+    track,
+    onGround: false,
+    wasAirborne: true,
+    klass: 'jet',
+  });
+  h.poll(cruise(220, 90), t0);
+  const report = h.history[0].position;
+  t.mock.timers.tick(300 * SEC);
+  h.poll(cruise(200, 100), t0);
+  assert.equal(h.history.length, 2);
+  const moved = Cesium.Cartesian3.distance(report, h.history[1].position);
+  assert.ok(
+    moved <= 220 * 300 + 1,
+    `the synthetic fix coasted ${moved.toFixed(0)} m`,
+  );
+  assert.equal(h.appended.length, 1, 'the synthetic fix joined the trail');
+});
+
+test('military: a report drawn at an invented height says so on its history fix', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({});
+  h.poll({ ...lightAircraft(60, 90), renderAltitudeInvented: true }, t0);
+  h.poll({ ...lightAircraft(60, 90), rawLon: -122.55 }, t0 + 10 * SEC);
+  assert.equal(h.history[0].inventedHeight, true);
+  assert.equal(h.history[1].inventedHeight, false);
+});
+
+test('military: with the ground unknown, a low barometric altitude alone does not hold a contact', (t) => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  t.mock.timers.enable({ apis: ['Date'], now: t0 });
+  const h = harness({});
+  h.poll(lightAircraft(36.4, 63), t0);
+  t.mock.timers.tick(10 * SEC);
+  h.poll(lightAircraft(30, 80), t0);
+  assert.equal(h.history.length, 2);
+  assert.equal(h.history[0].staleHolds, false);
+  assert.equal(h.appended.length, 1);
+});

src/layers/military/staleHold.test.mjs

@@ -0,0 +1,199 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createMotion } from './motion.js';
+import { createFlightState } from './state.js';
+import { createTracking } from './tracking.js';
+import { TRAIL_CONTINUITY_MAX_SPEED_KT } from '../../data/trailContinuity.js';
+import { RENDER_DELAY_SEC } from './policy.js';
+
+const SEC = 1000;
+
+function motionFixture({ info, fixes, nowMs, ground = () => null }) {
+  const flightState = {
+    records: { data: new Map([['00a003', info]]) },
+    _positionHistory: new Map([['00a003', fixes]]),
+    _scratchArc: {},
+    _scratchOffset: new Cesium.Cartesian3(),
+    _scratchEnu: new Cesium.Matrix4(),
+    _scratchRenderTime: new Cesium.JulianDate(),
+    now: () => nowMs,
+  };
+  const motion = createMotion({
+    flightState,
+    services: {
+      groundFloor: { cachedGroundFloor: ground },
+      camera: {},
+      focus: {},
+    },
+    parts: {},
+  });
+  return motion;
+}
+
+function fix(timeMs, lat, lon, heightM, velocity, track) {
+  return {
+    time: Cesium.JulianDate.fromDate(new Date(timeMs)),
+    epochMs: timeMs,
+    position: Cesium.Cartesian3.fromDegrees(lon, lat, heightM),
+    velocity,
+    track,
+  };
+}
+
+test('a military contact stale after a low approach holds at its last report', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const ground = 1_590; // An airbase, ellipsoidal.
+  const fixes = [
+    fix(t0, 35.03, -106.58, ground + 300, 75, 260),
+    fix(t0 + 10 * SEC, 35.03, -106.588, ground + 40, 70, 260),
+  ];
+  const info = {
+    altitudeFt: 5_350,
+    renderAltitudeM: ground + 40,
+    rawLat: 35.03,
+    rawLon: -106.588,
+    speedMps: 70,
+    track: 260,
+    onGround: false,
+  };
+  for (const afterSec of [10, 60, 200]) {
+    const motion = motionFixture({
+      info,
+      fixes,
+      nowMs: t0 + 10 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC,
+      ground: () => ground,
+    });
+    const drawn = motion._deadReckon('00a003', new Cesium.Cartesian3());
+    assert.ok(Cesium.Cartesian3.distance(drawn, fixes[1].position) < 0.01);
+  }
+});
+
+test('a cruising military contact still heard after its last position coasts past 90 s, for up to 5 minutes, then stops', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 36, -115, 9_000, 220, 90),
+    fix(t0 + 10 * SEC, 36, -114.975, 9_000, 220, 90),
+  ];
+  const info = {
+    altitudeFt: 29_500,
+    renderAltitudeM: 9_000,
+    rawLat: 36,
+    rawLon: -114.975,
+    speedMps: 220,
+    track: 90,
+    onGround: false,
+    lastContactEpochMs: t0 + 10 * SEC + 300 * SEC,
+  };
+  const at = (afterSec) =>
+    motionFixture({
+      info,
+      fixes,
+      nowMs: t0 + 10 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC,
+    })._deadReckon('00a003', new Cesium.Cartesian3());
+  const d = (p) => Cesium.Cartesian3.distance(p, fixes[1].position);
+  const near = (got, want) => Math.abs(got - want) < want * 0.005 + 50;
+  assert.ok(near(d(at(45)), 220 * 45));
+  assert.ok(near(d(at(250)), 220 * 250), 'still moving at 250 s');
+  assert.ok(near(d(at(400)), 220 * 300), 'stopped at 300 s');
+});
+
+test('a bad military speed coasts no faster than the 1,500 kt continuity limit', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [
+    fix(t0, 36, -115, 9_000, 2_000, 90),
+    fix(t0 + 10 * SEC, 36, -114.975, 9_000, 2_000, 90),
+  ];
+  const info = {
+    altitudeFt: 29_500,
+    renderAltitudeM: 9_000,
+    rawLat: 36,
+    rawLon: -114.975,
+    speedMps: 2_000,
+    track: 90,
+    onGround: false,
+  };
+  const drawn = motionFixture({
+    info,
+    fixes,
+    nowMs: t0 + 10 * SEC + RENDER_DELAY_SEC * SEC + 300 * SEC,
+  })._deadReckon('00a003', new Cesium.Cartesian3());
+  const limitMps = (TRAIL_CONTINUITY_MAX_SPEED_KT * 1852) / 3600;
+  const d = Cesium.Cartesian3.distance(drawn, fixes[1].position);
+  assert.ok(Math.abs(d - limitMps * 300) < 1_500, `${d.toFixed(0)} m`);
+});
+
+test('a holding military contact draws no head and its trail ends at the last report', () => {
+  const t0 = Date.UTC(2026, 10, 9, 20, 0);
+  const fixes = [0, 1, 2].map((i) =>
+    fix(t0 + i * 10 * SEC, 35.03, -106.57 - i * 0.008, 1_700, 70, 260),
+  );
+  const svc = {
+    ...Object.fromEntries(
+      [
+        'picking',
+        'sprites',
+        'aircraftPresentation',
+        'camera',
+        'militaryRegistry',
+        'labels',
+        'meshFloor',
+        'focus',
+        'readout',
+        'context',
+        'render',
+        'recession',
+      ].map((name) => [name, {}]),
+    ),
+    groundSnap: { createGroundSnap: () => ({ clear() {} }) },
+    geoid: { ensureGeoidReady: async () => {}, geoidHeight: () => 0 },
+    groundFloor: {
+      resolveGroundFloorCellsBounded: async () => {},
+      cachedGroundFloor: () => null,
+      floorAltitudeM: (alt) => alt,
+    },
+  };
+  const calls = [];
+  svc.trails = {
+    createTrail: () => ({
+      setSegments: (segments) => calls.push(segments.flat()),
+      setPositions: (positions) => calls.push(positions.slice()),
+      setVisible() {},
+      clear() {},
+      destroy() {},
+    }),
+  };
+  const flightState = createFlightState({ services: svc, source: {} });
+  const added = [];
+  flightState._viewer = {
+    isDestroyed: () => false,
+    entities: {
+      add: (definition) => (added.push(definition), definition),
+      remove() {},
+    },
+  };
+  flightState._trackedIcao = '00a003';
+  flightState.now = () => t0 + 20 * SEC + RENDER_DELAY_SEC * SEC + 120 * SEC;
+  flightState._positionHistory.set('00a003', fixes);
+  const last = Cesium.Cartesian3.clone(fixes[2].position);
+  const tracking = createTracking({
+    flightState,
+    services: svc,
+    parts: {
+      motion: {
+        _isTrackWarmingUp: () => false,
+        _trackedTrailCached: () => last,
+        _trackedDisplayPosition: () => last,
+        _trackedModelCenterWorld: () => null,
+        _trackedModelEnvelopeM: () => null,
+      },
+    },
+    layer: {},
+  });
+  tracking._startTrail('00a003');
+  assert.equal(calls.at(-1).length, 3);
+  const head = added.find((d) =>
+    String(d.id).startsWith('gev-trail:mil-head-'),
+  );
+  assert.deepEqual(head.polyline.positions.getValue(), []);
+});

src/layers/military/state.js

@@ -178,9 +178,32 @@ export function createFlightState({ source, services }) {
 
   flightState._trailHeadSeq = 0;
 
