ootk-core
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Orbital Object Toolkit. A modern typed replacement for satellite.js including SGP4 propagation, TLE parsing, Sun and Moon calculations, and more.
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JavaScript
/**
* @author Theodore Kruczek.
* @license MIT
* @copyright (c) 2022-2025 Theodore Kruczek Permission is
* hereby granted, free of charge, to any person obtaining a copy of this
* software and associated documentation files (the "Software"), to deal in the
* Software without restriction, including without limitation the rights to use,
* copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do
* so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
import { AngularDistanceMethod } from '../enums/AngularDistanceMethod.js';
import { Vector3D } from '../operations/Vector3D.js';
import { DEG2RAD, RAD2DEG, TAU } from '../utils/constants.js';
import { angularDistance } from '../utils/functions.js';
import { radecToPosition, radecToVelocity } from './ObservationUtils.js';
/**
* Represents a topocentric right ascension and declination observation.
*
* Topocentric coordinates take into account the observer's exact location on the Earth's surface. This model is crucial
* for precise measurements of local astronomical events and nearby celestial objects, where the observer's latitude,
* longitude, and altitude can significantly affect the observed position due to parallax. Topocentric coordinates are
* particularly important for observations of the Moon, planets, and artificial satellites.
*/
export class RadecTopocentric {
epoch;
rightAscension;
declination;
range;
rightAscensionRate;
declinationRate;
rangeRate;
constructor(epoch, rightAscension, declination, range, rightAscensionRate, declinationRate, rangeRate) {
this.epoch = epoch;
this.rightAscension = rightAscension;
this.declination = declination;
this.range = range;
this.rightAscensionRate = rightAscensionRate;
this.declinationRate = declinationRate;
this.rangeRate = rangeRate;
// Nothing to do here.
}
/**
* Create a new RadecTopocentric object, using degrees for the angular values.
* @param epoch UTC epoch.
* @param rightAscensionDegrees Right-ascension in degrees.
* @param declinationDegrees Declination in degrees.
* @param range Range in km.
* @param rightAscensionRateDegrees Right-ascension rate in degrees per second.
* @param declinationRateDegrees Declination rate in degrees per second.
* @param rangeRate Range rate in km/s.
* @returns A new RadecTopocentric object.
*/
static fromDegrees(epoch, rightAscensionDegrees, declinationDegrees, range, rightAscensionRateDegrees, declinationRateDegrees, rangeRate) {
const rightAscensionRate = rightAscensionRateDegrees
? rightAscensionRateDegrees * DEG2RAD
: null;
const declinationRate = declinationRateDegrees
? declinationRateDegrees * DEG2RAD
: null;
return new RadecTopocentric(epoch, rightAscensionDegrees * DEG2RAD, declinationDegrees * DEG2RAD, range, rightAscensionRate, declinationRate, rangeRate);
}
/**
* Create a new RadecTopocentric object from a J2000 state vector.
* @param state Inertial state vector.
* @param site Site vector.
* @returns A new RadecTopocentric object.
*/
static fromStateVector(state, site) {
const p = state.position.subtract(site.position);
const pI = p.x;
const pJ = p.y;
const pK = p.z;
const pMag = p.magnitude();
const declination = Math.asin(pK / pMag);
const pDot = state.velocity.subtract(site.velocity);
const pIDot = pDot.x;
const pJDot = pDot.y;
const pKDot = pDot.z;
const pIJMag = Math.sqrt(pI * pI + pJ * pJ);
let rightAscension;
if (pIJMag !== 0) {
rightAscension = Math.atan2(pJ, pI);
}
else {
rightAscension = Math.atan2(pJDot, pIDot);
}
const rangeRate = p.dot(pDot) / pMag;
const rightAscensionRate = (pIDot * pJ - pJDot * pI) / (-(pJ * pJ) - pI * pI);
const declinationRate = (pKDot - rangeRate * Math.sin(declination)) / pIJMag;
return new RadecTopocentric(state.epoch, rightAscension % TAU, declination, pMag, rightAscensionRate, declinationRate, rangeRate);
}
/**
* Gets the right ascension in degrees.
* @returns The right ascension in degrees.
*/
get rightAscensionDegrees() {
return this.rightAscension * RAD2DEG;
}
/**
* Gets the declination in degrees.
* @returns The declination in degrees.
*/
get declinationDegrees() {
return this.declination * RAD2DEG;
}
/**
* Gets the right ascension rate in degrees per second.
* @returns The right ascension rate in degrees per second, or null if it is not available.
*/
get rightAscensionRateDegrees() {
return this.rightAscensionRate
? this.rightAscensionRate * RAD2DEG
: null;
}
/**
* Gets the rate of change of declination in degrees per second.
* @returns The rate of change of declination in degrees per second, or null if the declination rate is not defined.
*/
get declinationRateDegrees() {
return this.declinationRate
? this.declinationRate * RAD2DEG
: null;
}
/**
* Return the position relative to the observer site.
*
* An optional range value can be passed to override the value contained in this observation.
* @param site Observer site.
* @param range Range in km.
* @returns A Vector3D object.
*/
position(site, range) {
const r = range ?? this.range ?? 1.0;
return radecToPosition(this.rightAscension, this.declination, r).add(site.position);
}
/**
* Return the velocity relative to the observer site.
*
* An optional range and rangeRate value can be passed to override the values contained in this observation.
* @param site Observer site.
* @param range Range in km.
* @param rangeRate Range rate in km/s.
* @returns A Vector3D object.
*/
velocity(site, range, rangeRate) {
if (!this.rightAscensionRate || !this.declinationRate) {
throw new Error('Velocity unsolvable, missing ra/dec rates.');
}
const r = range ?? this.range ?? 1.0;
const rd = rangeRate ?? this.rangeRate ?? 0.0;
return radecToVelocity(this.rightAscension, this.declination, r, this.rightAscensionRate, this.declinationRate, rd).add(site.velocity);
}
/**
* Calculates the line of sight vector in the topocentric coordinate system.
* The line of sight vector points from the observer's location towards the celestial object.
* @returns The line of sight vector as a Vector3D object.
*/
lineOfSight() {
const ca = Math.cos(this.rightAscension);
const cd = Math.cos(this.declination);
const sa = Math.sin(this.rightAscension);
const sd = Math.sin(this.declination);
return new Vector3D(cd * ca, cd * sa, sd);
}
/**
* Calculate the angular distance between this and another RadecTopocentric object.
* @param radec - The other RadecTopocentric object.
* @param method - The angular distance method to use.
* @returns The angular distance.
*/
angle(radec, method = AngularDistanceMethod.Cosine) {
return angularDistance(this.rightAscension, this.declination, radec.rightAscension, radec.declination, method);
}
/**
* Calculate the angular distance between this and another RadecTopocentric object.
* @param radec - The other RadecTopocentric object.
* @param method - The angular distance method to use.
* @returns The angular distance
*/
angleDegrees(radec, method = AngularDistanceMethod.Cosine) {
return this.angle(radec, method) * RAD2DEG;
}
}
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