PosVelChebyshev.java
/* Copyright 2002-2022 CS GROUP
* Licensed to CS GROUP (CS) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
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package org.orekit.bodies;
import java.io.Serializable;
import org.hipparchus.CalculusFieldElement;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.time.TimeScale;
import org.orekit.time.TimeStamped;
import org.orekit.utils.FieldPVCoordinates;
import org.orekit.utils.PVCoordinates;
/** Position-Velocity model based on Chebyshev polynomials.
* <p>This class represent the most basic element of the piecewise ephemerides
* for solar system bodies like JPL DE 405 ephemerides.</p>
* @see JPLEphemeridesLoader
* @author Luc Maisonobe
*/
class PosVelChebyshev implements TimeStamped, Serializable {
/** Serializable UID. */
private static final long serialVersionUID = 20151023L;
/** Time scale in which the ephemeris is defined. */
private final TimeScale timeScale;
/** Start of the validity range of the instance. */
private final AbsoluteDate start;
/** Duration of validity range of the instance. */
private final double duration;
/** Chebyshev polynomials coefficients for the X component. */
private final double[] xCoeffs;
/** Chebyshev polynomials coefficients for the Y component. */
private final double[] yCoeffs;
/** Chebyshev polynomials coefficients for the Z component. */
private final double[] zCoeffs;
/** Simple constructor.
* @param start start of the validity range of the instance
* @param timeScale time scale in which the ephemeris is defined
* @param duration duration of the validity range of the instance
* @param xCoeffs Chebyshev polynomials coefficients for the X component
* (a reference to the array will be stored in the instance)
* @param yCoeffs Chebyshev polynomials coefficients for the Y component
* (a reference to the array will be stored in the instance)
* @param zCoeffs Chebyshev polynomials coefficients for the Z component
* (a reference to the array will be stored in the instance)
*/
PosVelChebyshev(final AbsoluteDate start, final TimeScale timeScale, final double duration,
final double[] xCoeffs, final double[] yCoeffs, final double[] zCoeffs) {
this.start = start;
this.timeScale = timeScale;
this.duration = duration;
this.xCoeffs = xCoeffs;
this.yCoeffs = yCoeffs;
this.zCoeffs = zCoeffs;
}
/** {@inheritDoc} */
public AbsoluteDate getDate() {
return start;
}
/** Check if a date is in validity range.
* @param date date to check
* @return true if date is in validity range
*/
public boolean inRange(final AbsoluteDate date) {
final double dt = date.offsetFrom(start, timeScale);
return dt >= -0.001 && dt <= duration + 0.001;
}
/** Get the position-velocity-acceleration at a specified date.
* @param date date at which position-velocity-acceleration is requested
* @return position-velocity-acceleration at specified date
*/
public PVCoordinates getPositionVelocityAcceleration(final AbsoluteDate date) {
// normalize date
final double t = (2 * date.offsetFrom(start, timeScale) - duration) / duration;
final double twoT = 2 * t;
// initialize Chebyshev polynomials recursion
double pKm1 = 1;
double pK = t;
double xP = xCoeffs[0];
double yP = yCoeffs[0];
double zP = zCoeffs[0];
// initialize Chebyshev polynomials derivatives recursion
double qKm1 = 0;
double qK = 1;
double xV = 0;
double yV = 0;
double zV = 0;
// initialize Chebyshev polynomials second derivatives recursion
double rKm1 = 0;
double rK = 0;
double xA = 0;
double yA = 0;
double zA = 0;
// combine polynomials by applying coefficients
for (int k = 1; k < xCoeffs.length; ++k) {
// consider last computed polynomials on position
xP += xCoeffs[k] * pK;
yP += yCoeffs[k] * pK;
zP += zCoeffs[k] * pK;
// consider last computed polynomials on velocity
xV += xCoeffs[k] * qK;
yV += yCoeffs[k] * qK;
zV += zCoeffs[k] * qK;
// consider last computed polynomials on acceleration
xA += xCoeffs[k] * rK;
yA += yCoeffs[k] * rK;
zA += zCoeffs[k] * rK;
// compute next Chebyshev polynomial value
final double pKm2 = pKm1;
pKm1 = pK;
pK = twoT * pKm1 - pKm2;
// compute next Chebyshev polynomial derivative
final double qKm2 = qKm1;
qKm1 = qK;
qK = twoT * qKm1 + 2 * pKm1 - qKm2;
// compute next Chebyshev polynomial second derivative
final double rKm2 = rKm1;
rKm1 = rK;
rK = twoT * rKm1 + 4 * qKm1 - rKm2;
}
final double vScale = 2 / duration;
final double aScale = vScale * vScale;
return new PVCoordinates(new Vector3D(xP, yP, zP),
new Vector3D(xV * vScale, yV * vScale, zV * vScale),
new Vector3D(xA * aScale, yA * aScale, zA * aScale));
}
/** Get the position-velocity-acceleration at a specified date.