-  /** @type {Cesium.Cartesian3[]} Chronological tracked-aircraft fixes (oldest first) */
+  /** @type {{position: Cesium.Cartesian3|null, timeMs: number, nullAltitude: boolean, breakBefore: boolean, drawn?: boolean}[]}
+   *  Chronological tracked-aircraft trail (oldest first): reports, and
+   *  (`drawn`) where the icon was drawn between them; `breakBefore` starts a
+   *  new segment. */
 
-  flightState._trailPositions = [];
+  flightState._trailFixes = [];
+
+  /** @type {number} Body fixes drawn: those at or before the display time. */
+
+  flightState._trailBodyCount = 0;
+
+  /** @type {Cesium.Cartesian3|null} Where the tracked icon was last recorded into the trail. */
+
+  flightState._trailRecordPosition = null;
+
+  /** @type {number} Display time (epoch ms) of that recording. */
+
+  flightState._trailRecordMs = -Infinity;
+
+  /** @type {string|undefined} The stale cue last written into the tracked label (see `_staleCueTick`). */
+
+  flightState._trackedStaleCueText = undefined;
+
+  /** @type {{key: string, cue: {text: string, dimmed: boolean}}|null} This frame's tracked stale cue (see `_trackedStaleCue`). */
+
+  flightState._trackedStaleCueCache = null;
 
   /** @type {number} Monotonic token — invalidates in-flight backfill responses */
 

src/layers/military/tracking.js

@@ -1,6 +1,6 @@
 import * as Cesium from 'cesium';
-import { julianAt } from '../../time/julian.js';
-import { t } from '../../i18n/index.js';
+import { formatNumber, t } from '../../i18n/index.js';
+import { unitLabel } from '../../ui/unitLabels.js';
 import { nextCockpitNearContacts } from '../../data/cockpitAirLod.js';
 import { aircraftIcon, TRACKED_ICON_PX } from '../../data/aircraftIcons.js';
 import { trackedModelZoomActive } from '../../data/trackedModelRegime.js';
@@ -19,14 +19,33 @@ import {
   TRACKED_MODEL_MAX_LOAD_FAILS,
   TRACKED_MODEL_RETRY_BACKOFF_MS,
   TRAIL_MAX_POINTS,
+  DR_CORRECTION_MS,
   RENDER_DELAY_SEC,
   TRAIL_COLOR,
   BILLBOARD_SCALE,
   GROUND_SCALE,
   AMBER_TRANSPARENT,
   TRACKED_ICON_COLOR,
+  STALE_ALPHA,
 } from './policy.js';
 import { roundHeadingDegrees } from '../../data/headingDegrees.js';
+import {
+  drawTrailSegments,
+  historyFixEpochMs,
+  insertTrailFix,
+  joinTrailBackfill,
+  recordTrailPoint,
+  trailBackfillFixes,
+  trailBodyCount,
+  trailBodyCountOutsideModel,
+  trailBodySegments,
+  trailHeadStartFix,
+  trailRecordingBlocked,
+  TRAIL_RECORD_STEP_M,
+  TRAIL_RECORD_STEP_MS,
+} from '../../data/trailContinuity.js';
+
+const STALE_TRACKED_COLOR = TRACKED_ICON_COLOR.withAlpha(STALE_ALPHA);
 
 export function createTracking({
   flightState,
@@ -206,24 +225,98 @@ export function createTracking({
     _setCockpitContactMode(active);
   }
 
+  /**
+   * The tracked plane's stale cue and whether it is drawn with the stale fade.
+   * - SIGNAL LOST · <time since its last report>, dimmed: its coast has ended
+   *   (or it holds at its last report after a missed update) and no newer
+   *   report has come.
+   * - STALE, dimmed: it has missed an update and still coasts.
+   * - STALE, not dimmed: the whole feed is backing off.
+   * @param {string} icao24
+   * @returns {{text: string, dimmed: boolean}}
+   */
+
+  function _trackedStaleCue(icao24) {
+    const records = flightState.records;
+    const misses = records.missingPolls.get(icao24) || 0;
+    const repeated = records.fixRepeated?.has(icao24) === true;
+    const backoff = flightState.feed._backoff === true;
+    // Computed once per frame for the label, the icon colour and the model
+    // fade; a poll between frames changes the key.
+    const frame = flightState._viewer?.scene?.frameState?.frameNumber;
+    const key = Number.isFinite(frame)
+      ? `${frame}|${icao24}|${misses}|${repeated}|${backoff}`
+      : null;
+    const cached = flightState._trackedStaleCueCache;
+    if (key !== null && cached?.key === key) return cached.cue;
+    const cue = _computeTrackedStaleCue(
+      icao24,
+      misses > 0 || repeated,
+      backoff,
+    );
+    flightState._trackedStaleCueCache = key === null ? null : { key, cue };
+    return cue;
+  }
+
+  /** `_trackedStaleCue` without the per-frame cache. A repeated fix counts as missed. */
+
+  function _computeTrackedStaleCue(icao24, missed, backoff) {
+    const lost = parts.motion._signalLost(icao24);
+    if (lost && (missed || !lost.holds)) {
+      const sec = Math.max(
+        0,
+        Math.floor((flightState.now() - lost.reportEpochMs) / 1000),
+      );
+      const time =
+        sec < 60
+          ? `${formatNumber(sec, { useGrouping: false })} ${unitLabel('s')}`
+          : `${formatNumber(Math.floor(sec / 60), { useGrouping: false })} ${unitLabel('min')}`;
+      return { text: t('SIGNAL LOST · {time}', { time }), dimmed: true };
+    }
+    if (missed) return { text: t('STALE'), dimmed: true };
+    if (backoff) return { text: t('STALE'), dimmed: false };
+    return { text: '', dimmed: false };
+  }
+
+  /** The tracked billboard's colour: amber, faded while stale, clear while the 3D model is the visual. */
+
+  function _trackedIconColor() {
+    if (parts.rendering._modelOwnsVisual(flightState._trackedIcao))
+      return AMBER_TRANSPARENT;
+    return _trackedStaleCue(flightState._trackedIcao).dimmed
+      ? STALE_TRACKED_COLOR
+      : TRACKED_ICON_COLOR;
+  }
+
+  /**
+   * Re-word the tracked label when its stale cue changes (STALE, SIGNAL LOST,
+   * each new second or minute of it). Run from the layer's per-frame pass.
+   */
+
+  function _staleCueTick() {
+    const icao24 = flightState._trackedIcao;
+    if (!icao24 || !flightState._trackedEntity) return;
+    const text = _trackedStaleCue(icao24).text;
+    if (text === flightState._trackedStaleCueText) return;
+    flightState._trackedStaleCueText = text;
+    _updateTrackedLabelModel(icao24);
+  }
+
   /**
    * Build the multi-line presentation text for the protected tracked host card.
    * Lines: callsign, type/registration, operator/altitude.
-   * While the plane is in its missed-poll grace (coasting on dead reckoning
-   * with sticky metadata) the first line carries a "· STALE" cue — the fleet's
-   * 45%-alpha billboard fade doesn't apply to the tracked plane (its entity
-   * owns the visual), so without this the readout would present last-known
-   * altitude/speed as live.
+   * The first line carries the stale cue (`_trackedStaleCue`): STALE while
+   * the plane coasts on dead reckoning after a missed update, SIGNAL LOST
+   * once the coast has ended, so last-known altitude/speed are never
+   * presented as live.
    * @param {Object|null} info - Flight metadata from _flightData
    * @param {string} icao24 - ICAO hex identifier (fallback display name)
    * @returns {string} Newline-separated label text
    */
 
   function _buildTrackedLabel(info, icao24) {
-    const stale =
-      flightState.records.missingPolls.get(icao24) || flightState.feed._backoff
-        ? ` · ${t('STALE')}`
-        : '';
+    const cue = _trackedStaleCue(icao24).text;
+    const stale = cue ? ` · ${cue}` : '';
     const callsign =
       (parts.queries._toCleanText(info?.callsign) ||
         parts.queries._toCleanText(info?.registration) ||
@@ -456,33 +549,112 @@ export function createTracking({
   }
 
