Let a host application place live contacts

createApplicationCatalog takes an optional placement provider. Its layer(kind) hands Flights, Military Flights, AIS Vessels, Transit and Rail a place(id, wallMs), and optionally observe(id, fix) and forget(id). While live, each layer tells it every fix it accepts and draws a contact where it places it: position, height (the layer's own when none is given), course and speed. A null answer, a provider that throws, a kind it leaves out, or no provider at all keeps the layer's own motion unchanged, and following the world clock the layers never ask it. A transit vehicle or train it holds still lets the render loop idle and is asked again each second; vessels ask for frames only while it places one, and stop when the layer turns off. Tests show the layers draw and reconcile exactly as before without a provider and with one that places nothing, draw where a provider places them, and idle while it holds them still.
2026-11-15 07:42:23 +00:00 · 2026-11-14 15:07:25 -08:00
1 parent e53cfef1d1
commit 6f830a2e70
@@ -6,6 +6,7 @@ import { createWeatherLayer } from '../layers/weather/index.js';
 import { createApplicationCyclones } from './layers/cyclones.js';
 import { createWindLayer } from '../layers/wind/index.js';
 import { createLayerCatalog } from './catalog.js';
+import { layerPlacement } from '../data/placementProvider.js';
 import { LAYER_STATE_REGISTRY } from '../data/layerState.js';
 import { createMilitaryRegistry } from '../layers/aircraft/classification.js';
 import { createApplicationFlights } from './layers/flights.js';
@@ -63,6 +64,9 @@ export const APPLICATION_LAYER_METADATA = Object.freeze([
 /** Construct the current catalog without choosing any source provider.
 * Scene engines remain page-owned; layers and classification have this app's lifetime.
 * The manager owns layer destruction, while abort releases classification even if startup fails.
+ * `placement`, a host-supplied placement provider (../data/placementProvider.js),
+ * places live aircraft, vessels, transit and trains; without one each layer
+ * uses its own motion.
 */
 export function createApplicationCatalog({
  surface,
@@ -74,6 +78,7 @@ export function createApplicationCatalog({
  resolveAsset,
  nepalBoundaryResolver,
  streetLevelProviders,
+  placement = null,
 }) {
  if (!signal?.addEventListener)
    throw new TypeError('An application lifetime signal is required');
@@ -97,17 +102,20 @@ export function createApplicationCatalog({
    const flights = createApplicationFlights({
      surface,
      source: sources.flights,
+      placement: layerPlacement(placement, 'aircraft'),
      militaryRegistry,
      resolveAsset,
    });
    const military = createApplicationMilitary({
      surface,
      source: sources.military,
+      placement: layerPlacement(placement, 'military'),
      militaryRegistry,
      resolveAsset,
    });
    const vessels = createApplicationVessels({
      source: sources.vessels,
+      placement: layerPlacement(placement, 'vessels'),
      options: vesselOptions,
      displayParams: () => flights.getParams(),
      ...(resolveAsset ? { resolveAsset } : {}),
@@ -172,6 +180,7 @@ export function createApplicationCatalog({
        createApplicationTransit({
          surface,
          source: sources.transit,
+          placement: layerPlacement(placement, 'transit'),
          displayParams: () => flights.getParams(),
          ...(resolveAsset ? { resolveAsset } : {}),
        }),
@@ -210,6 +219,7 @@ export function createApplicationCatalog({
        createApplicationRail({
          surface,
          source: sources.rail,
+          placement: layerPlacement(placement, 'rail'),
          displayParams: () => flights.getParams(),
          ...(resolveAsset ? { resolveAsset } : {}),
        }),
@@ -16,9 +16,11 @@ import { fleetModelServices } from './fleetModels.js';
 * Construct one layer using the application scene owners and a supplied
 * source. `displayParams` reads the Flights layer's DISPLAY-rail 3D
 * preference, under which nearby vessels are drawn as fleet models.
+ * `placement`: a host-supplied placement for its live vessels, or null.
 */
 export function createApplicationVessels({
  source,
+  placement = null,
  options = {},
  displayParams = null,
  resolveAsset,
@@ -38,6 +40,7 @@ export function createApplicationVessels({
      worldFocus,
      render,
      readouts,
+      placement,
      fleet: fleetModelServices({
        display: displayParams,
        ...(resolveAsset ? { resolveAsset } : {}),
@@ -12,11 +12,13 @@ import * as context from '../../data/contextStore.js';
 import * as render from '../../renderGovernor.js';
 import * as recession from '../../data/aircraftRecession.js';

-/** Construct one layer using the application scene owners and a supplied source. */
+/** Construct one layer using the application scene owners and a supplied source.
+ * `placement`: a host-supplied placement for its live aircraft, or null. */
 export function createApplicationFlights({
  surface,
  source,
  militaryRegistry,
+  placement = null,
  resolveAsset = (url) =>
    `${import.meta.env?.BASE_URL || '/'}${url.replace(/^\//, '')}`,
 }) {
@@ -41,6 +43,7 @@ export function createApplicationFlights({
      render,
      groundSnap,
      recession,
+      placement,
    },
  });
 }
@@ -12,11 +12,13 @@ import * as context from '../../data/contextStore.js';
 import * as render from '../../renderGovernor.js';
 import * as recession from '../../data/aircraftRecession.js';

