fix(cockpit): no swath readout while the orbital view looks past the Earth's limb

src/cockpitOrbital.js

@@ -364,6 +364,21 @@ export function offNadirAngleDeg({ northDeg = 0, eastDeg = 0 } = {}) {
   return (Math.acos(cosine) * 180) / Math.PI;
 }
 
+/**
+ * Whether the view's centre, tilted off nadir, still meets the Earth: its
+ * angle from nadir (`offNadirAngleDeg`) is within the Earth's angular radius
+ * from the satellite, asin(R / (R + altitude)).
+ * @param {number} altitudeM
+ * @param {{northDeg?: number, eastDeg?: number}} [offset]
+ * @returns {boolean}
+ */
+export function offNadirCentreMeetsEarth(altitudeM, offset = {}) {
+  if (!Number.isFinite(altitudeM) || altitudeM <= 0) return false;
+  const R = ORBITAL_EARTH_RADIUS_M;
+  const limbDeg = (Math.asin(R / (R + altitudeM)) * 180) / Math.PI;
+  return offNadirAngleDeg(offset) < limbDeg;
+}
+
 /**
  * Roughly how much wider the ground across the frame is at the centre of a
  * tilted view than at nadir, on a flat Earth: tilting across the frame (east,

src/cockpitOrbital.test.mjs

@@ -13,6 +13,7 @@ import {
   nadirOrientation,
   nextOrbitalZoom,
   offNadirAngleDeg,
+  offNadirCentreMeetsEarth,
   offNadirSwathScale,
   offNadirOrientation,
   orbitalBaseFovRad,
@@ -300,3 +301,16 @@ test("a tilted view's width grows faster across the frame than along it", () =>
   }
   assert.equal(offNadirSwathScale({ northDeg: Number.NaN }), 1);
 });
+
+test('a view tilted past the limb no longer meets the Earth', () => {
+  // GEO: the Earth spans about 8.7 degrees either side of nadir.
+  assert.equal(offNadirCentreMeetsEarth(35_786_000, { northDeg: 5 }), true);
+  assert.equal(offNadirCentreMeetsEarth(35_786_000, { northDeg: 15 }), false);
+  // LEO: a 30 degree tilt still lands on the ground.
+  assert.equal(
+    offNadirCentreMeetsEarth(420_000, { northDeg: 20, eastDeg: 20 }),
+    true,
+  );
+  assert.equal(offNadirCentreMeetsEarth(420_000, { eastDeg: 80 }), false);
+  assert.equal(offNadirCentreMeetsEarth(NaN, {}), false);
+});

src/ui/cockpitOrbitalView.js

@@ -14,6 +14,7 @@ import {
   nextOrbitalZoom,
   offNadirAngleDeg,
   offNadirOrientation,
+  offNadirCentreMeetsEarth,
   offNadirSwathScale,
   orbitalBaseFovRad,
   previousOrbitalZoom,
@@ -410,9 +411,12 @@ export function publishOrbitalView(info, transition = IDLE_TRANSITION) {
   const settledFov = fovForZoom(orbitalBaseFovRad(info.altitudeM), state.zoom);
   const footprintKm = footprintWidthKm(info.altitudeM, settledFov, aspect);
   const trackUp = Number.isFinite(info.groundTrackDeg);
-  const swathScale = offset
+  const tilt = offset
-    ? offNadirSwathScale(lookTilt(orbitalLookReadoutParts(state.lookTarget)))
+    ? lookTilt(orbitalLookReadoutParts(state.lookTarget))
-    : 1;
+    : null;
+  const swathScale = offset ? offNadirSwathScale(tilt) : 1;
+  // Tilted past the limb, the view's centre looks into space: no swath.
+  const seesEarth = !offset || offNadirCentreMeetsEarth(info.altitudeM, tilt);
   this.publishOrbital({
     name: info.name,
     classLabel: info.classLabel || '',
@@ -432,7 +436,7 @@ export function publishOrbitalView(info, transition = IDLE_TRANSITION) {
       // the nadir footprint (`offNadirSwathScale`); 1 and null at nadir.
       swathScale,
       swathKm:
-        offset && Number.isFinite(footprintKm)
+        offset && seesEarth && Number.isFinite(footprintKm)
           ? footprintKm * swathScale
           : null,
     },