   /**
-   * Append one fix to the tracked aircraft's trail accumulation and refresh
-   * the rendered trail. Caller passes an owned (cloned) Cartesian3.
-   * @param {Cesium.Cartesian3} position - New fix position, appended at the head.
+   * Put one report into the tracked aircraft's trail, in time order, and
+   * refresh the rendered trail. Caller passes an owned (cloned) Cartesian3. A
+   * report that breaks from its neighbour starts a new segment (see
+   * trailContinuity).
+   * @param {Cesium.Cartesian3} position - The report's position.
+   * @param {number} [epochMs] - The report's time; defaults to the tracked
+   *   aircraft's newest position-history fix, which every caller has just pushed.
+   * @param {{nullAltitude?: boolean}} [options] - `nullAltitude`: the report
+   *   had no altitude, so its position is not kept and the line breaks there.
+   *   A report for a time the display has already passed replaces what was
+   *   drawn after it (see `insertTrailFix`).
    */
 
-  function _appendTrailFix(position) {
-    flightState._trailPositions.push(_trailFloorPosition(position));
-    if (flightState._trailPositions.length > TRAIL_MAX_POINTS)
-      flightState._trailPositions.shift();
+  function _appendTrailFix(position, epochMs, { nullAltitude = false } = {}) {
+    const history = flightState._positionHistory.get(flightState._trackedIcao);
+    const timeMs = Number.isFinite(epochMs)
+      ? epochMs
+      : historyFixEpochMs(history?.[history.length - 1]);
+    insertTrailFix(
+      flightState._trailFixes,
+      _trailFloorPosition(position),
+      timeMs,
+      TRAIL_MAX_POINTS,
+      {
+        nullAltitude,
+        displayMs: flightState.now() - RENDER_DELAY_SEC * 1000,
+        settleMs: DR_CORRECTION_MS,
+      },
+    );
     _refreshTrailDisplay();
   }
 
   /**
-   * Renders the trail BODY — the accumulated fixes EXCLUDING the newest raw one. That
-   * newest fix is at ~now, ~one poll interval AHEAD of the delayed icon (rendered at
-   * now − RENDER_DELAY_SEC), so drawing it would push the trail in FRONT of the plane.
-   * The cheap per-frame _trailHeadEntity segment bridges the last body point to the
-   * delayed dead-reckoned head, so this primitive only rebuilds on a real fix (poll
-   * cadence), never at motion cadence.
+   * Per-frame trail work, run from the layer's preRender pass whether or not
+   * the head is visible: record where the tracked icon is drawn (at most once
+   * a second or every 150 m, floored like the head) and move the body on as
+   * the display time passes reports. The head callback only reads the result.
+   */
+
+  function _trailTick() {
+    if (!flightState._trackedIcao || !flightState._trail) return;
+    const displayMs = flightState.now() - RENDER_DELAY_SEC * 1000;
+    let changed = false;
+    if (
+      flightState._trailFixes.length &&
+      !parts.motion._isTrackWarmingUp() &&
+      !trailRecordingBlocked(flightState._trailFixes, displayMs)
+    ) {
+      const drawn = parts.motion._trackedDisplayPosition(
+        flightState._trackedIcao,
+      );
+      const last = flightState._trailRecordPosition;
+      if (
+        drawn &&
+        (!last ||
+          displayMs - flightState._trailRecordMs >= TRAIL_RECORD_STEP_MS ||
+          Cesium.Cartesian3.distance(last, drawn) >= TRAIL_RECORD_STEP_M)
+      ) {
+        flightState._trailRecordMs = displayMs;
+        flightState._trailRecordPosition = Cesium.Cartesian3.clone(
+          drawn,
+          flightState._trailRecordPosition ?? undefined,
+        );
+        changed = recordTrailPoint(
+          flightState._trailFixes,
+          _trailFloorPosition(Cesium.Cartesian3.clone(drawn)),
+          displayMs,
+          TRAIL_MAX_POINTS,
+        );
+      }
+    }
+    if (changed || _trailBodyCountNow() !== flightState._trailBodyCount)
+      _refreshTrailDisplay();
+  }
+
+  /**
+   * Renders the trail BODY — the accumulated fixes at or before the delayed display
+   * time (now − RENDER_DELAY_SEC). Newer fixes are AHEAD of the icon, so drawing them
+   * would push the trail in FRONT of the plane. The cheap per-frame _trailHeadEntity
+   * segment bridges the last body point to the delayed dead-reckoned head, so the
+   * body rebuilds only on a real fix or when the display time passes one, never at
+   * motion cadence. A break lifts the pen: each segment is
+   * its own line, nothing between them.
    */
 
   function _refreshTrailDisplay() {
     if (!flightState._trail) return;
-    flightState._trail.setPositions(
-      flightState._trailPositions.length > 1
-        ? flightState._trailPositions.slice(0, -1)
-        : flightState._trailPositions,
+    flightState._trailBodyCount = _trailBodyCountNow();
+    drawTrailSegments(
+      flightState._trail,
+      trailBodySegments(flightState._trailFixes, flightState._trailBodyCount),
+    );
+  }
+
+  /** Fixes at or before the time the aircraft is drawn (now − RENDER_DELAY_SEC),
+   *  less the newest recorded points inside its 3D model. */
+
+  function _trailBodyCountNow() {
+    return trailBodyCountOutsideModel(
+      flightState._trailFixes,
+      trailBodyCount(
+        flightState._trailFixes,
+        flightState.now() - RENDER_DELAY_SEC * 1000,
+      ),
+      parts.motion._trackedModelCenterWorld(),
+      parts.motion._trackedModelEnvelopeM(),
     );
   }
 
@@ -495,25 +667,27 @@ export function createTracking({
 
   function _startTrail(icao24) {
     flightState._trailBackfillToken += 1;
-    flightState._trailPositions = [];
+    flightState._trailFixes = [];
+    flightState._trailRecordPosition = null;
+    flightState._trailRecordMs = -Infinity;
     const history = flightState._positionHistory.get(icao24) || [];
-    // Seed only fixes at/behind the DELAYED display time (now − RENDER_DELAY_SEC). The
-    // newest ~RENDER_DELAY_SEC of fixes are AHEAD of the displayed icon; including them
-    // would draw the trail in front of the plane. They join via _appendTrailFix as they age.
-    const seedRenderTime = Cesium.JulianDate.addSeconds(
-      julianAt(flightState.now()),
-      -RENDER_DELAY_SEC,
-      flightState._scratchWarmupTime,
-    );
+    // Seed every report. The body only draws those at or before the delayed
+    // display time (now − RENDER_DELAY_SEC); newer ones join it as the display
+    // passes them. A synthetic forward fix (it carries `reportEpochMs`) is a
+    // motion aid, never a report. A report drawn at an invented height keeps
+    // no position and breaks the line.
     for (const fix of history) {
-      if (Cesium.JulianDate.lessThanOrEquals(fix.time, seedRenderTime)) {
-        // Floor grounded-history seeds (round 2): a grounded contact's stored
-        // fixes carry the pre-datum default height — the trail copy sits on
-        // the surface instead (pure lift; airborne fixes pass through).
-        flightState._trailPositions.push(
-          _trailFloorPosition(Cesium.Cartesian3.clone(fix.position)),
-        );
-      }
+      if (Number.isFinite(fix.reportEpochMs)) continue;
+      // Floor grounded-history seeds (round 2): a grounded contact's stored
+      // fixes carry the pre-datum default height — the trail copy sits on
+      // the surface instead (pure lift; airborne fixes pass through).
+      insertTrailFix(
+        flightState._trailFixes,
+        _trailFloorPosition(Cesium.Cartesian3.clone(fix.position)),
+        historyFixEpochMs(fix),
+        TRAIL_MAX_POINTS,
+        { nullAltitude: fix.inventedHeight === true },
+      );
     }
     if (!flightState._trail && flightState._viewer) {
       flightState._trail = createTrail(flightState._viewer, {
@@ -534,14 +708,13 @@ export function createTracking({
         show: !flightState._cockpitContactMode,
         polyline: {
           positions: new Cesium.CallbackProperty(() => {
-            // Need ≥2 accumulated points: the body draws all-but-newest, so the head must
-            // start at the last DISPLAYED body point (index n−2). With a single fix that
-            // point would be the sole raw fix — ~now, AHEAD of the delayed icon — so the
-            // segment would draw IN FRONT of the plane. Likewise during warm-up the icon
-            // predates all real history, so there is no valid body point behind it.
+            // The head starts at the last DISPLAYED body point, the newest fix at or
+            // before the delayed display time; a fix ahead of the icon would draw the
+            // segment IN FRONT of the plane. During warm-up the icon predates all real
+            // history, so there is no valid body point behind it.
             if (
               !flightState._trackedIcao ||
-              flightState._trailPositions.length < 2 ||
+              !flightState._trailFixes.length ||
               parts.motion._isTrackWarmingUp()
             )
               return [];
@@ -549,12 +722,20 @@ export function createTracking({
               parts.motion._trackedTrailCached() ||
               parts.motion._trackedDisplayPosition(flightState._trackedIcao);
             if (!head) return [];
-            // body[n−2] (last displayed body point) → delayed head: runs FORWARD, never a
-            // backward/reversing segment.
-            const start =
-              flightState._trailPositions[
-                flightState._trailPositions.length - 2
-              ];
+            // The last body point (a report, or where the icon was drawn a
+            // moment ago) → the icon. The body is moved on by _trailTick, never
+            // here: a callback must not add or remove entities.
+            // No head across a break: a body point more than 150 km away, or
+            // too long ago or too far to be one path, stays detached; none while
+            // the aircraft holds at its last point.
+            const last = trailHeadStartFix(
+              flightState._trailFixes,
+              head,
+              flightState.now(),
+              flightState._trailBodyCount,
+            );
+            if (!last) return [];
+            const start = last.position;
             // Floor the head too (round 2): a grounded tracked contact's DR
             // display height is the pre-datum default — without this the last
             // segment dives underground while taxiing.
@@ -595,10 +776,10 @@ export function createTracking({
   }
 