-/** Construct one layer using the application scene owners and a supplied source. */
+/** Construct one layer using the application scene owners and a supplied source.
+ * `placement`: a host-supplied placement for its live aircraft, or null. */
 export function createApplicationMilitary({
  surface,
  source,
  militaryRegistry,
+  placement = null,
  resolveAsset = (url) =>
    `${import.meta.env?.BASE_URL || '/'}${url.replace(/^\//, '')}`,
 }) {
@@ -41,6 +43,7 @@ export function createApplicationMilitary({
      render,
      groundSnap,
      recession,
+      placement,
    },
  });
 }
@@ -16,13 +16,15 @@ import {
 * ground floor, the same floor Transit and the aircraft stand on.
 * `displayParams` reads the Flights layer's DISPLAY-rail 3D preference, under
 * which nearby trains are drawn as fleet models.
- * @param {{surface: object, source?: object,
+ * `placement` is a host-supplied placement for its live trains, or null.
+ * @param {{surface: object, source?: object, placement?: object|null,
 *   displayParams?: () => object|null, resolveAsset?: (url: string) => string}} options
 * @returns {object}
 */
 export function createApplicationRail({
  surface,
  source,
+  placement = null,
  displayParams = null,
  resolveAsset,
 }) {
@@ -36,6 +38,7 @@ export function createApplicationRail({
      sprites,
      picking,
      overlays,
+      placement,
      credits: { registerDynamicCredit, railFeedCredit },
      ground: {
        GROUND_FLOOR_LIFT_M: groundFloor.GROUND_FLOOR_LIFT_M,
@@ -23,13 +23,15 @@ import {
 * `displayParams` reads the Flights layer's DISPLAY-rail 3D preference, under
 * which nearby vehicles are drawn as fleet models.
 *
- * @param {{surface: object, source?: object,
+ * `placement` is a host-supplied placement for its live vehicles, or null.
+ * @param {{surface: object, source?: object, placement?: object|null,
 *   displayParams?: () => object|null, resolveAsset?: (url: string) => string}} options
 * @returns {object}
 */
 export function createApplicationTransit({
  surface,
  source,
+  placement = null,
  displayParams = null,
  resolveAsset,
 }) {
@@ -43,6 +45,7 @@ export function createApplicationTransit({
      sprites,
      picking,
      overlays,
+      placement,
      credits: { registerDynamicCredit, transitFeedCredit },
      ground: {
        GROUND_FLOOR_LIFT_M: groundFloor.GROUND_FLOOR_LIFT_M,
@@ -0,0 +1,131 @@
+/**
+ * A host-supplied placement provider: an application embedding the catalog
+ * may decide where its moving contacts are drawn while live. Each layer that
+ * takes one tells it every fix it accepts (`observe`) and asks it, each time
+ * it draws a contact, where to draw it now (`place`). A layer given none, or
+ * asking about a contact the provider does not place (`null`), draws the
+ * contact with its own motion, exactly as without a provider. A provider
+ * that throws is treated as having no answer for that call.
+ *
+ * Layers consult a provider only while live: following the world clock, the
+ * layer's own motion places every contact.
+ *
+ * @typedef {'aircraft'|'military'|'vessels'|'transit'|'rail'} PlacementKind
+ *
+ * @typedef {object} PlacementFix
+ * @property {number} timeMs The fix's own time, Unix ms.
+ * @property {number} lat
+ * @property {number} lon
+ * @property {number} [altitudeM] The height the layer draws the fix at,
+ *   metres above the ellipsoid; NaN when the layer keeps its own height.
+ * @property {number} [speedMps] Reported ground speed, NaN for none.
+ * @property {number} [courseDeg] Reported course over ground, NaN for none.
+ * @property {number} [headingDeg] Reported heading, NaN for none.
+ * @property {boolean} [onGround] Reported on the ground (aircraft).
+ * @property {string} [feedId] The feed the contact belongs to (transit, rail).
+ *
+ * @typedef {object} Placement
+ * @property {number} lat
+ * @property {number} lon
+ * @property {number} altitudeM Metres above the ellipsoid, as the observed
+ *   fixes' `altitudeM`; NaN keeps the layer's own height.
+ * @property {number} courseDeg Direction of travel, NaN for none.
+ * @property {number} speedMps NaN for none.
+ * @property {boolean} extrapolating Past the contact's newest fix.
+ *
+ * @typedef {object} LayerPlacement What a layer is handed: never throws.
+ * @property {(id: string, fix: PlacementFix) => void} observe
+ * @property {(id: string, wallMs: number) => Placement|null} place The
+ *   answer is one object reused by every call: read it before the next.
+ * @property {(id: string) => void} forget The contact left the layer.
+ *
+ * @typedef {object} PlacementProvider
+ * @property {(kind: PlacementKind) => object|null|undefined} layer The
+ *   placement for one kind of contact: an object with `place(id, wallMs)`
+ *   and optionally `observe(id, fix)` and `forget(id)`, or null to leave that
+ *   kind to its layer's own motion.
+ */
+
+/** The kinds of contact a provider may place. */
+export const PLACEMENT_KINDS = Object.freeze([
+  'aircraft',
+  'military',
+  'vessels',
+  'transit',
+  'rail',
+]);
+
+const finiteOr = (value, fallback) =>
+  typeof value === 'number' && Number.isFinite(value) ? value : fallback;
+
+/**
+ * One kind's placement from a provider, guarded so a layer can call it every
+ * frame: null when there is no provider or it does not place `kind`.
+ * @param {PlacementProvider|null|undefined} provider
+ * @param {PlacementKind} kind
+ * @returns {LayerPlacement|null}
+ */
+export function layerPlacement(provider, kind) {
+  if (!PLACEMENT_KINDS.includes(kind))
+    throw new TypeError(`Unknown placement kind: ${kind}`);
+  if (provider == null) return null;
+  if (typeof provider.layer !== 'function')
+    throw new TypeError('A placement provider needs a layer(kind) function');
+  const placement = provider.layer(kind);
+  if (placement == null) return null;
+  if (typeof placement.place !== 'function')
+    throw new TypeError(`The ${kind} placement needs a place(id, wallMs)`);
+  const observe =
+    typeof placement.observe === 'function' ? placement.observe : null;
+  const forget =
+    typeof placement.forget === 'function' ? placement.forget : null;
+  // Asked for every drawn contact every frame: one answer, reused.
+  const answer = {
+    lat: NaN,
+    lon: NaN,
+    altitudeM: NaN,
+    courseDeg: NaN,
+    speedMps: NaN,
+    extrapolating: false,
+  };
+  return Object.freeze({
+    observe(id, fix) {
+      if (!observe) return;
+      try {
+        observe.call(placement, id, fix);
+      } catch {
+        // A provider that fails to take a fix only answers less.
+      }
+    },
+    place(id, wallMs) {
+      let placed;
+      try {
+        placed = placement.place(id, wallMs);
+      } catch {
+        return null;
+      }
+      if (
+        !placed ||
+        !Number.isFinite(placed.lat) ||
+        !Number.isFinite(placed.lon) ||
+        Math.abs(placed.lat) > 90
+      )
+        return null;
+      answer.lat = placed.lat;
+      answer.lon = placed.lon;
+      answer.altitudeM = finiteOr(placed.altitudeM, NaN);
+      answer.courseDeg = finiteOr(placed.courseDeg, NaN);
+      answer.speedMps = finiteOr(placed.speedMps, NaN);
+      answer.extrapolating = placed.extrapolating === true;
+      return answer;
+    },
+    forget(id) {
+      if (!forget) return;
+      try {
+        forget.call(placement, id);
+      } catch {
+        // Nothing to undo.
+      }
+    },
+  });
+}
@@ -0,0 +1,108 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { PLACEMENT_KINDS, layerPlacement } from './placementProvider.js';
+
+test('no provider, or a provider that leaves a kind alone, gives no placement', () => {
+  for (const kind of PLACEMENT_KINDS) {
+    assert.equal(layerPlacement(null, kind), null);
+    assert.equal(layerPlacement(undefined, kind), null);
+    assert.equal(layerPlacement({ layer: () => null }, kind), null);
+  }
+  assert.throws(() => layerPlacement(null, 'satellites'), TypeError);
+  assert.throws(() => layerPlacement({}, 'aircraft'), TypeError);
+  assert.throws(
+    () => layerPlacement({ layer: () => ({ observe() {} }) }, 'aircraft'),
+    TypeError,
+  );
+});
+
+test('a placement is asked by id and wall time, and its answer is checked', () => {
+  const asked = [];
+  const seen = [];
+  const forgotten = [];
+  const answers = new Map([
+    [
+      'a',
+      { lat: 51.5, lon: -0.1, altitudeM: 900, courseDeg: 270, speedMps: 80 },
+    ],
+    ['partial', { lat: 10, lon: 20, altitudeM: 'high', extrapolating: true }],
+    ['off-globe', { lat: 91, lon: 0 }],
+    ['nan', { lat: NaN, lon: 0 }],
+  ]);
+  const provider = {
+    layer: (kind) =>
+      kind === 'aircraft'
+        ? {
+            observe: (id, fix) => seen.push([id, fix.timeMs]),
+            place(id, wallMs) {
+              asked.push([id, wallMs]);
+              return answers.get(id) ?? null;
+            },
+            forget: (id) => forgotten.push(id),
+          }
+        : null,
+  };
+  assert.equal(layerPlacement(provider, 'vessels'), null);
+  const placement = layerPlacement(provider, 'aircraft');
+  placement.observe('a', { timeMs: 5, lat: 0, lon: 0 });
+  assert.deepEqual(seen, [['a', 5]]);
+  const placed = placement.place('a', 1000);
+  assert.deepEqual(
+    { ...placed },
+    {
+      lat: 51.5,
+      lon: -0.1,
+      altitudeM: 900,
+      courseDeg: 270,
+      speedMps: 80,
+      extrapolating: false,
+    },
+  );
+  const partial = placement.place('partial', 2000);
+  assert.equal(partial, placed, 'one answer object, reused');
+  assert.ok(Number.isNaN(partial.altitudeM));
+  assert.ok(Number.isNaN(partial.courseDeg));
+  assert.ok(Number.isNaN(partial.speedMps));
+  assert.equal(partial.extrapolating, true);
+  assert.equal(placement.place('off-globe', 3000), null);
+  assert.equal(placement.place('nan', 3000), null);
+  assert.equal(placement.place('unknown', 3000), null);
+  assert.deepEqual(
+    asked.map(([id]) => id),
+    ['a', 'partial', 'off-globe', 'nan', 'unknown'],
+  );
+  placement.forget('a');
+  assert.deepEqual(forgotten, ['a']);
+});
+
+test('a provider that throws has no answer, and observe and forget are optional', () => {
+  const placement = layerPlacement(
+    {
+      layer: () => ({
+        place() {
+          throw new Error('not ready');
+        },
+      }),
+    },
+    'transit',
+  );
+  assert.equal(placement.place('x', 0), null);
+  assert.doesNotThrow(() => placement.observe('x', { timeMs: 0 }));
+  assert.doesNotThrow(() => placement.forget('x'));
+  const failing = layerPlacement(
+    {
+      layer: () => ({
+        place: () => null,
+        observe() {
+          throw new Error('full');
+        },
+        forget() {
+          throw new Error('gone');
+        },
+      }),
+    },
+    'rail',
+  );
+  assert.doesNotThrow(() => failing.observe('x', { timeMs: 0 }));
+  assert.doesNotThrow(() => failing.forget('x'));
+});
@@ -1,14 +1,20 @@
+import { forgetPlacedAircraft } from './placed.js';
+
 /**
 * What the layer drops when the world clock moves it to another time (its
 * source declares a timeline; see src/time/snapshotTimeline.js).
 */
-export function createClockFollow({ flightState, parts }) {
+export function createClockFollow({ flightState, services, parts }) {
  /**
   * Forget how every contact moved, before the snapshot at another time takes
   * the place of every contact: their fixes, courses, floors and the tracked
   * trail belong to the time the clock left.
   */
  function resetMotion() {
+    forgetPlacedAircraft(
+      services?.placement,
+      flightState._positionHistory.keys(),
+    );
    flightState.records.reset();
    flightState._positionHistory.clear();
    flightState._displayCourse.clear();
@@ -217,6 +217,25 @@ test('a jump forgets how contacts moved; clearing drops every contact', () => {
  assert.ok(calls.includes('model:abc123'));
 });

+test('leaving the live edge forgets every contact a host placement was told of', () => {
+  const forgotten = [];
+  const flightState = {
+    records: records(),
+    _positionHistory: new Map([
+      ['abc123', []],
+      ['def456', []],
+    ]),
+    _displayCourse: new Map(),
+    _groundSnap: { clear() {} },
+    _displayFloorState: new Map(),
+  };
+  const parts = { tracking: { _clearTrail() {} } };
+  const services = { placement: { forget: (id) => forgotten.push(id) } };
+  createClockFollow({ flightState, services, parts }).resetMotion();
+  assert.deepEqual(forgotten, ['abc123', 'def456']);
+  assert.equal(flightState._positionHistory.size, 0);
+});
+
 test('a snapshot at a time the source drops for a newer one sets no error', async () => {
  const probe = ingestion({
    async getSnapshot() {
@@ -35,6 +35,7 @@ export function createCivilFlightLayer({
  parts.clockFollow = createClockFollow(context);
  const applySnapshot = createFlightSnapshotRenderer({
    flightState,
+    placement: services.placement ?? null,
    records: flightState.records,
    militaryRegistry: services.militaryRegistry,
    groundFloor: services.groundFloor,
@@ -1,5 +1,6 @@
 import * as Cesium from 'cesium';
 import { loadFleetModelSet } from '../../data/fleetModels.js';
+import { forgetPlacedAircraft } from './placed.js';

 export function createLifecycle({
  flightState,
@@ -72,6 +73,10 @@ export function createLifecycle({
      flightState._cullPositions.clear();
      flightState._detectionObjects = new Map();
      flightState.records.data = new Map();
+      forgetPlacedAircraft(
+        services.placement,
+        flightState._positionHistory.keys(),
+      );
      flightState._positionHistory = new Map();
      flightState._displayCourse.clear();
      flightState._groundSnap.clear();
@@ -291,6 +296,10 @@ export function createLifecycle({
      flightState._cullPositions.clear();
      flightState._detectionObjects.clear();
      flightState.records.data.clear();
+      forgetPlacedAircraft(
+        services.placement,
+        flightState._positionHistory.keys(),
+      );
      flightState._positionHistory.clear();
      flightState._displayCourse.clear();
      flightState._groundSnap.clear();
@@ -16,6 +16,7 @@ import {
  COAST_MAX_SPEED_MPS,
 } from '../../data/motionModel.js';
 import { CLASS_SCALE_2D } from '../../data/aircraftClass.js';
+import { placeLiveAircraft } from './placed.js';
 import {
  modelVisualAnchor,
  modelAnchorWorld,
@@ -173,6 +174,19 @@ export function createMotion({
    }

    const out = result || new Cesium.Cartesian3();
+    // A host-supplied placement decides where a live contact is drawn; with
+    // none, or no answer for this contact, the layer's own motion below does.
+    if (
+      placeLiveAircraft(
+        flightState,
+        services.placement,
+        icao24,
+        info,
+        history,
+        out,
+      )
+    )
+      return out;
    // Render one poll interval behind real time so we interpolate between
    // two KNOWN fixes whenever possible (see RENDER_DELAY_SEC rationale).
    const renderTime =
@@ -3,8 +3,11 @@ import assert from 'node:assert/strict';
 import * as Cesium from 'cesium';
 import { createMotion } from './motion.js';
 import { arcOffsetEnu } from '../../data/motionModel.js';
+import { createMotion as createMilitaryMotion } from '../military/motion.js';
+import { layerPlacement } from '../../data/placementProvider.js';
+import { observeLiveAircraft } from './placed.js';