* @param date date at which position-velocity-acceleration is requested
* @param <T> type fo the field elements
* @return position-velocity-acceleration at specified date
*/
public <T extends CalculusFieldElement<T>> FieldPVCoordinates<T> getPositionVelocityAcceleration(final FieldAbsoluteDate<T> date) {
final T zero = date.getField().getZero();
final T one = date.getField().getOne();
// normalize date
final T t = date.offsetFrom(new FieldAbsoluteDate<>(date.getField(), start), timeScale).multiply(2).subtract(duration).divide(duration);
final T twoT = t.add(t);
// initialize Chebyshev polynomials recursion
T pKm1 = one;
T pK = t;
T xP = zero.add(xCoeffs[0]);
T yP = zero.add(yCoeffs[0]);
T zP = zero.add(zCoeffs[0]);
// initialize Chebyshev polynomials derivatives recursion
T qKm1 = zero;
T qK = one;
T xV = zero;
T yV = zero;
T zV = zero;
// initialize Chebyshev polynomials second derivatives recursion
T rKm1 = zero;
T rK = zero;
T xA = zero;
T yA = zero;
T zA = zero;
// combine polynomials by applying coefficients
for (int k = 1; k < xCoeffs.length; ++k) {
// consider last computed polynomials on position
xP = xP.add(pK.multiply(xCoeffs[k]));
yP = yP.add(pK.multiply(yCoeffs[k]));
zP = zP.add(pK.multiply(zCoeffs[k]));
// consider last computed polynomials on velocity
xV = xV.add(qK.multiply(xCoeffs[k]));
yV = yV.add(qK.multiply(yCoeffs[k]));
zV = zV.add(qK.multiply(zCoeffs[k]));
// consider last computed polynomials on acceleration
xA = xA.add(rK.multiply(xCoeffs[k]));
yA = yA.add(rK.multiply(yCoeffs[k]));
zA = zA.add(rK.multiply(zCoeffs[k]));
// compute next Chebyshev polynomial value
final T pKm2 = pKm1;
pKm1 = pK;
pK = twoT.multiply(pKm1).subtract(pKm2);
// compute next Chebyshev polynomial derivative
final T qKm2 = qKm1;
qKm1 = qK;
qK = twoT.multiply(qKm1).add(pKm1.multiply(2)).subtract(qKm2);
// compute next Chebyshev polynomial second derivative
final T rKm2 = rKm1;
rKm1 = rK;
rK = twoT.multiply(rKm1).add(qKm1.multiply(4)).subtract(rKm2);
}
final double vScale = 2 / duration;
final double aScale = vScale * vScale;
return new FieldPVCoordinates<>(new FieldVector3D<>(xP, yP, zP),
new FieldVector3D<>(xV.multiply(vScale), yV.multiply(vScale), zV.multiply(vScale)),
new FieldVector3D<>(xA.multiply(aScale), yA.multiply(aScale), zA.multiply(aScale)));
}
}