   /**
-   * Fire-and-forget optional source history backfill (PRD F2). Trace points are
-   * stride-thinned to the remaining vertex budget, then spliced strictly older
-   * than the oldest seeded fix AHEAD of the locally accumulated fine segment,
-   * capped at TRAIL_MAX_POINTS (newest kept). Any failure (404/timeout/
+   * Fire-and-forget optional source history backfill (PRD F2). Every trace
+   * point strictly older than the oldest seeded fix is spliced AHEAD of the
+   * locally accumulated fine segment, each leg its own segment, capped at
+   * TRAIL_MAX_POINTS (newest kept). Any failure (404/timeout/
    * malformed) silently keeps the local-only trail.
    * @param {string} icao24 - ICAO hex identifier being tracked.
    * @param {number} token - Backfill token captured at request time.
@@ -644,7 +825,13 @@ export function createTracking({
         lon = point.longitude;
       if (!Number.isFinite(lat) || !Number.isFinite(lon)) continue;
       if (!Number.isFinite(t) || t >= oldestFixEpochSec) continue;
-      parsed.push({ lat, lon, baroAltitudeM: point.baroAltitudeM });
+      parsed.push({
+        lat,
+        lon,
+        baroAltitudeM: point.baroAltitudeM,
+        onGround: point.onGround === true,
+        timeMs: point.observedAtMs,
+      });
     }
     if (!parsed.length) return;
 
@@ -667,39 +854,34 @@ export function createTracking({
     )
       return;
 
-    let older = [];
-    let lastAltM = null; // carry-forward for ground points whose cell isn't warm yet
-    for (const { lat, lon, baroAltitudeM } of parsed) {
-      const baroM =
-        baroAltitudeM == null ? null : baroAltitudeM + geoidHeight(lat, lon);
-      let altM = floorAltitudeM(baroM, cachedGroundFloor(lat, lon));
-      // Ground/no-alt point with an unresolved floor: hold the previous
-      // waypoint's altitude (continuity — never a dive to a made-up depth).
-      // Leading points with nothing to carry keep the old low breadcrumb
-      // sentinel (an arbitrary placeholder, never a reported altitude).
-      if (altM == null) altM = lastAltM != null ? lastAltM : 50;
-      lastAltM = altM;
-      older.push(Cesium.Cartesian3.fromDegrees(lon, lat, altM));
-    }
-
-    // Stride-thin the backfill so it plus the live fine segment fit the cap.
-    const budget = Math.max(
-      1,
-      TRAIL_MAX_POINTS - flightState._trailPositions.length,
+    // Every trace point is kept, each leg its own segment. A 'ground' point
+    // is at altitude 0, floored at the ground. A point with no altitude is a
+    // break on both sides: it sits on the ground where the floor is known and
+    // is skipped otherwise; a skipped point still breaks the line.
+    const backfill = trailBackfillFixes(
+      parsed.map(({ lat, lon, baroAltitudeM, onGround, timeMs }) => {
+        const known = Number.isFinite(baroAltitudeM)
+          ? baroAltitudeM
+          : onGround
+            ? 0
+            : null;
+        const altM = floorAltitudeM(
+          known == null ? null : known + geoidHeight(lat, lon),
+          cachedGroundFloor(lat, lon),
+        );
+        return {
+          position:
+            altM == null ? null : Cesium.Cartesian3.fromDegrees(lon, lat, altM),
+          timeMs,
+          nullAltitude: known == null,
+        };
+      }),
+    );
+    flightState._trailFixes = joinTrailBackfill(
+      backfill,
+      flightState._trailFixes,
+      TRAIL_MAX_POINTS,
     );
-    if (older.length > budget) {
-      const stride = Math.ceil(older.length / budget);
-      const thinned = [];
-      for (let i = 0; i < older.length; i += stride) thinned.push(older[i]);
-      older = thinned;
-    }
-
-    flightState._trailPositions = older.concat(flightState._trailPositions);
-    if (flightState._trailPositions.length > TRAIL_MAX_POINTS) {
-      flightState._trailPositions = flightState._trailPositions.slice(
-        flightState._trailPositions.length - TRAIL_MAX_POINTS,
-      );
-    }
     _refreshTrailDisplay();
   }
 
@@ -709,7 +891,9 @@ export function createTracking({
 
   function _clearTrail() {
     flightState._trailBackfillToken += 1;
-    flightState._trailPositions = [];
+    flightState._trailFixes = [];
+    flightState._trailRecordPosition = null;
+    flightState._trailRecordMs = -Infinity;
     if (flightState._trail) flightState._trail.clear();
     if (
       flightState._trailHeadEntity &&
@@ -926,15 +1110,9 @@ export function createTracking({
           (CLASS_SCALE_2D[
             flightState.records.data.get(flightState._trackedIcao)?.klass
           ] || 1),
-        // Solid amber when the billboard is the visual (zoomed out, 3D off, or model still loading);
-        // transparent once the STANDALONE tracked model is actually up (ready + shown).
-        color: new Cesium.CallbackProperty(
-          () =>
-            parts.rendering._modelOwnsVisual(flightState._trackedIcao)
-              ? AMBER_TRANSPARENT
-              : TRACKED_ICON_COLOR,
-          false,
-        ),
+        // Solid amber (faded while stale) when the billboard is the visual (zoomed out, 3D off,
+        // or model still loading); transparent once the STANDALONE tracked model is actually up.
+        color: new Cesium.CallbackProperty(_trackedIconColor, false),
         sizeInMeters: false,
         scaleByDistance: new Cesium.NearFarScalar(1000, 3.0, 8000000, 0.5),
         alignedAxis: Cesium.Cartesian3.ZERO,
@@ -1157,6 +1335,9 @@ export function createTracking({
     _setCockpitContactMode,
     _applyCockpitState,
     _buildTrackedLabel,
+    _trackedStaleCue,
+    _trackedIconColor,
+    _staleCueTick,
     _updateTrackedLabelModel,
     _syncTrackedBillboardImage,
     _refreshTr3bContact,
@@ -1168,6 +1349,7 @@ export function createTracking({
     _syncTracked2dRotation,
     _trailFloorPosition,
     _appendTrailFix,
+    _trailTick,
     _refreshTrailDisplay,
     _startTrail,
     _backfillTrail,