-function fixture() {
+function fixture(placement = null) {
  const flightState = {
    records: { data: new Map() },
    _positionHistory: new Map(),
@@ -16,7 +19,7 @@ function fixture() {
  };
  const motion = createMotion({
    flightState,
-    services: { groundFloor: {}, camera: {}, focus: {} },
+    services: { groundFloor: {}, camera: {}, focus: {}, placement },
    parts: {},
  });
  return { flightState, motion };
@@ -184,3 +187,189 @@ test('tracked callers still sample a fresh delayed time without a fleet epoch',
  );
  assert.equal(reads, 2);
 });
+
+/** Two fixes a minute apart, read across the minute from a clock at `start`. */
+function placedScenario(create, placement, record) {
+  const flightState = {
+    records: { data: new Map() },
+    _positionHistory: new Map(),
+    _focusEvidenceIds: new Set(),
+    _scratchArc: {},
+    _scratchOffset: new Cesium.Cartesian3(),
+    _scratchRenderTime: new Cesium.JulianDate(),
+    now: () => 0,
+  };
+  const motion = create({
+    flightState,
+    services: { groundFloor: {}, camera: {}, focus: {}, placement },
+    parts: {},
+  });
+  const start = Date.parse('2026-10-24T12:00:00Z');
+  const at = (seconds) =>
+    Cesium.JulianDate.fromDate(new Date(start + seconds * 1000));
+  flightState.records.data.set('abc123', record);
+  flightState._positionHistory.set('abc123', [
+    {
+      time: at(0),
+      position: Cesium.Cartesian3.fromDegrees(-97, 30, 1000),
+      track: 90,
+      velocity: 100,
+    },
+    {
+      time: at(60),
+      position: Cesium.Cartesian3.fromDegrees(-96.99, 30, 1000),
+      track: 90,
+      velocity: 100,
+    },
+  ]);
+  const drawn = [];
+  for (let seconds = 0; seconds <= 150; seconds += 7.5) {
+    flightState.now = () => start + seconds * 1000;
+    const position = motion._deadReckon('abc123', new Cesium.Cartesian3());
+    drawn.push({
+      position,
+      course: flightState._drCourseDeg,
+      speed: flightState._drSpeedMps,
+      hold: flightState._drCourseHold,
+    });
+  }
+  return { flightState, motion, drawn, start };
+}
+
+const declining = (asked) =>
+  layerPlacement(
+    {
+      layer: () => ({
+        place: (id, wallMs) => {
+          asked.push([id, wallMs]);
+          return null;
+        },
+      }),
+    },
+    'aircraft',
+  );
+
+// Each kind's record carries only its own kinematics fields.
+for (const [name, create, record] of [
+  [
+    'civil',
+    createMotion,
+    { velocity: 100, true_track: 90, renderAltitudeM: 1234 },
+  ],
+  [
+    'military',
+    createMilitaryMotion,
+    { speedMps: 100, track: 90, renderAltitudeM: 1234 },
+  ],
+]) {
+  test(`${name} aircraft move as before without a placement or with one that places nothing`, () => {
+    const own = placedScenario(create, null, record).drawn;
+    const asked = [];
+    const declined = placedScenario(create, declining(asked), record).drawn;
+    assert.equal(asked.length, own.length);
+    assert.equal(declined.length, own.length);
+    for (let i = 0; i < own.length; i++) {
+      assert.ok(
+        Cesium.Cartesian3.equals(declined[i].position, own[i].position),
+      );
+      assert.equal(declined[i].course, own[i].course);
+      assert.equal(declined[i].speed, own[i].speed);
+      assert.equal(declined[i].hold, own[i].hold);
+    }
+  });
+
+  test(`${name} aircraft are drawn where a placement places them, live only`, () => {
+    const asked = [];
+    const placement = layerPlacement(
+      {
+        layer: () => ({
+          place(id, wallMs) {
+            asked.push(wallMs);
+            return wallMs % 2 === 0
+              ? {
+                  lat: 30.5,
+                  lon: -96.5,
+                  altitudeM: 2000,
+                  courseDeg: 45,
+                  speedMps: 0.5,
+                  extrapolating: true,
+                }
+              : { lat: 30.6, lon: -96.4 };
+          },
+        }),
+      },
+      'aircraft',
+    );
+    const { flightState, motion, start } = placedScenario(
+      create,
+      placement,
+      record,
+    );
+    flightState.now = () => start + 2000;
+    const out = motion._deadReckon('abc123', new Cesium.Cartesian3());
+    close(out, Cesium.Cartesian3.fromDegrees(-96.5, 30.5, 2000));
+    assert.equal(flightState._drCourseDeg, 45);
+    assert.equal(flightState._drSpeedMps, 0.5);
+    assert.equal(flightState._drCourseHold, true);
+    // No height, course or speed placed: the record's drawn height and kinematics.
+    flightState.now = () => start + 2001;
+    close(
+      motion._deadReckon('abc123', new Cesium.Cartesian3()),
+      Cesium.Cartesian3.fromDegrees(-96.4, 30.6, 1234),
+    );
+    assert.equal(flightState._drCourseDeg, 90);
+    assert.equal(flightState._drSpeedMps, 100);
+    // Following the world clock the layer's own motion places it, unasked.
+    const before = asked.length;
+    flightState.timeline = { active: () => true };
+    const own = motion._deadReckon('abc123', new Cesium.Cartesian3());
+    assert.equal(asked.length, before);
+    assert.ok(Cesium.Cartesian3.distance(own, out) > 1000);
+  });
+}
+
+test('an aircraft fix is told to the placement live, with its drawn height', () => {
+  const told = [];
+  const placement = layerPlacement(
+    {
+      layer: () => ({
+        place: () => null,
+        observe: (id, fix) => told.push([id, fix]),
+      }),
+    },
+    'aircraft',
+  );
+  const state = { timeline: { active: () => false } };
+  const meta = { rawLat: 1, rawLon: 2, renderAltitudeM: 300, onGround: false };
+  observeLiveAircraft(state, placement, 'abc123', 5000, meta, 80, 270);
+  observeLiveAircraft(
+    state,
+    placement,
+    'abc123',
+    6000,
+    { ...meta, renderAltitudeInvented: true, onGround: true },
+    null,
+    undefined,
+  );
+  observeLiveAircraft(state, null, 'abc123', 7000, meta, 80, 270);
+  state.timeline.active = () => true;
+  observeLiveAircraft(state, placement, 'abc123', 8000, meta, 80, 270);
+  assert.equal(told.length, 2);
+  assert.deepEqual(told[0], [
+    'abc123',
+    {
+      timeMs: 5000,
+      lat: 1,
+      lon: 2,
+      altitudeM: 300,
+      speedMps: 80,
+      courseDeg: 270,
+      headingDeg: NaN,
+      onGround: false,
+    },
+  ]);
+  assert.ok(Number.isNaN(told[1][1].altitudeM));
+  assert.ok(Number.isNaN(told[1][1].speedMps));
+  assert.ok(Number.isNaN(told[1][1].courseDeg));
+  assert.equal(told[1][1].onGround, true);
+});
@@ -0,0 +1,112 @@
+import * as Cesium from 'cesium';
+import { COURSE_HOLD_SPEED_MPS } from '../../data/motionModel.js';
+import { followsClock } from './policy.js';
+
+/** Scratch for a placed contact's held height. */
+const _placedCarto = new Cesium.Cartographic();
+
+/**
+ * Where the host-supplied placement (`services.placement`,
+ * ../../data/placementProvider.js) draws a live aircraft now, written into
+ * `out` with the course scratches `_deadReckon` sets on `flightState`; false
+ * when there is no placement, the layer follows the world clock, or it has
+ * no answer for this aircraft, and the layer's own motion places it. A
+ * placement without a height keeps the aircraft's newest drawn height.
+ * @param {object} flightState The civil or military flight state.
+ * @param {object|null|undefined} placement
+ * @param {string} icao24
+ * @param {object|undefined} info The aircraft's record.
+ * @param {{position: Cesium.Cartesian3}[]} history Its fixes, newest last.
+ * @param {Cesium.Cartesian3} out
+ * @returns {boolean}
+ */
+export function placeLiveAircraft(
+  flightState,
+  placement,
+  icao24,
+  info,
+  history,
+  out,
+) {
+  if (!placement || followsClock(flightState)) return false;
+  const placed = placement.place(icao24, flightState.now());
+  if (!placed) return false;
+  let heightM = placed.altitudeM;
+  if (!Number.isFinite(heightM)) heightM = info?.renderAltitudeM;
+  if (!Number.isFinite(heightM))
+    heightM = Cesium.Cartographic.fromCartesian(
+      history[history.length - 1].position,
+      Cesium.Ellipsoid.WGS84,
+      _placedCarto,
+    ).height;
+  const speed = Number.isFinite(placed.speedMps)
+    ? placed.speedMps
+    : // Civil records carry velocity and true_track, military speedMps and track.
+      Number.isFinite(info?.velocity)
+      ? info.velocity
+      : info?.speedMps || 0;
+  flightState._drCourseDeg = Number.isFinite(placed.courseDeg)
+    ? placed.courseDeg
+    : Number.isFinite(info?.true_track)
+      ? info.true_track
+      : info?.track || 0;
+  flightState._drSpeedMps = speed;
+  flightState._drCourseHold = speed < COURSE_HOLD_SPEED_MPS;
+  flightState._drExtrapolating = placed.extrapolating;
+  Cesium.Cartesian3.fromDegrees(
+    placed.lon,
+    placed.lat,
+    heightM,
+    Cesium.Ellipsoid.WGS84,
+    out,
+  );
+  return true;
+}
+
+/**
+ * Tell the host-supplied placement, if any, that the aircraft `ids` left the
+ * layer (removed, the layer off, or the world clock moved it to another time).
+ * @param {object|null|undefined} placement
+ * @param {Iterable<string>} ids
+ */
+export function forgetPlacedAircraft(placement, ids) {
+  if (!placement) return;
+  for (const id of ids) placement.forget(id);
+}
+
+/**
+ * Tell the host-supplied placement an aircraft's fix as the layer took it
+ * (nothing while the layer follows the world clock): its own time, its
+ * position, the height the layer draws it at (NaN when that height is
+ * invented), and its reported speed, course and ground state.
+ * @param {object} flightState
+ * @param {object|null|undefined} placement
+ * @param {string} icao24
+ * @param {number} fixEpochMs
+ * @param {{rawLat: number, rawLon: number, renderAltitudeM: number,
+ *   renderAltitudeInvented?: boolean, onGround?: boolean}} meta
+ * @param {number|null|undefined} speedMps
+ * @param {number|null|undefined} courseDeg
+ */
+export function observeLiveAircraft(
+  flightState,
+  placement,
+  icao24,
+  fixEpochMs,
+  meta,
+  speedMps,
+  courseDeg,
+) {
+  if (!placement || followsClock(flightState)) return;
+  placement.observe(icao24, {
+    timeMs: fixEpochMs,
+    lat: meta.rawLat,
+    lon: meta.rawLon,
+    altitudeM:
+      meta.renderAltitudeInvented === true ? NaN : meta.renderAltitudeM,
+    speedMps: Number.isFinite(speedMps) ? speedMps : NaN,
+    courseDeg: Number.isFinite(courseDeg) ? courseDeg : NaN,
+    headingDeg: NaN,
+    onGround: meta.onGround === true,
+  });
+}
@@ -9,10 +9,12 @@ import {
 import { aircraftIcon } from '../../data/aircraftIcons.js';
 import { POSITION_HISTORY_LIMIT } from './recordPolicy.js';
 import { followsClock } from './policy.js';
+import { observeLiveAircraft } from './placed.js';

