fix getOrbitTrack computing start times for previous and next orbits

Author: davidcalhoun

__tests__/sgp4.js

@@ -29,6 +29,10 @@ const tleStr = `ISS (ZARYA)
 1 25544U 98067A   17206.51418347  .00001345  00000-0  27503-4 0  9993
 2 25544  51.6396 207.2711 0006223  72.3525  71.7719 15.54224686 67715`;
 
+const proxima2 = `PROXIMA II              
+1 43696U 18088G   21129.25183023  .00001100  00000-0  45012-4 0  9990
+2 43696  85.0353 221.2351 0019671 158.2001 202.0083 15.22857070138409`;
+
 const tleArr = tleStr.split("\n");
 
 describe("getSatelliteInfo", () => {
@@ -190,8 +194,22 @@ describe("getGroundTracks", () => {
 		const firstLng = coords[0][0][0];
 		const lastLng = coords[0][coords[0].length - 1][0];
 
-		expect(firstLng).toBe(-179.95882804237493);
-		expect(lastLng).toBe(179.96378140395484);
+		expect(firstLng).toBe(-179.9996305056871);
+		expect(lastLng).toBe(179.9939688862288);
+	});
+
+	test("2", async () => {
+		const timestamp = 1620583838732;
+		const result = await getGroundTracks({
+			tle: proxima2,
+			startTimeMS: timestamp
+		});
+		expect(result[0][0][0]).toBeCloseTo(-179.65354);
+		expect(result[0][0][1]).toBeCloseTo(84.57353);
+		expect(result[1][0][0]).toBeCloseTo(-179.68200);
+		expect(result[1][0][1]).toBeCloseTo(85.06215);
+		expect(result[2][0][0]).toBeCloseTo(-179.89417);
+		expect(result[2][0][1]).toBeCloseTo(84.63849);
 	});
 });
 
@@ -209,8 +227,22 @@ describe("getGroundTracksSync", () => {
 
 		const firstLng = coords[0][0][0];
 		const lastLng = coords[0][coords[0].length - 1][0];
-		expect(firstLng).toBe(-179.95882804237493);
-		expect(lastLng).toBe(179.96378140395484);
+		expect(firstLng).toBe(-179.9996305056871);
+		expect(lastLng).toBe(179.9939688862288);
+	});
+
+	test("2", async () => {
+		const timestamp = 1620583838732;
+		const result = await getGroundTracksSync({
+			tle: proxima2,
+			optionalTimeMS: timestamp
+		});
+		expect(result[0][0][0]).toBeCloseTo(-179.65354);
+		expect(result[0][0][1]).toBeCloseTo(84.57353);
+		expect(result[1][0][0]).toBeCloseTo(-179.68200);
+		expect(result[1][0][1]).toBeCloseTo(85.06215);
+		expect(result[2][0][0]).toBeCloseTo(-179.89417);
+		expect(result[2][0][1]).toBeCloseTo(84.63849);
 	});
 });
 
@@ -236,6 +268,20 @@ describe("problematic TLES (geosync, decayed)", () => {
 		expect(result).toEqual(expectedResult);
 	});
 
+	test("getLastAntemeridianCrossingTimeMS 2", () => {
+		const timestamp = 1620579956208;
+		const result = getLastAntemeridianCrossingTimeMS(proxima2, timestamp);
+		const expectedResult = -1;
+		expect((timestamp - result) / 1000 / 60).toBeCloseTo(72.50);
+	});
+
+	test("getLastAntemeridianCrossingTimeMS 3", () => {
+		const timestamp = 1620581856788;
+		const result = getLastAntemeridianCrossingTimeMS(proxima2, timestamp);
+		const expectedResult = -1;
+		expect((timestamp - result) / 1000 / 60).toBeCloseTo(8.976);
+	});
+
 	test("getOrbitTrack", async () => {
 		const timestamp = 1501039265000;
 		const result = await getOrbitTrack({

src/sgp4.js

@@ -233,7 +233,7 @@ export function getLastAntemeridianCrossingTimeMS(tle, timeMS) {
 
 	const time = timeMS || Date.now();
 
-	let step = 1000 * 60 * 10;
+	let step = 1000 * 60 * 3;
 	let curLngLat = [];
 	let lastLngLat = [];
 	let curTimeMS = time;
@@ -247,10 +247,13 @@ export function getLastAntemeridianCrossingTimeMS(tle, timeMS) {
 
 		didCrossAntemeridian = _crossesAntemeridian(lastLngLat[0], curLng);
 		if (didCrossAntemeridian) {
-			// Back up a bit, then keep halving the step increment till we get close enough.
+			// Back up to before we crossed the line.
 			curTimeMS += step;
-			step = step > 20000 ? 20000 : step / 2;
+
+			// Keep narrowing by halving increments.
+			step = step / 2;
 		} else {
+			// Didn't cross yet, so keep incrementing.
 			curTimeMS -= step;
 			lastLngLat = curLngLat;
 		}
@@ -543,17 +546,19 @@ export function getGroundTracks({
 		]);
 	}
 
+	/**
+	 * Buffer time that will be sure to place us well within the previous or next orbit.
+	 */
+	const bufferMS = orbitTimeMS / 5;
+
 	const lastOrbitStartMS = getLastAntemeridianCrossingTimeMS(
 		parsedTLE,
-
-		// TODO: fix this magic math
-		curOrbitStartMS - 10000
+		curOrbitStartMS - bufferMS
 	);
+
 	const nextOrbitStartMS = getLastAntemeridianCrossingTimeMS(
 		parsedTLE,
-
-		// TODO: fix this magic math
-		curOrbitStartMS + orbitTimeMS + 1000 * 60 * 30
+		curOrbitStartMS + orbitTimeMS + bufferMS
 	);
 
 	const groundTrackPromises = [
@@ -637,11 +642,11 @@ export function getGroundTracksSync({
 
 	const lastOrbitStartMS = getLastAntemeridianCrossingTimeMS(
 		parsedTLE,
-		curOrbitStartMS - 10000
+		curOrbitStartMS - (orbitTimeMS / 5)
 	);
 	const nextOrbitStartMS = getLastAntemeridianCrossingTimeMS(
 		parsedTLE,
-		curOrbitStartMS + orbitTimeMS + 1000 * 60 * 30
+		curOrbitStartMS + orbitTimeMS + (orbitTimeMS / 5)
 	);
 
 	const orbitStartTimes = [