src/layers/military/trail.test.mjs

@@ -0,0 +1,260 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createFlightState } from './state.js';
+import { createTracking } from './tracking.js';
+import { TRAIL_MAX_POINTS } from './policy.js';
+
+const MINUTE = 60_000;
+const HOUR = 60 * MINUTE;
+
+function services({ trails, floor = () => null } = {}) {
+  const names = [
+    'picking',
+    'sprites',
+    'aircraftPresentation',
+    'camera',
+    'militaryRegistry',
+    'labels',
+    'meshFloor',
+    'focus',
+    'readout',
+    'context',
+    'render',
+    'recession',
+  ];
+  return {
+    ...Object.fromEntries(names.map((name) => [name, {}])),
+    groundSnap: { createGroundSnap: () => ({ clear() {} }) },
+    trails,
+    geoid: { ensureGeoidReady: async () => {}, geoidHeight: () => 0 },
+    groundFloor: {
+      resolveGroundFloorCellsBounded: async () => {},
+      cachedGroundFloor: floor,
+      floorAltitudeM: (alt, ground) =>
+        Number.isFinite(alt) && Number.isFinite(ground)
+          ? Math.max(alt, ground + 1)
+          : Number.isFinite(alt)
+            ? alt
+            : Number.isFinite(ground)
+              ? ground + 1
+              : null,
+    },
+  };
+}
+
+/** A fake trail that records what draws: segments of two or more points. */
+function recordingTrail() {
+  const calls = [];
+  return {
+    last: () =>
+      (calls[calls.length - 1] ?? []).filter((segment) => segment.length >= 2),
+    createTrail: () => ({
+      setSegments(segments) {
+        calls.push(segments.map((segment) => segment.slice()));
+      },
+      setPositions(positions) {
+        calls.push([positions.slice()]);
+      },
+      setVisible() {},
+      clear() {
+        calls.push([]);
+      },
+      destroy() {},
+    }),
+  };
+}
+
+function trackedHarness({ liveFixes = [], trace = null, nowMs, floor, head }) {
+  const trail = recordingTrail();
+  const svc = services({ trails: trail, floor });
+  const flightState = createFlightState({
+    services: svc,
+    source: {
+      label: 'Fixture military aircraft',
+      async getTrack() {
+        if (!trace) throw new Error('404');
+        return { records: trace };
+      },
+    },
+  });
+  const added = [];
+  flightState._viewer = {
+    isDestroyed: () => false,
+    entities: {
+      add(definition) {
+        added.push(definition);
+        return definition;
+      },
+      remove() {},
+    },
+  };
+  flightState._trackedIcao = 'ae0001';
+  flightState.now = () => nowMs;
+  flightState._positionHistory.set(
+    'ae0001',
+    liveFixes.map((fix) => ({
+      time: Cesium.JulianDate.fromDate(new Date(fix.timeMs)),
+      epochMs: fix.timeMs,
+      position: Cesium.Cartesian3.fromDegrees(fix.lon, fix.lat, 9_000),
+    })),
+  );
+  const tracking = createTracking({
+    flightState,
+    services: svc,
+    parts: {
+      motion: {
+        _isTrackWarmingUp: () => false,
+        _trackedTrailCached: () => head ?? null,
+        _trackedDisplayPosition: () => head ?? null,
+        _trackedModelCenterWorld: () => null,
+        _trackedModelEnvelopeM: () => null,
+      },
+    },
+    layer: {},
+  });
+  const headPositions = () =>
+    added
+      .find((definition) =>
+        String(definition.id).startsWith('gev-trail:mil-head-'),
+      )
+      .polyline.positions.getValue();
+  return { trail, tracking, flightState, headPositions };
+}
+
+const settle = () => new Promise((resolve) => setTimeout(resolve, 0));
+
+/** readsb-shaped trace points as the military source parses them. */
+function point(timeMs, lat, lon, baroAltitudeM = 9_000, onGround = false) {
+  return {
+    observedAtMs: timeMs,
+    latitude: lat,
+    longitude: lon,
+    baroAltitudeM,
+    onGround,
+  };
+}
+
+function leg(startMs, lat, lon, count) {
+  return Array.from({ length: count }, (_, i) =>
+    point(startMs + i * MINUTE, lat, lon + i * 0.12),
+  );
+}
+
+test('a multi-leg military trace becomes separate segments with no line across a gap', async () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const first = leg(t0, 36, -115, 5);
+  const second = leg(t0 + 5 * MINUTE + 3 * HOUR, 35, -106, 5);
+  const third = leg(t0 + 10 * MINUTE + 5 * HOUR, 38, -77, 5);
+  const trace = [...first, ...second, ...third];
+  const liveStart = t0 + 15 * MINUTE + 5 * HOUR;
+  const liveFixes = [0, 1, 2].map((i) => ({
+    timeMs: liveStart + i * MINUTE,
+    lat: 38,
+    lon: -77 + 0.6 + i * 0.12,
+  }));
+  const { trail, flightState, tracking } = trackedHarness({
+    liveFixes,
+    trace,
+    nowMs: liveStart + 3 * MINUTE,
+  });
+  tracking._startTrail('ae0001');
+  await settle();
+  assert.equal(flightState._trailFixes.length, trace.length + 3);
+  assert.deepEqual(
+    trail.last().map((segment) => segment.length),
+    [5, 5, 8],
+  );
+});
+
+test('a military trace point with no altitude breaks the line and is never given a made-up height', async () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const trace = leg(t0, 35, -106, 7);
+  trace[3] = point(trace[3].observedAtMs, 35, -105.64, null);
+  const liveFixes = [{ timeMs: t0 + 7 * MINUTE, lat: 35, lon: -105.16 }];
+  const { trail, flightState, tracking } = trackedHarness({
+    liveFixes,
+    trace,
+    nowMs: t0 + 9 * MINUTE,
+  });
+  tracking._startTrail('ae0001');
+  await settle();
+  // Skipped (no floor known): 3 + 3 + the live fix drawn.
+  assert.equal(flightState._trailFixes.length, trace.length - 1 + 1);
+  assert.deepEqual(
+    trail.last().map((segment) => segment.length),
+    [3, 4],
+  );
+  for (const fix of flightState._trailFixes) {
+    const height = Cesium.Cartographic.fromCartesian(fix.position).height;
+    assert.ok(Math.abs(height - 9_000) < 1, `no invented height: ${height}`);
+  }
+});
+
+test('a military ground point is drawn on the ground and stays joined', async () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  // Taxiing: 'ground' points a few hundred metres apart, one minute apart.
+  const trace = Array.from({ length: 4 }, (_, i) =>
+    point(t0 + i * MINUTE, 35, -106.6 + i * 0.004, null, true),
+  );
+  const liveFixes = [{ timeMs: t0 + 4 * MINUTE, lat: 35, lon: -106.584 }];
+  const { trail, flightState, tracking } = trackedHarness({
+    liveFixes,
+    trace,
+    nowMs: t0 + 6 * MINUTE,
+    floor: () => 1_590,
+  });
+  tracking._startTrail('ae0001');
+  await settle();
+  assert.deepEqual(
+    trail.last().map((segment) => segment.length),
+    [5],
+  );
+  const height = Cesium.Cartographic.fromCartesian(
+    flightState._trailFixes[0].position,
+  ).height;
+  assert.ok(Math.abs(height - 1_591) < 1);
+});
+
+test('the military head is drawn to a nearby aircraft and not to one more than 150 km away', () => {
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const liveFixes = [0, 1, 2].map((i) => ({
+    timeMs: t0 + i * MINUTE,
+    lat: 35,
+    lon: -106 + i * 0.12,
+  }));
+  const near = Cesium.Cartesian3.fromDegrees(-105.7, 35, 9_000);
+  const joined = trackedHarness({
+    liveFixes,
+    nowMs: t0 + 3 * MINUTE,
+    head: near,
+  });
+  joined.tracking._startTrail('ae0001');
+  assert.equal(joined.headPositions().length, 2);
+
+  const far = Cesium.Cartesian3.fromDegrees(-103, 35, 9_000);
+  const detached = trackedHarness({
+    liveFixes,
+    nowMs: t0 + 3 * MINUTE,
+    head: far,
+  });
+  detached.tracking._startTrail('ae0001');
+  assert.deepEqual(detached.headPositions(), []);
+});
+
+test('a full 24 h military trace fits within TRAIL_MAX_POINTS without thinning', async () => {
+  const day = 24 * 60;
+  const t0 = Date.UTC(2026, 10, 8, 0, 0);
+  const trace = leg(t0, 20, -150, day);
+  const liveStart = t0 + day * MINUTE;
+  const liveFixes = [{ timeMs: liveStart, lat: 20, lon: -150 + day * 0.12 }];
+  const { flightState, tracking } = trackedHarness({
+    liveFixes,
+    trace,
+    nowMs: liveStart + MINUTE,
+  });
+  tracking._startTrail('ae0001');
+  await settle();
+  assert.ok(TRAIL_MAX_POINTS >= day + 500);
+  assert.equal(flightState._trailFixes.length, day + 1);
+});