 /** Apply reconciled aircraft records to Cesium resources and follow state. */
 export function createFlightSnapshotRenderer({
  flightState,
+  placement = null,
  records,
  militaryRegistry,
  groundFloor,
@@ -65,6 +67,7 @@ export function createFlightSnapshotRenderer({
          flightState._billboards.delete(icao24);
          rendering._releaseModel(icao24); // military-suppression: drop any 3D model too
          records.forget(icao24);
+          placement?.forget(icao24);
          flightState._cullPositions.delete(icao24);
          flightState._positionHistory.delete(icao24);
          flightState._displayCourse.delete(icao24);
@@ -98,6 +101,17 @@ export function createFlightSnapshotRenderer({
      );
      const isTracked = icao24 === flightState._trackedIcao;

+      // A host-supplied placement, if any, is told every fix as taken.
+      observeLiveAircraft(
+        flightState,
+        placement,
+        icao24,
+        fixEpochMs,
+        meta,
+        meta.velocity,
+        meta.true_track,
+      );
+
      // Append to position history stamped with the FEED's fix epoch
      // (time_position), not client receipt time — OpenSky positions arrive
      // 5-15s stale and receipt-time stamping is what caused the
@@ -311,6 +325,7 @@ export function createFlightSnapshotRenderer({
      flightState._billboards.delete(icao24);
      rendering._releaseModel(icao24); // aged-out aircraft: drop its 3D model (no orphan / cap leak)
      records.forget(icao24);
+      placement?.forget(icao24);
      flightState._cullPositions.delete(icao24);
      flightState._positionHistory.delete(icao24);
      flightState._displayCourse.delete(icao24);
@@ -1289,6 +1289,7 @@ export function createTracking({
        flightState._billboards.delete(icao24);
        parts.rendering._releaseModel(icao24); // military-suppression: drop any 3D model too
        flightState.records.data.delete(icao24);
+        services.placement?.forget(icao24);
        flightState._cullPositions.delete(icao24);
        flightState._positionHistory.delete(icao24);
        flightState._displayCourse.delete(icao24);
@@ -1,14 +1,20 @@
+import { forgetPlacedAircraft } from '../flights/placed.js';
+
 /**
 * What the layer drops when the world clock moves it to another time (its
 * source declares a timeline; see src/time/snapshotTimeline.js).
 */
-export function createClockFollow({ flightState, parts }) {
+export function createClockFollow({ flightState, services, parts }) {
  /**
   * Forget how every contact moved, before the snapshot at another time takes
   * the place of every contact: their fixes, courses and the tracked trail
   * belong to the time the clock left.
   */
  function resetMotion() {
+    forgetPlacedAircraft(
+      services?.placement,
+      flightState._positionHistory.keys(),
+    );
    flightState.records.reset();
    flightState._positionHistory.clear();
    flightState._displayCourse.clear();
@@ -31,6 +31,7 @@ export function createMilitaryFlightLayer({
  parts.clockFollow = createClockFollow(context);
  const applySnapshot = createMilitarySnapshotRenderer({
    flightState,
+    placement: services.placement ?? null,
    records: flightState.records,
    groundFloor: services.groundFloor,
    meshFloor: services.meshFloor,
@@ -1,5 +1,6 @@
 import * as Cesium from 'cesium';
 import { loadFleetModelSet } from '../../data/fleetModels.js';
+import { forgetPlacedAircraft } from '../flights/placed.js';

 export function createLifecycle({
  flightState,
@@ -66,6 +67,10 @@ export function createLifecycle({
      flightState._cullPositions.clear();
      flightState._detectionObjects = new Map();
      flightState.records.data = new Map();
+      forgetPlacedAircraft(
+        services.placement,
+        flightState._positionHistory.keys(),
+      );
      flightState._positionHistory = new Map();
      flightState._displayCourse.clear();
      flightState._groundSnap.clear();
@@ -264,6 +269,10 @@ export function createLifecycle({
      flightState._cullPositions.clear();
      flightState._detectionObjects.clear();
      flightState.records.data.clear();
+      forgetPlacedAircraft(
+        services.placement,
+        flightState._positionHistory.keys(),
+      );
      flightState._positionHistory.clear();
      flightState._displayCourse.clear();
      flightState._groundSnap.clear();
@@ -15,6 +15,7 @@ import {
  COAST_MAX_SPEED_MPS,
 } from '../../data/motionModel.js';
 import { CLASS_SCALE_2D } from '../../data/aircraftClass.js';
+import { placeLiveAircraft } from '../flights/placed.js';
 import {
  modelVisualAnchor,
  modelAnchorWorld,
@@ -70,6 +71,19 @@ export function createMotion({
    }

    const out = result || new Cesium.Cartesian3();
+    // A host-supplied placement decides where a live contact is drawn; with
+    // none, or no answer for this contact, the layer's own motion below does.
+    if (
+      placeLiveAircraft(
+        flightState,
+        services.placement,
+        icao24,
+        info,
+        history,
+        out,
+      )
+    )
+      return out;
    // Render one poll interval behind real time so we interpolate between
    // two KNOWN fixes whenever possible (see RENDER_DELAY_SEC rationale).
    const renderTime = Cesium.JulianDate.addSeconds(
@@ -15,10 +15,12 @@ import {
  TRACKED_ICON_COLOR,
  followsClock,
 } from './policy.js';
+import { observeLiveAircraft } from '../flights/placed.js';

 /** Apply military record changes to Cesium resources and existing follow operations. */
 export function createMilitarySnapshotRenderer({
  flightState,
+  placement = null,
  records,
  groundFloor,
  meshFloor,
@@ -73,6 +75,17 @@ export function createMilitarySnapshotRenderer({
      );
      const isTracked = icao24 === flightState._trackedIcao;

+      // A host-supplied placement, if any, is told every fix as taken.
+      observeLiveAircraft(
+        flightState,
+        placement,
+        icao24,
+        fixEpochMs,
+        meta,
+        meta.speedMps,
+        meta.track,
+      );
+
      // Append to position history stamped with the FEED's fix epoch.
      // adsb.lol's seen_pos is the AGE in seconds of the last position
      // report, so the fix epoch is receipt time minus that age. Only
@@ -324,6 +337,7 @@ export function createMilitarySnapshotRenderer({
      flightState._billboards.delete(icao24);
      rendering._releaseModel(icao24); // aged-out aircraft: drop its 3D model (no orphan / cap leak)
      records.forget(icao24);
+      placement?.forget(icao24);
      flightState._cullPositions.delete(icao24);
      flightState._positionHistory.delete(icao24);
      flightState._displayCourse.delete(icao24);
@@ -19,6 +19,7 @@ import { RAIL_SELECTED_OVERLAY_SOURCE_ID } from './policy.js';
 import { publicRailCatalog } from '../../data/railFeeds.js';
 import { pointAtS } from '../../data/railPaths.js';
 import { fleetModelFakes } from '../../testSupport/fleetModelFakes.mjs';
+import { PLACED_WAKE_MS } from '../transit/movement.js';

