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Coordinate Transforms

These functions implement reference frame and unit conversions.

Units

Usually:

  • Positions, ranges, and heights are in km
  • Velocities are in km/s
  • Angles are in radians

The library provides synonyms for

  • number type: Kilometer, AU, KilometerPerSecond, Radians etc
  • { x: number, y: number, z: number } type: EciVec3, EcfVec3 etc

They are not structurally distinct, they are only here to remind you of units passed around.

GMST​

Several transforms need Greenwich Mean Sidereal Time.

import { gstime } from 'satellite.js';

// From a JS Date
const gmst = gstime(new Date());

// From a Julian Day number
const gmst2 = gstime(jday);

// From individual UTC components
const gmst3 = gstime(year, month, day, hour, minute, second);

Frame transforms​

eciToEcf - ECI → ECF​

import { eciToEcf } from 'satellite.js';

const positionEcf = eciToEcf(positionEci, gmst);
const velocityEcf = eciToEcf(velocityEci, gmst);

ecfToEci - ECF → ECI​

import { ecfToEci } from 'satellite.js';

const positionEci = ecfToEci(positionEcf, gmst);

eciToGeodetic - ECI → Geodetic​

import { eciToGeodetic } from 'satellite.js';

const geodetic = eciToGeodetic(positionEci, gmst);

const { longitude, latitude, height } = geodetic;

geodeticToEcf - Geodetic → ECF​

import { geodeticToEcf, degreesToRadians } from 'satellite.js';

const observerGeodetic = {
  longitude: degreesToRadians(-122.03),
  latitude: degreesToRadians(36.96),
  height: 0.370, // km
};

const observerEcf = geodeticToEcf(observerGeodetic);

ecfToLookAngles - ECF → Look Angles​

import { ecfToLookAngles } from 'satellite.js';

const lookAngles = ecfToLookAngles(observerGeodetic, satelliteEcf);

const { azimuth, elevation, rangeSat } = lookAngles;

Angle helpers​

FunctionDescription
degreesToRadians(deg)Degrees → radians
radiansToDegrees(rad)Radians → degrees
degreesLat(rad)Radians → degrees, throws if |rad| > π/2
degreesLong(rad)Radians → degrees, throws if |rad| > π
radiansLat(deg)Degrees → radians, throws if |deg| > 90
radiansLong(deg)Degrees → radians, throws if |deg| > 180