src/layers/military/trailRecord.test.mjs

@@ -0,0 +1,697 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { createMotion } from './motion.js';
+import { createFlightState } from './state.js';
+import { createTracking } from './tracking.js';
+import { RENDER_DELAY_SEC, STALE_ALPHA } from './policy.js';
+
+// A tracked military contact's trail between reports is the path its icon
+// was drawn along, recorded from the real display path frame by frame.
+
+const SEC = 1000;
+const ICAO = '00a004';
+const T0 = Date.UTC(2026, 10, 9, 21, 0);
+
+function fix(timeMs, lat, lon, heightM, velocity, track) {
+  return {
+    time: Cesium.JulianDate.fromDate(new Date(timeMs)),
+    epochMs: timeMs,
+    position: Cesium.Cartesian3.fromDegrees(lon, lat, heightM),
+    velocity,
+    track,
+  };
+}
+
+function services(trail, ground) {
+  const names = [
+    'picking',
+    'sprites',
+    'aircraftPresentation',
+    'camera',
+    'militaryRegistry',
+    'labels',
+    'meshFloor',
+    'focus',
+    'readout',
+    'context',
+    'render',
+    'recession',
+  ];
+  return {
+    ...Object.fromEntries(names.map((name) => [name, {}])),
+    aircraftPresentation: {
+      isTr3b: () => false,
+      tr3bTypeLabel: (id, label) => label,
+    },
+    focus: {
+      nearFarScalarValueAtDistance: () => 1,
+      clearFocusTarget() {},
+      publishFocusTargetFromCachedPosition() {},
+    },
+    groundSnap: { createGroundSnap: () => ({ clear() {} }) },
+    trails: trail,
+    geoid: { ensureGeoidReady: async () => {}, geoidHeight: () => 0 },
+    groundFloor: {
+      resolveGroundFloorCellsBounded: async () => {},
+      cachedGroundFloor: ground,
+      floorAltitudeM: (alt, g) =>
+        Number.isFinite(g) && Number.isFinite(alt)
+          ? Math.max(alt, g + 1.5)
+          : alt,
+    },
+  };
+}
+
+/**
+ * Real motion, real display position and real tracking over one contact.
+ * `frame()` runs one rendered frame: the layer's preRender work, then the
+ * head callback, as Cesium does.
+ */
+function harness({ fixes, info, nowMs, ground = () => null }) {
+  const segmentsCalls = [];
+  let inHead = false;
+  const trail = {
+    createTrail: () => ({
+      setSegments(segments) {
+        assert.ok(
+          !inHead,
+          'the trail body was rebuilt inside the head callback',
+        );
+        segmentsCalls.push(segments.map((segment) => segment.slice()));
+      },
+      setPositions(positions) {
+        assert.ok(
+          !inHead,
+          'the trail body was rebuilt inside the head callback',
+        );
+        segmentsCalls.push([positions.slice()]);
+      },
+      setVisible() {},
+      clear() {},
+      destroy() {},
+    }),
+  };
+  const svc = services(trail, ground);
+  const flightState = createFlightState({ services: svc, source: {} });
+  flightState.records.data.set(ICAO, info);
+  let frameNumber = 0;
+  const added = [];
+  let entityChanges = 0;
+  flightState._viewer = {
+    isDestroyed: () => false,
+    scene: {
+      frameState: {
+        get frameNumber() {
+          return frameNumber;
+        },
+      },
+    },
+    entities: {
+      add(definition) {
+        entityChanges += 1;
+        added.push(definition);
+        return definition;
+      },
+      remove() {
+        entityChanges += 1;
+      },
+    },
+  };
+  flightState._trackedIcao = ICAO;
+  let clock = nowMs;
+  flightState.now = () => clock;
+  flightState._positionHistory.set(ICAO, fixes);
+  const parts = {
+    rendering: { _modelOwnsVisual: () => false, _modelSpec: () => ({}) },
+    queries: {
+      _toCleanText: (value) => (typeof value === 'string' ? value.trim() : ''),
+      _formatAltitude: (ft) => `${ft} ft`,
+    },
+  };
+  parts.motion = createMotion({ flightState, services: svc, parts });
+  const tracking = createTracking({
+    flightState,
+    services: svc,
+    parts,
+    layer: {},
+  });
+  parts.tracking = tracking;
+  const realDateNow = Date.now;
+  Date.now = () => clock;
+  const headEntity = () =>
+    added.find((definition) =>
+      String(definition.id).startsWith('gev-trail:mil-head-'),
+    );
+  const head = () => {
+    const before = entityChanges;
+    inHead = true;
+    try {
+      return headEntity().polyline.positions.getValue();
+    } finally {
+      inHead = false;
+      assert.equal(
+        entityChanges,
+        before,
+        'the head callback added or removed an entity',
+      );
+    }
+  };
+  /** Drawn icon positions, one per frame. */
+  const drawn = [];
+  const frame = ({ readHead = true } = {}) => {
+    frameNumber += 1;
+    tracking._trailTick();
+    const icon = parts.motion._trackedDisplayPosition(ICAO);
+    if (icon) drawn.push(Cesium.Cartesian3.clone(icon));
+    if (readHead) head();
+  };
+  const run = (toMs, { frameMs = 100, readHead = true } = {}) => {
+    while (clock < toMs) {
+      clock = Math.min(toMs, clock + frameMs);
+      frame({ readHead });
+    }
+  };
+  return {
+    flightState,
+    tracking,
+    motion: parts.motion,
+    head,
+    drawn,
+    run,
+    body: () => segmentsCalls.at(-1) ?? [],
+    bodyPoints: () => (segmentsCalls.at(-1) ?? []).flat(),
+    restore: () => {
+      Date.now = realDateNow;
+    },
+  };
+}
+
+/** Distance from p to the nearest point of a polyline. */
+function distanceToPath(p, path) {
+  let best = Infinity;
+  const ab = new Cesium.Cartesian3();
+  const ap = new Cesium.Cartesian3();
+  const closest = new Cesium.Cartesian3();
+  if (path.length === 1) return Cesium.Cartesian3.distance(p, path[0]);
+  for (let i = 1; i < path.length; i++) {
+    Cesium.Cartesian3.subtract(path[i], path[i - 1], ab);
+    Cesium.Cartesian3.subtract(p, path[i - 1], ap);
+    const len2 = Cesium.Cartesian3.magnitudeSquared(ab);
+    const t =
+      len2 > 0
+        ? Math.max(0, Math.min(1, Cesium.Cartesian3.dot(ap, ab) / len2))
+        : 0;
+    Cesium.Cartesian3.add(
+      path[i - 1],
+      Cesium.Cartesian3.multiplyByScalar(ab, t, closest),
+      closest,
+    );
+    best = Math.min(best, Cesium.Cartesian3.distance(p, closest));
+  }
+  return best;
+}
+
+/** No vertex turns the line back sharper than 120° over a short step. */
+function assertNoSpur(line, label) {
+  const u = new Cesium.Cartesian3();
+  const v = new Cesium.Cartesian3();
+  for (let i = 1; i < line.length - 1; i++) {
+    Cesium.Cartesian3.subtract(line[i], line[i - 1], u);
+    Cesium.Cartesian3.subtract(line[i + 1], line[i], v);
+    const lu = Cesium.Cartesian3.magnitude(u);
+    const lv = Cesium.Cartesian3.magnitude(v);
+    if (!(lu > 0 && lv > 0) || Math.min(lu, lv) > 500) continue;
+    const cos = Cesium.Cartesian3.dot(u, v) / (lu * lv);
+    assert.ok(
+      cos >= -0.5,
+      `${label}: spur at vertex ${i} of ${line.length} (turn ${(
+        (Math.acos(cos) * 180) /
+        Math.PI
+      ).toFixed(0)}°)`,
+    );
+  }
+}
+
+const indexOfPoint = (line, p) =>
+  line.findIndex((q) => Cesium.Cartesian3.distance(p, q) < 0.01);
+
+// A departing jet: four northbound climb reports, then a left turn through
+// west to south with no report while the icon flies it.
+function departure() {
+  const fixes = [
+    fix(T0, 37.6329, -122.3661, 213, 85, 8),
+    fix(T0 + 30 * SEC, 37.6542, -122.3616, 495, 95, 0),
+    fix(T0 + 60 * SEC, 37.6813, -122.3615, 808, 104, 350),
+    fix(T0 + 90 * SEC, 37.7066, -122.3755, 1257, 112, 340),
+  ];
+  const info = {
+    altitudeFt: 4_124,
+    renderAltitudeM: 1257,
+    rawLat: 37.7066,
+    rawLon: -122.3755,
+    speedMps: 112,
+    track: 340,