 const M_PER_DEG = 111_320;
 const LAT = 42.36;
@@ -502,3 +503,31 @@ test('turning the layer off ends the hold', async (t) => {
  assert.equal(app.state()._cockpitLostFeedId, null);
  assert.equal(app.layer.getCockpitSubject(), null);
 });
+
+test('a train a placement holds still stops animating, and moves on when the placement does', async (t) => {
+  let spot = { lat: LAT, lon: LON0 + 3000 / KX };
+  const app = await onTheLine(t, {
+    placement: {
+      layer: () => ({
+        place: (id) => (id === T1 ? { ...spot, speedMps: 0 } : null),
+      }),
+    },
+  });
+  app.run(1_000);
+  const entry = app.entry(T1);
+  assert.equal(entry.placed, true);
+  assert.equal(app.state()._moving.has(entry), false, 'parked: not animating');
+  assert.notEqual(entry.wakeTimer, null, 'a wake asks the placement again');
+  assert.ok(app.state()._moving.has(app.entry(T2)), 'its own motion runs on');
+
+  spot = { lat: LAT, lon: spot.lon + 10 / KX };
+  t.mock.timers.tick(PLACED_WAKE_MS);
+  assert.ok(app.state()._moving.has(entry), 'moving again');
+  for (let i = 0; i < 3; i += 1) {
+    spot = { lat: LAT, lon: spot.lon + 1 / KX };
+    app.run(250);
+    assert.ok(app.state()._moving.has(entry), 'while it moves');
+  }
+  app.run(250);
+  assert.equal(app.state()._moving.has(entry), false, 'stopped again');
+});
@@ -82,6 +82,7 @@ export function createIngestion({ state, services, parts, source }) {
    state._moving.delete(entry);
    parts.rendering.cancelWake(entry);
    destroyTrack(entry.track);
+    entry.placement?.forget(key);
    state._trains.delete(key);
    state._revision += 1;
    // A copy it hid may now be the one to draw.
@@ -274,6 +275,8 @@ export function createIngestion({ state, services, parts, source }) {
    const entry = {
      key,
      feedId: feed.id,
+      // A host-supplied placement, if any, draws it while live.
+      placement: services.placement ?? null,
      marker: null,
      category: record.category,
      railMode: record.mode,
@@ -35,6 +35,7 @@ export function createLifecycle({ state, services, parts, timetable }) {
    for (const entry of state._trains.values()) {
      parts.rendering.cancelWake(entry);
      destroyTrack(entry.track);
+      entry.placement?.forget(entry.key);
    }
    state._trains.clear();
    state._moving.clear();
@@ -1,5 +1,10 @@
 import * as Cesium from 'cesium';
-import { applyDisplayCourse, updatePlayback } from '../transit/movement.js';
+import {
+  PLACED_WAKE_MS,
+  applyDisplayCourse,
+  playbackMoving,
+  updatePlayback,
+} from '../transit/movement.js';
 import { HEIGHT_SAMPLE_MAX_ALTITUDE_M } from '../transit/policy.js';
 import {
  COLD_CELL_FLUSH_MS,
@@ -551,8 +556,11 @@ export function createRendering({ state, services, parts }) {
   */
  function place(entry) {
    const sample = entry.sample;
+    // Placed by a host-supplied placement, it is drawn where that puts it.
    const onPath =
-      entry.path && placeOnPath(entry.path, sample, entry.segment, pathPoint);
+      !entry.placed &&
+      entry.path &&
+      placeOnPath(entry.path, sample, entry.segment, pathPoint);
    const lat = onPath
      ? pathPoint.lat
      : Number.isFinite(sample?.lat)
@@ -629,16 +637,26 @@ export function createRendering({ state, services, parts }) {
    if (!entry.marker) return;
    place(entry);
    const sample = entry.sample;
-    const moving =
-      playing(entry) &&
-      sample?.phase === 'playing' &&
-      sample.segmentSpeedMps > 0;
+    const moving = playing(entry) && playbackMoving(entry);
    if (moving) {
      state._moving.add(entry);
      return;
    }
    state._moving.delete(entry);
-    if (playing(entry) && Number.isFinite(sample?.nextWakeMonoMs)) {
+    if (!playing(entry)) return;
+    // A train a host-supplied placement holds still is asked again every
+    // PLACED_WAKE_MS: it moves on once the placement does.
+    const monoNow = performance.now();
+    const placedWake = entry.placed === true;
+    const boundary = sample?.nextWakeMonoMs;
+    const wakeMonoMs = placedWake
+      ? Math.min(
+          Number.isFinite(boundary) ? boundary : Infinity,
+          monoNow + PLACED_WAKE_MS,
+        )
+      : boundary;
+    if (Number.isFinite(wakeMonoMs)) {
+      const atBoundary = !placedWake || boundary <= wakeMonoMs;
      entry.wakeTimer = setTimeout(
        () => {
          entry.wakeTimer = null;
@@ -646,9 +664,10 @@ export function createRendering({ state, services, parts }) {
          updatePlayback(entry, Date.now(), performance.now());
          schedulePlayback(entry);
          syncRenderHold();
-          governorRequestRender('rail-motion-boundary');
+          if (atBoundary || state._moving.has(entry))
+            governorRequestRender('rail-motion-boundary');
        },
-        Math.max(1, sample.nextWakeMonoMs - performance.now()),
+        Math.max(1, wakeMonoMs - monoNow),
      );
    }
  }
@@ -851,7 +870,8 @@ export function createRendering({ state, services, parts }) {
      place(entry);
      if (
        entry.sample.phase !== 'playing' ||
-        entry.sample.segmentSpeedMps === 0
+        entry.sample.segmentSpeedMps === 0 ||
+        (entry.placed === true && entry.placedMoving !== true)
      )
        schedulePlayback(entry);
    }
@@ -86,6 +86,7 @@ export function createIngestion({ state, services, parts, source }) {
    state._heightDirty.delete(entry);
    parts.rendering.cancelWake(entry);
    destroyTrack(entry.track);
+    entry.placement?.forget(key);
    state._vehicles.delete(key);
    state._detectRevision += 1;
  }
@@ -327,6 +328,8 @@ export function createIngestion({ state, services, parts, source }) {
        entry = {
          key,
          feedId: feed.id,
+          // A host-supplied placement, if any, draws it while live.
+          placement: services.placement ?? null,
          marker: null,
          mode,
          // The GTFS route type its model is chosen by, when it has one.
@@ -147,6 +147,7 @@ export function createLifecycle({ state, services, parts }) {
        entry.detectContact = null;
        parts.rendering.cancelWake(entry);
        destroyTrack(entry.track);
+        entry.placement?.forget(entry.key);
      }
      state._vehicles.clear();
      parts.height.clear();
@@ -242,6 +243,7 @@ export function createLifecycle({ state, services, parts }) {
        entry.detectContact = null;
        parts.rendering.cancelWake(entry);
        destroyTrack(entry.track);
+        entry.placement?.forget(entry.key);
      }
      state._vehicles.clear();
      parts.rendering.stopModels();
@@ -12,6 +12,8 @@ import {
 } from './policy.js';
 import { FLOOR_WARM_PER_POLL } from './policy.js';
 import { fleetModelFakes } from '../../testSupport/fleetModelFakes.mjs';
+import { layerPlacement } from '../../data/placementProvider.js';
+import { PLACED_WAKE_MS } from './movement.js';
 import {
  FUTURE_STAMP_TOLERANCE_MS,
  ORDER_REJECTS_BEFORE_RESET,
@@ -183,6 +185,8 @@ function harness(
    // Fleet-model services (testSupport/fleetModelFakes.mjs), with a camera
    // looking straight down for them; without them vehicles stay icons.
    fleet = null,
+    // A host-supplied placement (../../data/placementProvider.js), or none.
+    placement = null,
  } = {},
 ) {
  const viewRect = { ...view };
@@ -291,6 +295,7 @@ function harness(
    source,
    services: {
      ...(fleet ? { fleet } : {}),
+      ...(placement ? { placement: layerPlacement(placement, 'transit') } : {}),
      ground,
      mesh: {
        sampleMeshFloorCells(scene, points, options) {
@@ -1156,6 +1161,110 @@ test('a parked fleet that keeps reporting the same position stops asking for fra
    'only the vehicle that actually moved is animating',
  );
 });
+test('a vehicle a placement holds still stops asking for frames, and moves on when the placement does', async (t) => {
+  // The placement parks bus-1 at a terminal until `spot` moves.
+  let spot = { lat: 42.3605, lon: -71.0605 };
+  let asked = 0;
+  const app = harness(t, {
+    placement: {
+      layer: () => ({
+        place: () => {
+          asked += 1;
+          return { ...spot, courseDeg: 0, speedMps: 0 };
+        },
+      }),
+    },
+  });
+  app.serve('mbta', () => ({
+    status: 200,
+    body: snapshot(
+      'mbta',
+      'MBTA',
+      [vehicle('bus-1', 42.36, -71.06, reported())],
+      { fetchedAt: Date.now() },
+    ),
+  }));
+  app.layer.enable(app.viewer);
+  await app.layer.update();
+  app.settle();
+  const entry = app.vehicles()[0];
+  assert.equal(entry.placed, true);
+  for (let i = 0; i < 3; i += 1) {
+    app.advance(16);
+    app.frame();
+  }
+  assert.equal(app.state()._moving.has(entry), false, 'parked: not animating');
+  assert.equal(app.holds().includes('transit'), false, 'and the loop idles');
+  assert.notEqual(entry.wakeTimer, null, 'a wake asks the placement again');
+  assert.ok(Math.abs(lat(entry) - 42.3605) < 1e-6, 'drawn where placed');
+
+  // Asked again while it stays put: still idle, the wake re-armed.
+  const before = asked;
+  app.advance(PLACED_WAKE_MS);
+  assert.ok(asked > before, 'the wake asked the placement');
+  assert.equal(app.state()._moving.has(entry), false);
+  assert.equal(app.holds().includes('transit'), false);
+
+  // The placement moves it: the next wake picks that up and the loop runs.
+  spot = { lat: 42.361, lon: -71.0605 };
+  app.advance(PLACED_WAKE_MS);
+  assert.equal(app.state()._moving.has(entry), true, 'moving again');
+  assert.ok(app.holds().includes('transit'), 'and holds the loop');
+  assert.ok(Math.abs(lat(entry) - 42.361) < 1e-6);
+  for (let i = 0; i < 3; i += 1) {
+    spot = { lat: spot.lat + 1e-5, lon: spot.lon };
+    app.advance(16);
+    app.frame();
+    assert.equal(app.state()._moving.has(entry), true, 'while it moves');
+  }
+
+  // It stops again: the next frame that sees it unchanged lets go.
+  app.advance(16);
+  app.frame();
+  assert.equal(app.state()._moving.has(entry), false, 'stopped again');
+  assert.equal(app.holds().includes('transit'), false);
+});
+
+test('a vehicle a placement draws is shown from a high camera, as one it does not', async (t) => {
+  // Far above the ground no floor is asked for, so the fixes carry no height
+  // and the vehicle stands on the ellipsoid, placed or not.
+  const app = harness(t, {
+    at: { lat: 52.09, lon: 5.12 },
+    altitude: 1_500_000,
+    view: { west: -78.084, south: 20, east: 88.084, north: 84 },
+    placement: {
+      layer: () => ({
+        place: (id) =>
+          id.endsWith('nl-1')
+            ? { lat: 52.091, lon: 5.12, courseDeg: 0, speedMps: 0 }
+            : null,
+      }),
+    },
+  });
+  const now = Date.now();
+  app.serve('ovapi-nl', () => ({
+    status: 200,
+    body: snapshot(
+      'ovapi-nl',
+      'OVapi',
+      [
+        vehicle('nl-1', 52.09, 5.12, now / 1000),
+        vehicle('nl-2', 51.92, 4.48, now / 1000),
+      ],
+      { fetchedAt: Date.now() },
+    ),
+  }));
+  app.layer.enable(app.viewer);
+  await app.layer.update();
+  app.advance(250);
+  app.frame();
+  const placed = app.vehicles().find((entry) => entry.placed === true);
+  assert.ok(placed, 'one vehicle is placed');
+  const shown = app.vehicles().filter((entry) => entry.marker?.show !== false);
+  assert.equal(shown.length, 2, 'both vehicles are drawn');
+  assert.ok(Math.abs(lat(placed) - 52.091) < 1e-6, 'where placed');
+});
+
 test('the last vehicle leaving mid-segment takes the render hold with it', async (t) => {
  const app = harness(t);
  const fetchedAt = Date.now();
@@ -186,13 +186,57 @@ export function recordFix(entry, fix, observation = {}) {
    );
    entry.currentEpoch = latest.epoch;
    if (context) entry.track.epochs.set(latest.epoch, context);
+    entry.placement?.observe(entry.key, {
+      timeMs: fix.t,
+      lat: fix.lat,
+      lon: fix.lon,
+      altitudeM: NaN,
+      speedMps: NaN,
+      courseDeg: Number.isFinite(fix.bearingDeg) ? fix.bearingDeg : NaN,
+      headingDeg: NaN,
+      feedId: entry.feedId,
+    });
  }
  return result;
 }
+/**
+ * How often a vehicle a host-supplied placement holds still is asked again
+ * whether it moves, so it resumes without the frame loop running for it.
+ */
+export const PLACED_WAKE_MS = 1000;
+
+/**
+ * Advance a vehicle's playback to now. With a host-supplied placement
+ * (`entry.placement`, ../../data/placementProvider.js) that places it, it is
+ * drawn there, its course and speed the placement's (`entry.placed`); the
+ * playback's own phase, motion and segment stand. `entry.placedMoving` says
+ * whether the placed position changed since the placement was last asked at
+ * an earlier time (true for its first answer, so one more frame can tell).
+ */
 export function updatePlayback(entry, wallNowMs, monoNowMs) {
  entry.clocks.wallNowMs = wallNowMs;
  entry.clocks.monoNowMs = monoNowMs;
  advance(entry.track, entry.clocks, entry.sample);
+  const placed = entry.placement?.place(entry.key, wallNowMs) ?? null;
+  const wasPlaced = entry.placed === true;
+  entry.placed = placed !== null;
+  if (!placed) entry.placedMoving = false;