+    turnRateDps: -2,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: T0 + 200 * SEC,
+  };
+  return { fixes, info };
+}
+
+test('military: a report newer than the display time arrives mid-turn, and the trail between the reports stays on the drawn arc', () => {
+  const { fixes, info } = departure();
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    // 40 s of turn with no new report: the icon flies the arc.
+    h.run(start + 40 * SEC);
+    const arc = h.drawn.slice();
+    // The next report is where the plane really was 60 s after the last one,
+    // still ahead of the display time (normal 30 s delay, slow feed). The icon
+    // now interpolates the straight line between the two reports and is eased
+    // onto it.
+    const at60 = h.motion._deadReckon(
+      ICAO,
+      new Cesium.Cartesian3(),
+      Cesium.JulianDate.fromDate(new Date(T0 + 150 * SEC)),
+    );
+    const c = Cesium.Cartographic.fromCartesian(at60);
+    const report = fix(
+      T0 + 150 * SEC,
+      Cesium.Math.toDegrees(c.latitude),
+      Cesium.Math.toDegrees(c.longitude),
+      c.height,
+      112,
+      220,
+    );
+    fixes.push(report);
+    info.track = 220;
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    // The display passes the new report.
+    h.run(start + 75 * SEC);
+    const line = h.bodyPoints();
+    const from = indexOfPoint(line, fixes[3].position);
+    const to = indexOfPoint(line, report.position);
+    assert.ok(from >= 0 && to > from, 'both reports are on the trail');
+    const between = line.slice(from, to + 1);
+    // Every vertex between the reports is where the icon was drawn...
+    const drawnPath = [fixes[3].position, ...h.drawn, report.position];
+    for (const vertex of between) {
+      const off = distanceToPath(vertex, drawnPath);
+      assert.ok(
+        off < 50,
+        `trail vertex ${off.toFixed(0)} m off the drawn path`,
+      );
+    }
+    // ...and the arc the icon flew is still there: no straight chord. (The
+    // arc strays far from the chord, so a chord would fail this.)
+    const chord = [fixes[3].position, report.position];
+    assert.ok(Math.max(...arc.map((p) => distanceToPath(p, chord))) > 300);
+    // The last 10 s of the arc are exempt: the correction pulled the icon back
+    // over them, and keeping them would fold the line back on itself.
+    for (const point of arc.slice(0, -100)) {
+      const off = distanceToPath(point, between);
+      assert.ok(off < 50, `drawn arc point ${off.toFixed(0)} m from the trail`);
+    }
+    assertNoSpur(line, 'trail');
+    assertNoSpur(h.head(), 'head');
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a report the display already passed replaces the guess drawn after it, with no spike', () => {
+  const { fixes, info } = departure();
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(start + 40 * SEC);
+    const displayMs = T0 + 130 * SEC;
+    // A report for 10 s before the display time, 400 m inside the drawn arc.
+    const guess = h.motion._deadReckon(
+      ICAO,
+      new Cesium.Cartesian3(),
+      Cesium.JulianDate.fromDate(new Date(T0 + 120 * SEC)),
+    );
+    const c = Cesium.Cartographic.fromCartesian(guess);
+    const report = fix(
+      T0 + 120 * SEC,
+      Cesium.Math.toDegrees(c.latitude) - 400 / 111_320,
+      Cesium.Math.toDegrees(c.longitude),
+      c.height,
+      112,
+      250,
+    );
+    fixes.push(report);
+    info.track = 250;
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    h.run(start + 55 * SEC);
+    const recorded = h.flightState._trailFixes.filter((entry) => entry.drawn);
+    // Nothing recorded between the report and the end of the ease.
+    assert.deepEqual(
+      recorded
+        .filter(
+          (entry) =>
+            entry.timeMs > report.epochMs && entry.timeMs < displayMs + 900,
+        )
+        .map((entry) => entry.timeMs - T0),
+      [],
+    );
+    const line = h.bodyPoints();
+    assert.ok(indexOfPoint(line, report.position) >= 0);
+    assertNoSpur(line, 'trail');
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a history report at an invented height is seeded with no position, and nothing is recorded next to it', () => {
+  const { fixes, info } = departure();
+  fixes[3].inventedHeight = true;
+  const start = T0 + 90 * SEC + RENDER_DELAY_SEC * SEC;
+  const h = harness({ fixes, info, nowMs: start });
+  try {
+    h.tracking._startTrail(ICAO);
+    const seeded = h.flightState._trailFixes.find(
+      (entry) => entry.timeMs === fixes[3].epochMs,
+    );
+    assert.equal(seeded.position, null);
+    assert.equal(seeded.nullAltitude, true);
+    h.run(start + 10 * SEC);
+    assert.equal(
+      h.flightState._trailFixes.filter((entry) => entry.drawn).length,
+      0,
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+// A turboprop at FL210 flying east out of receiver coverage: its last
+// position is at T0 + 30 s, and contact goes on for four more minutes.
+function outOfCoverage() {
+  const fixes = [
+    fix(T0, 43, -108.1, 6_400, 140, 90),
+    fix(T0 + 30 * SEC, 43, -108.05, 6_400, 140, 90),
+  ];
+  const info = {
+    altitudeFt: 21_000,
+    renderAltitudeM: 6_400,
+    rawLat: 43,
+    rawLon: -108.05,
+    speedMps: 140,
+    track: 90,
+    callsign: 'TEST7',
+    turnRateDps: 0,
+    onGround: false,
+    wasAirborne: true,
+    lastContactEpochMs: T0 + 30 * SEC + 240 * SEC,
+  };
+  return { fixes, info };
+}
+
+/** Display time (ms) `afterSec` past the last report. */
+const pastReport = (afterSec) =>
+  T0 + 30 * SEC + RENDER_DELAY_SEC * SEC + afterSec * SEC;
+
+test('military: a plane that stops reporting keeps moving dimmed as STALE from its first missed update, then stops as SIGNAL LOST', () => {
+  const { fixes, info } = outOfCoverage();
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(30));
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: '',
+      dimmed: false,
+    });
+    assert.equal(h.tracking._trackedIconColor().alpha, 1);
+    // The first missed update.
+    h.flightState.records.missingPolls.set(ICAO, 1);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: 'STALE',
+      dimmed: true,
+    });
+    assert.match(
+      h.tracking._buildTrackedLabel(h.flightState.records.data.get(ICAO), ICAO),
+      /STALE/,
+    );
+    // Drawn with the fleet's stale fade.
+    assert.equal(h.tracking._trackedIconColor().alpha, STALE_ALPHA);
+    // Still moving well past 90 s.
+    h.run(pastReport(200));
+    assert.ok(
+      Cesium.Cartesian3.distance(h.drawn.at(-1), h.drawn.at(-11)) > 100,
+    );
+    // The coast ends 300 s past the report; the plane stops there.
+    h.run(pastReport(310));
+    const recorded = h.flightState._trailFixes.filter((e) => e.drawn).length;
+    h.run(pastReport(330));
+    assert.ok(
+      Cesium.Cartesian3.distance(h.drawn.at(-1), h.drawn.at(-50)) < 0.01,
+    );
+    assert.equal(
+      h.flightState._trailFixes.filter((e) => e.drawn).length,
+      recorded,
+      'the trail kept recording a stopped plane',
+    );
+    // 330 s of display past the report is 330 s + the display delay of wall time.
+    const minutes = Math.floor((330 + RENDER_DELAY_SEC) / 60);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: `SIGNAL LOST · ${minutes} min`,
+      dimmed: true,
+    });
+    assert.match(
+      h.tracking._buildTrackedLabel(h.flightState.records.data.get(ICAO), ICAO),
+      new RegExp(`SIGNAL LOST · ${minutes} min`),
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a plane held at its last report shows SIGNAL LOST in seconds under a minute', () => {