+  else if (!wasPlaced) entry.placedMoving = true;
+  else if (wallNowMs !== entry.placedAtWallMs)
+    entry.placedMoving =
+      placed.lat !== entry.placedLat || placed.lon !== entry.placedLon;
+  if (placed) {
+    entry.placedLat = placed.lat;
+    entry.placedLon = placed.lon;
+    entry.placedAtWallMs = wallNowMs;
+    const sample = entry.sample;
+    sample.lat = placed.lat;
+    sample.lon = placed.lon;
+    if (Number.isFinite(placed.courseDeg))
+      sample.segmentCourseDeg = placed.courseDeg;
+    if (Number.isFinite(placed.speedMps))
+      sample.segmentSpeedMps = placed.speedMps;
+  }
  entry.resets = entry.track.resets;
  const segment = entry.segment;
  readFix(
@@ -217,6 +261,16 @@ export function syncPlayback(entry, wallNowMs, monoNowMs = performance.now()) {
  }
  updatePlayback(entry, wallNowMs, monoNowMs);
 }
+/**
+ * Whether a vehicle's drawn position changes on its own: a vehicle a
+ * host-supplied placement draws moves while that placement's position
+ * changes; any other while its playback glides along a segment.
+ */
+export function playbackMoving(entry) {
+  if (entry.placed === true) return entry.placedMoving === true;
+  const sample = entry.sample;
+  return sample?.phase === 'playing' && sample.segmentSpeedMps > 0;
+}
 export function playbackPosition(entry) {
  if (!entry?.track) return null;
  return {
@@ -10,6 +10,7 @@ import {
 } from './movement.js';
 import { buildTransitSelectionCopy } from './policy.js';
 import { sampleAt } from '../../data/contactPlayback.js';
+import { layerPlacement } from '../../data/placementProvider.js';
 const entry = () => {
  const e = { mode: 'bus', modeInferred: false, courseDeg: null };
  initializePlayback(e);
@@ -163,3 +164,101 @@ test('course fallback prefers held course, then finite reported bearing, then un
    assert.equal(e.courseDeg, expected);
  }
 });
+
+const placedEntry = (placement) => {
+  const e = {
+    key: 'feed:7',
+    feedId: 'feed',
+    mode: 'bus',
+    modeInferred: false,
+    courseDeg: null,
+    placement,
+  };
+  initializePlayback(e);
+  return e;
+};
+const drive = (e) => {
+  recordFix(e, { ...fix(0), bearingDeg: 10 });
+  recordFix(e, { ...fix(15000, 42.001), bearingDeg: 20 });
+  recordFix(e, fix(30000, 42.002));
+  const samples = [];
+  for (let wall = 0; wall <= 120000; wall += 2500) {
+    updatePlayback(e, wall, wall);
+    samples.push({ ...e.sample });
+  }
+  return samples;
+};
+test('without a placement, or one that places nothing, playback is the layer own', () => {
+  const own = drive(placedEntry(null));
+  const declined = [];
+  const placement = layerPlacement(
+    {
+      layer: () => ({
+        observe: (id, f) => declined.push([id, f.timeMs]),
+        place: () => null,
+      }),
+    },
+    'transit',
+  );
+  const e = placedEntry(placement);
+  assert.deepEqual(drive(e), own);
+  assert.equal(e.placed, false);
+  assert.deepEqual(declined, [
+    ['feed:7', 0],
+    ['feed:7', 15000],
+    ['feed:7', 30000],
+  ]);
+});
+test('a placement draws the vehicle where it places it, keeping the playback state', () => {
+  const own = drive(placedEntry(null));
+  const e = placedEntry(
+    layerPlacement(
+      {
+        layer: () => ({
+          place: (id, wallMs) =>
+            wallMs >= 60000
+              ? { lat: 1, lon: 2, courseDeg: 45, speedMps: 7 }
+              : { lat: 3, lon: 4 },
+        }),
+      },
+      'transit',
+    ),
+  );
+  const placed = drive(e);
+  assert.equal(e.placed, true);
+  for (let i = 0; i < own.length; i++) {
+    const late = i * 2500 >= 60000;
+    assert.equal(placed[i].lat, late ? 1 : 3);
+    assert.equal(placed[i].lon, late ? 2 : 4);
+    assert.equal(placed[i].phase, own[i].phase);
+    assert.equal(placed[i].motion, own[i].motion);
+    if (late) {
+      assert.equal(placed[i].segmentCourseDeg, 45);
+      assert.equal(placed[i].segmentSpeedMps, 7);
+    } else {
+      // No course or speed placed: the playback's own stand.
+      assert.deepEqual(
+        [placed[i].segmentCourseDeg, placed[i].segmentSpeedMps],
+        [own[i].segmentCourseDeg, own[i].segmentSpeedMps],
+      );
+    }
+  }
+});
+test('a refused fix is never told to the placement', () => {
+  const told = [];
+  const e = placedEntry(
+    layerPlacement(
+      {
+        layer: () => ({
+          observe: (id, f) => told.push(f.lat),
+          place: () => null,
+        }),
+      },
+      'transit',
+    ),
+  );
+  recordFix(e, fix(0));
+  recordFix(e, fix(15000, 42.001));
+  assert.equal(recordFix(e, fix(30000, 43)).accepted, false);
+  assert.deepEqual(told, [42, 42.001]);
+});
@@ -4,8 +4,10 @@ import {
  perspectiveProjectedRotation,
 } from '../../data/iconOrientation.js';
 import {
+  PLACED_WAKE_MS,
  updatePlayback,
  applyDisplayCourse,
+  playbackMoving,
  syncPlayback,
 } from './movement.js';
 import {
@@ -165,7 +167,7 @@ export function createRendering({ state, services, parts }) {
      migrateMarker(entry, false);
      return;
    }
-    const positional = sample.phase === 'playing' && sample.segmentSpeedMps > 0;
+    const positional = playbackMoving(entry);
    const turning =
      Number.isFinite(sample.segmentCourseDeg) &&
      Number.isFinite(entry.courseDeg) &&
@@ -178,7 +180,19 @@ export function createRendering({ state, services, parts }) {
    } else {
      state._moving.delete(entry);
      migrateMarker(entry, false);
-      if (Number.isFinite(sample.nextWakeMonoMs)) {
+      // A vehicle a host-supplied placement holds still is asked again every
+      // PLACED_WAKE_MS: it moves on once the placement does.
+      const monoNow = performance.now();
+      const placedWake = entry.placed === true;
+      const boundary = sample.nextWakeMonoMs;
+      const wakeMonoMs = placedWake
+        ? Math.min(
+            Number.isFinite(boundary) ? boundary : Infinity,
+            monoNow + PLACED_WAKE_MS,
+          )
+        : boundary;
+      if (Number.isFinite(wakeMonoMs)) {
+        const atBoundary = !placedWake || boundary <= wakeMonoMs;
        entry.wakeTimer = setTimeout(
          () => {
            entry.wakeTimer = null;
@@ -187,9 +201,10 @@ export function createRendering({ state, services, parts }) {
            placeSample(entry);
            schedulePlayback(entry);
            syncRenderHold();
-            governorRequestRender('transit-motion-boundary');
+            if (atBoundary || state._moving.has(entry))
+              governorRequestRender('transit-motion-boundary');
          },
-          Math.max(1, sample.nextWakeMonoMs - performance.now()),
+          Math.max(1, wakeMonoMs - monoNow),
        );
      }
    }
@@ -441,7 +456,8 @@ export function createRendering({ state, services, parts }) {
      if (
        !entry.surfaceReady ||
        entry.sample.phase !== 'playing' ||
-        entry.sample.segmentSpeedMps === 0
+        entry.sample.segmentSpeedMps === 0 ||
+        (entry.placed === true && entry.placedMoving !== true)
      )
        schedulePlayback(entry);
      if (!state._moving.has(entry)) settleSelectedCard(entry);
@@ -198,6 +198,24 @@ export function createTrails({ state, services, parts, source }) {
    nearGround = parts.height.nearGround(),
  ) {
    pruneMarkerPaths(entry, sample);
+    // Placed by a host-supplied placement, it is drawn at the placed point,
+    // at the height its own playback has reached; far above the ground, where
+    // no floor is asked for, on the ellipsoid as an unplaced vehicle is.
+    if (entry.placed === true) {
+      const heightM = Number.isFinite(sample.heightM)
+        ? sample.heightM
+        : nearGround
+          ? NaN
+          : 0;
+      if (!Number.isFinite(heightM)) return null;
+      return Cesium.Cartesian3.fromDegrees(
+        sample.lon,
+        sample.lat,
+        heightM,
+        undefined,
+        out,
+      );
+    }
    if (!nearGround) {
      const a = entry.fromCart,
        b = entry.toCart,
@@ -31,6 +31,7 @@ export function createVesselLayer({ source, services, options = {} } = {}) {
  parts.queries = createQueries(context);
  parts.snapshots = createVesselSnapshotRenderer({
    state: vesselState.state,
+    placement: services?.placement ?? null,
    records: vesselState.state.records,
    rendering: parts.rendering,
    tracking: parts.tracking,
@@ -115,6 +115,8 @@ export function createLifecycle({
    state.feed.firstConnectTimer = null;
    state.feed.abort = null;
    state.billboardCollection = null;
+    if (services.placement)
+      for (const id of state.records.byId.keys()) services.placement.forget(id);
    state.records.all = [];
    state.records.byId = new Map();
    state.records.unkeyed = [];
@@ -211,6 +213,7 @@ export function createLifecycle({
      invalidateAisSession();
      releaseContinuousRender('ais-vessels');
      components.rendering.cancelTrailingVisibilityPass();
+      components.rendering.cancelPlacementFrame();
      unregisterPickOwner('ais-live-vessels');
      components.rendering.setVisible(false);
      vesselState._vesselOverlayHost.clearSource(VESSEL_OVERLAY_SOURCE_ID);
@@ -232,6 +235,7 @@ export function createLifecycle({
      invalidateAisSession();
      releaseContinuousRender('ais-vessels'); // direct-destroy path (perf wave 2 fix)
      components.rendering.cancelTrailingVisibilityPass();
+      components.rendering.cancelPlacementFrame();
      if (state.feed.abort) state.feed.abort.abort();
      unregisterPickOwner('ais-live-vessels');
      components.selection.clearVesselInspection();
@@ -0,0 +1,106 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { VesselRecords } from './records.js';
+import { createVesselSnapshotRenderer } from './snapshotRenderer.js';
+import { layerPlacement } from '../../data/placementProvider.js';
+
+const row = (id, fields = {}) => ({
+  id,
+  mmsi: id,
+  lat: 51.93,
+  lon: 4.05,
+  speed: 10,
+  course: 90,
+  heading: 92,
+  last_position_epoch: 1_000,
+  ...fields,
+});
+
+function harness({ placement = null, following = false } = {}) {
+  const visuals = [];
+  const state = { selectedRecord: null, trailId: null };
+  const records = new VesselRecords({ now: () => 5_000_000 });
+  const rendering = {
+    ensureCollections() {},
+    makeOccluder: () => null,
+    renderRowLimit: () => Infinity,
+    prepareRecordVisual(record) {
+      visuals.push(record.id);
+      return { billboard: null };
+    },
+    addRecordPrimitives() {},
+    removeRecordPrimitives() {},
+    shipIcon: () => 'icon',
+    shipScale: () => 1,
+    updateVisibility() {},
+  };
+  const renderer = createVesselSnapshotRenderer({
+    state,
+    placement,
+    records,
+    rendering,
+    tracking: {},
+    selection: {},
+    cards: {},
+    followsClock: () => following,
+  });
+  return {
+    visuals,
+    reconcile: (rows, options) => renderer.reconcileVessels({}, rows, options),
+  };
+}
+
+function recording() {
+  const told = [];
+  const forgotten = [];
+  const placement = layerPlacement(
+    {
+      layer: () => ({
+        observe: (id, fix) => told.push([id, fix]),
+        place: () => null,
+        forget: (id) => forgotten.push(id),
+      }),
+    },
+    'vessels',
+  );
+  return { told, forgotten, placement };
+}
+
+test('vessels reconcile as before with no placement', () => {
+  const own = harness();
+  own.reconcile([row('a'), row('b')]);
+  own.reconcile([row('a', { lat: 52, last_position_epoch: 1_060 })]);
+  const { placement } = recording();
+  const placed = harness({ placement });
+  placed.reconcile([row('a'), row('b')]);
+  placed.reconcile([row('a', { lat: 52, last_position_epoch: 1_060 })]);
+  assert.deepEqual(placed.visuals, own.visuals);
+});
+
+test('each live vessel fix is told to the placement, in m/s, and a vessel that leaves is forgotten', () => {
+  const { told, forgotten, placement } = recording();
+  const probe = harness({ placement });
+  probe.reconcile([row('a'), row('b', { speed: null, course: null })]);
+  probe.reconcile([row('a', { lat: 52, last_position_epoch: 1_060 })]);
+  assert.deepEqual(
+    told.map(([id, fix]) => [id, fix.timeMs, fix.lat]),
+    [
+      ['a', 1_000_000, 51.93],
+      ['b', 1_000_000, 51.93],
+      ['a', 1_060_000, 52],
+    ],
+  );
+  assert.ok(Math.abs(told[0][1].speedMps - 10 * (1852 / 3600)) < 1e-12);
+  assert.equal(told[0][1].courseDeg, 90);
+  assert.equal(told[0][1].headingDeg, 92);
+  assert.ok(Number.isNaN(told[1][1].speedMps));
+  assert.ok(Number.isNaN(told[1][1].courseDeg));
+  assert.deepEqual(forgotten, ['b']);
+});
+
+test('following the world clock, or rows at another time, tell the placement nothing', () => {
+  const { told, placement } = recording();
+  harness({ placement, following: true }).reconcile([row('a')]);
+  harness({ placement }).reconcile([row('a')], { replace: true });
+  assert.deepEqual(told, []);
+});
@@ -2,6 +2,7 @@ import test from 'node:test';
 import assert from 'node:assert/strict';
 import { createVesselLayer } from './index.js';
 import { VISIBILITY_UPDATE_MS } from './policy.js';
+import { layerPlacement } from '../../data/placementProvider.js';

 // Vessels move only when a poll lands, so the layer must let the scene idle:
 // it asks for frames for forced passes and one trailing frame after the camera
@@ -9,7 +10,7 @@ import { VISIBILITY_UPDATE_MS } from './policy.js';

 const noop = () => {};

-function setup() {
+function setup({ placement = null } = {}) {
  let clock = 10_000;
  const requests = [];
  const holds = new Set();
@@ -46,6 +47,7 @@ function setup() {
        releaseContinuousRender: (id) => holds.delete(id),
        governorRequestRender: (reason) => requests.push(reason),
      },
+      ...(placement ? { placement: layerPlacement(placement, 'vessels') } : {}),
    },
  });
  const camera = {
@@ -136,3 +138,61 @@ test('destroy cancels a pending trailing frame', (t) => {
  advance(VISIBILITY_UPDATE_MS);
  assert.deepEqual(requests, []);
 });
+
+test('a placement that places nothing asks for no frames; one that places a drawn vessel moves it in place', (t) => {
+  let mono = 0;
+  t.mock.method(performance, 'now', () => mono);
+  const originalDocument = globalThis.document;
+  globalThis.document = { getElementById: () => null };
+  t.after(() => {
+    if (originalDocument === undefined) delete globalThis.document;
+    else globalThis.document = originalDocument;
+  });
+  let spot = null;
+  const { layer, camera, requests, timers, advance } = setup({
+    placement: { layer: () => ({ place: () => spot }) },
+  });
+  const record = {
+    mmsi: '111',
+    lat: 51.93,
+    lon: 4.05,
+    billboard: { show: true, position: null },
+  };
+  layer.testing._setVesselStateForTest({
+    viewer: { camera, scene: {} },
+    records: [record],
+  });
+  const visual = layer.testing._prepareVesselVisualForTest(record);
+  const position = visual.position;
+  const before = { ...position };
+
+  // Nothing placed: no frame is asked for, and the vessel stays put.
+  for (let i = 0; i < 8; i += 1) {
+    mono += 250;
+    layer.testing._runPlacementPassForTest();
+    advance(250);
+  }
+  assert.equal(timers.size, 0);
+  assert.deepEqual(requests, []);
+  assert.deepEqual({ ...visual.position }, before);
+
+  // Placed: moved in place, no new positions, and a frame asked for.
+  spot = { lat: 51.94, lon: 4.05 };
+  mono += 250;
+  layer.testing._runPlacementPassForTest();
+  assert.equal(visual.position, position, 'written in place');
+  assert.notDeepEqual({ ...visual.position }, before);
+  assert.equal(record.billboard.position, position);
+  assert.equal(timers.size, 1);
+  advance(1_000);
+  assert.deepEqual(requests, ['ais-vessels-placement']);
+
+  // Turning the layer off drops the pending frame.
+  mono += 250;
+  layer.testing._runPlacementPassForTest();
+  assert.equal(timers.size, 1);
+  layer.destroy();
+  assert.equal(timers.size, 0);
+  advance(1_000);
+  assert.deepEqual(requests, ['ais-vessels-placement']);
+});
@@ -33,6 +33,8 @@ import { isAidToNavigation } from '../../data/vesselTypes.js';

 /** How often vessels move on as the world clock plays (wall ms). */
 const FOLLOW_POSITION_MS = 250;
+/** A frame is asked for this often while a placement moves live vessels. */
+const PLACEMENT_FRAME_MS = 1000;

 /** How often the vessels near enough for a model are gathered again (wall ms). */
 const MODEL_CANDIDATES_MS = 500;
@@ -58,10 +60,12 @@ export function createRendering({
      visual = { billboard: record.billboard || null };
      visualRecords.set(record, visual);
    }
-    // Following the world clock, a vessel is drawn where it was at the time.
+    // Following the world clock, a vessel is drawn where it was at the time;
+    // live, where a host-supplied placement draws it, if one does.
    const { lat, lon } = followsClock(vesselState)
      ? vesselPositionAt(record, vesselState.worldNow())
-      : record;
+      : (services.placement?.place(record.id, vesselState._aisRuntime.now()) ??
+        record);
    const heightM = components.queries.vesselDatumHeightM(
      components.tracking.currentGeoidN(lat, lon),
      VESSEL_LIFT_M,
@@ -78,9 +82,50 @@ export function createRendering({
      visual.position,
      new Cesium.Cartesian3(),
    );
+    if (services.placement) {
+      visual.drawnLat = lat;
+      visual.drawnLon = lon;
+    }
    return visual;
  }