+  // Its last report is 100 m above a known ground height: it holds there.
+  const { fixes, info } = outOfCoverage();
+  const h = harness({
+    fixes,
+    info,
+    nowMs: pastReport(-5),
+    ground: () => 6_300,
+  });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.flightState.records.missingPolls.set(ICAO, 1);
+    h.run(pastReport(10));
+    assert.ok(10 + RENDER_DELAY_SEC < 60);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: `SIGNAL LOST · ${10 + RENDER_DELAY_SEC} s`,
+      dimmed: true,
+    });
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a report arriving 200 s into a coast: the plane eases onto it and the trail drops what was drawn after it', () => {
+  const { fixes, info } = outOfCoverage();
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(200));
+    // Heard again: the report is for 10 s before the display time, 2 km north
+    // of where the coast put it.
+    const at = T0 + 30 * SEC + 190 * SEC;
+    const guess = h.motion._deadReckon(
+      ICAO,
+      new Cesium.Cartesian3(),
+      Cesium.JulianDate.fromDate(new Date(at)),
+    );
+    const c = Cesium.Cartographic.fromCartesian(guess);
+    const report = fix(
+      at,
+      Cesium.Math.toDegrees(c.latitude) + 2_000 / 111_320,
+      Cesium.Math.toDegrees(c.longitude),
+      c.height,
+      140,
+      90,
+    );
+    fixes.push(report);
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    assert.deepEqual(
+      h.flightState._trailFixes.filter(
+        (e) => e.drawn && e.timeMs > report.epochMs,
+      ),
+      [],
+    );
+    const from = h.drawn.length;
+    h.run(pastReport(203));
+    const after = h.drawn.slice(from - 1);
+    for (let i = 1; i < after.length; i++)
+      assert.ok(
+        Cesium.Cartesian3.distance(after[i], after[i - 1]) < 400,
+        'the plane jumped instead of easing',
+      );
+    // Eased onto the path from the report: 13 s on from it at 140 m/s.
+    const off = Cesium.Cartesian3.distance(after.at(-1), report.position);
+    assert.ok(
+      Math.abs(off - 140 * 13) < 150,
+      `${off.toFixed(0)} m from the report`,
+    );
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a tracked plane that stops reporting: STALE from the first missed poll, SIGNAL LOST after the five-minute coast, kept at 9 min, removed at 10 min', () => {
+  const { fixes, info } = outOfCoverage();
+  const fixMs = fixes.at(-1).epochMs;
+  // Contact stops with the position: the plane left coverage.
+  info.lastContactEpochMs = fixMs;
+  info.newestFixMs = fixMs;
+  const h = harness({ fixes, info, nowMs: fixMs + 5 * SEC });
+  try {
+    h.tracking._startTrail(ICAO);
+    const records = h.flightState.records;
+    const polls = new Map();
+    let removedAtSec = null;
+    // A poll every 10 s that lists the plane without a position.
+    for (let afterSec = 10; afterSec <= 660; afterSec += 10) {
+      h.run(fixMs + afterSec * SEC, { frameMs: 500, readHead: false });
+      const result = records.absence(ICAO, {
+        complete: true,
+        retainIds: new Set([ICAO]),
+        likelyLanded: false,
+        tracked: true,
+      });
+      if (result === 'remove') {
+        removedAtSec = afterSec;
+        break;
+      }
+      polls.set(afterSec, {
+        cue: h.tracking._trackedStaleCue(ICAO),
+        drawn: Cesium.Cartesian3.clone(h.drawn.at(-1)),
+      });
+    }
+    // Wall time when the display reaches the end of the coast.
+    const coastEndSec = 300 + RENDER_DELAY_SEC;
+    for (const [afterSec, { cue }] of polls) {
+      if (afterSec <= coastEndSec) {
+        assert.deepEqual(cue, { text: 'STALE', dimmed: true }, `${afterSec} s`);
+      } else {
+        assert.deepEqual(
+          cue,
+          {
+            text: `SIGNAL LOST · ${Math.floor(afterSec / 60)} min`,
+            dimmed: true,
+          },
+          `${afterSec} s`,
+        );
+      }
+    }
+    const moved = (a, b) =>
+      Cesium.Cartesian3.distance(polls.get(a).drawn, polls.get(b).drawn);
+    // Moving through the coast, still at its last speed at four minutes.
+    assert.ok(Math.abs(moved(230, 240) - 140 * 10) < 20);
+    // Stopped once the coast has ended.
+    const after = Math.ceil(coastEndSec / 10) * 10;
+    assert.ok(moved(after, 540) < 0.01);
+    assert.equal(polls.get(300).cue.text, 'STALE');
+    assert.deepEqual(polls.get(540).cue, {
+      text: 'SIGNAL LOST · 9 min',
+      dimmed: true,
+    });
+    assert.equal(removedAtSec, 600);
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a repeated old fix counts as a missed update for the tracked plane', () => {
+  const { fixes, info } = outOfCoverage();
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(-2));
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: '',
+      dimmed: false,
+    });
+    // The feed re-sends the last position: its fix time did not advance.
+    h.flightState.records.fixRepeated.add(ICAO);
+    assert.deepEqual(h.tracking._trackedStaleCue(ICAO), {
+      text: 'STALE',
+      dimmed: true,
+    });
+    assert.equal(h.tracking._trackedIconColor().alpha, STALE_ALPHA);
+    assert.equal(h.flightState.records.missingPolls.has(ICAO), false);
+  } finally {
+    h.restore();
+  }
+});
+
+test('military: a report arriving after the plane has stopped: it eases in with no spur, and the trail keeps everything up to the stop', () => {
+  const { fixes, info } = outOfCoverage();
+  info.lastContactEpochMs = fixes.at(-1).epochMs;
+  const h = harness({ fixes, info, nowMs: pastReport(-5) });
+  try {
+    h.tracking._startTrail(ICAO);
+    h.run(pastReport(360));
+    const stop = Cesium.Cartesian3.clone(h.drawn.at(-1));
+    assert.ok(Cesium.Cartesian3.distance(stop, h.drawn.at(-100)) < 0.01);
+    const kept = h.flightState._trailFixes
+      .filter((e) => e.drawn)
+      .map((e) => Cesium.Cartesian3.clone(e.position));
+    // The thinned straight coast, recorded up to where the plane stopped.
+    assert.ok(kept.length >= 2);
+    assert.ok(Cesium.Cartesian3.distance(kept.at(-1), stop) < 300);
+    // Heard again, for 20 s before the display time: the plane flew on, a
+    // little north of its old heading.
+    const at = T0 + 30 * SEC + 340 * SEC;
+    const lonDeg =
+      -108.05 + (140 * 340) / (111_320 * Math.cos((43 * Math.PI) / 180));
+    const report = fix(at, 43 + 1_000 / 111_320, lonDeg, 6_400, 140, 90);
+    fixes.push(report);
+    h.tracking._appendTrailFix(
+      Cesium.Cartesian3.clone(report.position),
+      report.epochMs,
+    );
+    // Nothing was recorded after the stop, so nothing is dropped.
+    const after = h.flightState._trailFixes
+      .filter((e) => e.drawn)
+      .map((e) => e.position);
+    assert.equal(after.length, kept.length);
+    kept.forEach((p, i) =>
+      assert.ok(Cesium.Cartesian3.distance(p, after[i]) < 0.01),
+    );
+    const from = h.drawn.length;
+    h.run(pastReport(365));
+    const eased = h.drawn.slice(from - 1);
+    const jump = Cesium.Cartesian3.distance(eased[0], eased.at(-1));
+    for (let i = 1; i < eased.length; i++)
+      assert.ok(
+        Cesium.Cartesian3.distance(eased[i], eased[i - 1]) < jump / 2,
+        'the plane jumped instead of easing',
+      );
+    // On the path from the report: 25 s on from it at 140 m/s.
+    const off = Cesium.Cartesian3.distance(eased.at(-1), report.position);
+    assert.ok(Math.abs(off - 140 * 25) < 150, `${off.toFixed(0)} m`);
+    assertNoSpur(h.bodyPoints(), 'trail');
+    assertNoSpur(h.head(), 'head');
+    assert.ok(indexOfPoint(h.bodyPoints(), report.position) >= 0);
+  } finally {
+    h.restore();
+  }
+});