+  /**
+   * Move a prepared vessel visual to `lat`/`lon`, writing into its own
+   * positions rather than allocating new ones: the placement pass runs for
+   * every drawn vessel several times a second.
+   */
+  function moveRecordVisual(visual, lat, lon) {
+    const heightM = components.queries.vesselDatumHeightM(
+      components.tracking.currentGeoidN(lat, lon),
+      VESSEL_LIFT_M,
+    );
+    const ellipsoid = Cesium.Ellipsoid.WGS84;
+    Cesium.Cartesian3.fromDegrees(
+      lon,
+      lat,
+      heightM,
+      ellipsoid,
+      visual.position,
+    );
+    Cesium.Cartesian3.fromDegrees(
+      lon,
+      lat,
+      heightM - VESSEL_LIFT_M,
+      ellipsoid,
+      visual.waterline,
+    );
+    Cesium.Cartesian3.fromDegrees(
+      lon,
+      lat,
+      0,
+      ellipsoid,
+      visual.surfacePosition,
+    );
+    ellipsoid.geodeticSurfaceNormal(visual.position, visual.normal);
+    visual.drawnLat = lat;
+    visual.drawnLon = lon;
+  }
+
  function resetRecordVisuals() {
    visualRecords = new WeakMap();
  }
@@ -391,6 +436,7 @@ export function createRendering({
    if (state.preRenderRemover || !viewer) return;
    state.preRenderRemover = viewer.scene.preRender.addEventListener(() => {
      if (followsClock(vesselState)) followClockPositions();
+      else if (services.placement && state.feed.enabled) followPlacements();
      updateVisibility();
      if (state.feed.enabled) updateModels();
    });
@@ -413,6 +459,50 @@ export function createRendering({
    }
  }

+  // Live with a host-supplied placement, vessels move on between refreshes:
+  // drawn where it places them at most every FOLLOW_POSITION_MS, with a frame
+  // asked for every PLACEMENT_FRAME_MS while it places any drawn vessel, so
+  // they move while the scene idles.
+  let placedAtWallMs = -Infinity;
+  let placing = false;
+  let placementFrameTimer = null;
+  function followPlacements() {
+    const wallMs = performance.now();
+    if (wallMs - placedAtWallMs >= FOLLOW_POSITION_MS) {
+      placedAtWallMs = wallMs;
+      placing = false;
+      const nowMs = vesselState._aisRuntime.now();
+      // Drawn vessels only, as a sprite or a model: a hidden one is placed
+      // again when it is drawn.
+      for (const record of state.records.all) {
+        const visual = visualRecords.get(record);
+        if (!visual?.position) continue;
+        if (!visual.billboard?.show && !visual.modelOwnsVisual) continue;
+        const placed = services.placement.place(record.id, nowMs);
+        if (placed) placing = true;
+        const { lat, lon } = placed ?? record;
+        if (lat === visual.drawnLat && lon === visual.drawnLon) continue;
+        moveRecordVisual(visual, lat, lon);
+        if (visual.billboard) visual.billboard.position = visual.position;
+      }
+    }
+    if (placing && placementFrameTimer === null)
+      placementFrameTimer = vesselState._aisRuntime.setTimeout(() => {
+        placementFrameTimer = null;
+        if (state.feed.enabled && !followsClock(vesselState))
+          governorRequestRender('ais-vessels-placement');
+      }, PLACEMENT_FRAME_MS);
+  }
+
+  /** Stop asking for placement frames (the layer turned off or went away). */
+  function cancelPlacementFrame() {
+    placedAtWallMs = -Infinity;
+    placing = false;
+    if (placementFrameTimer === null) return;
+    vesselState._aisRuntime.clearTimeout(placementFrameTimer);
+    placementFrameTimer = null;
+  }
+
  function scheduleTrailingVisibilityPass(now) {
    if (state.trailingVisibilityTimer) return;
    const waitMs = Math.max(
@@ -787,6 +877,8 @@ export function createRendering({
    installRuntime,
    updateVisibility,
    cancelTrailingVisibilityPass,
+    cancelPlacementFrame,
+    followPlacements,
    applyVesselFocusDeemphasis,
    makeOccluder,
    isVisible,
@@ -1,6 +1,30 @@
-/** Connect portable vessel reconciliation to scene and selection operations. */
+const KNOT_MPS = 1852 / 3600;
+const finiteOrNaN = (value) =>
+  typeof value === 'number' && Number.isFinite(value) ? value : NaN;
+
+/**
+ * A vessel record's fix as a host-supplied placement takes it: its own time
+ * (when it was received, if the row gives none), position, speed in m/s
+ * (rows give knots), course and heading.
+ */
+function placementFix(record) {
+  const epoch = record.lastPositionEpoch;
+  return {
+    timeMs: Number.isFinite(epoch) ? epoch * 1000 : record.receivedAtMs,
+    lat: record.lat,
+    lon: record.lon,
+    altitudeM: NaN,
+    speedMps: Number.isFinite(record.speed) ? record.speed * KNOT_MPS : NaN,
+    courseDeg: finiteOrNaN(record.course),
+    headingDeg: finiteOrNaN(record.heading),
+  };
+}
+
+/** Connect portable vessel reconciliation to scene and selection operations.
+ * `placement`, a host-supplied placement, is told each live fix. */
 export function createVesselSnapshotRenderer({
  state,
+  placement = null,
  records,
  rendering,
  tracking,
@@ -17,6 +41,13 @@ export function createVesselSnapshotRenderer({
    rendering.ensureCollections(viewer);
    const occluder = rendering.makeOccluder();
    const follow = followsClock();
+    // Live fixes only: following the clock, the layer places every vessel.
+    const observe =
+      placement && !follow && !replace
+        ? (record) => {
+            if (record.id) placement.observe(record.id, placementFix(record));
+          }
+        : () => {};
    records.reconcile(
      rows,
      {
@@ -28,6 +59,7 @@ export function createVesselSnapshotRenderer({
      },
      {
        add(record) {
+          observe(record);
          rendering.prepareRecordVisual(record);
          rendering.addRecordPrimitives(record, occluder);
        },
@@ -35,6 +67,7 @@ export function createVesselSnapshotRenderer({
          return rendering.shipIcon(record, record === state.selectedRecord);
        },
        updated(record, prevIcon) {
+          observe(record);
          const selected = record === state.selectedRecord;
          const visual = rendering.prepareRecordVisual(record);
          if (visual.billboard) {
@@ -56,6 +89,7 @@ export function createVesselSnapshotRenderer({
          rendering.removeRecordPrimitives(record);
        },
        removed(id) {
+          placement?.forget(id);
          if (state.trailId === id) tracking.clearSelectedVesselTrail();
        },
        staleSelected(record) {
@@ -162,6 +162,18 @@ export function createTesting({
    components.rendering.updateVisibility(force);
  }

+  /** Draw a record's visual where it stands now. Test-only seam. */
+
+  function _prepareVesselVisualForTest(record) {
+    return components.rendering.prepareRecordVisual(record);
+  }
+
+  /** Run one live placement pass (a host-supplied placement). Test-only seam. */
+
+  function _runPlacementPassForTest() {
+    components.rendering.followPlacements();
+  }
+
  /** True while a trailing visibility frame is scheduled. Test-only seam. */

  function _hasTrailingVisibilityPassForTest() {
@@ -194,6 +206,8 @@ export function createTesting({
    _getVesselFeedStateForTest,
    _getVesselStateForTest,
    _runVisibilityPassForTest,
+    _prepareVesselVisualForTest,
+    _runPlacementPassForTest,
    _hasTrailingVisibilityPassForTest,
  };
 }
@@ -73,6 +73,11 @@ export function createTracking({
        heightM,
      );
      if (visual.billboard) visual.billboard.position = visual.position;
+      // A placed vessel is drawn again at its placement on the next pass.
+      if (services.placement) {
+        visual.drawnLat = NaN;
+        visual.drawnLon = NaN;
+      }
    }
  }

@@ -4,6 +4,7 @@
 // height, layer stats, selected route lines).
 import * as Cesium from 'cesium';
 import { createRailLayer } from '../layers/rail/index.js';
+import { layerPlacement } from '../data/placementProvider.js';
 import { CAMERA_DEBOUNCE_MS } from '../layers/rail/policy.js';
 import { WORLD_FOCUS_FRAMING } from '../worldFocus.js';
 import { publicRailCatalog } from '../data/railFeeds.js';
@@ -82,7 +83,8 @@ const headersOf = (values = {}) => ({
 *   catalog?: object[]|Promise<object[]>|Function|false,
 *   ground?: object,
 *   stops?: object|null, scheduleStops?: boolean,
- *   paths?: Record<string, object>|null, fleet?: object|null}} [options]
+ *   paths?: Record<string, object>|null, fleet?: object|null,
+ *   placement?: object|null}} [options]
 *   `ground`: a floor service in place of the always-cold stub; `catalog`:
 *   the server's feed catalog (it may list fewer feeds than are bundled; a
 *   promise answers when it settles, a function is called per request and
@@ -94,7 +96,8 @@ const headersOf = (values = {}) => ({
 *   failure status it answers with (404 for any other feed; without it the
 *   source has no path route at all); `fleet`: the fleet-model services
 *   (testSupport/fleetModelFakes.mjs), with a camera that looks straight
- *   down for them (none by default: trains stay icons).
+ *   down for them (none by default: trains stay icons); `placement`: a
+ *   host-supplied placement provider (../data/placementProvider.js).
 */
 export function railLayerHarness(
  t,
@@ -107,6 +110,7 @@ export function railLayerHarness(
    scheduleStops = false,
    paths = null,
    fleet = null,
+    placement = null,
  } = {},
 ) {
  const T0 = RAIL_HARNESS_T0;
@@ -332,6 +336,7 @@ export function railLayerHarness(
        warmGroundFloor() {},
      },
      ...(fleet ? { fleet } : {}),
+      ...(placement ? { placement: layerPlacement(placement, 'rail') } : {}),
      camera: {
        framing: WORLD_FOCUS_FRAMING.train,
        requestWorldFocus(detail) {