J2OnlyPerturbation.java
/* Copyright 2022-2024 Romain Serra
* 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,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.orekit.forces.gravity;
import org.hipparchus.CalculusFieldElement;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.util.FastMath;
import org.orekit.forces.ForceModel;
import org.orekit.forces.gravity.potential.UnnormalizedSphericalHarmonicsProvider;
import org.orekit.frames.FieldStaticTransform;
import org.orekit.frames.Frame;
import org.orekit.frames.StaticTransform;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.time.TimeScalarFunction;
import org.orekit.utils.ParameterDriver;
import java.util.Collections;
import java.util.List;
/** J2-only force model.
* This class models the oblateness part alone of the central body's potential (degree 2 and order 0),
* whilst avoiding the computational overhead of generic NxM spherical harmonics.
*
* <p>
* This J2 coefficient has same magnitude and opposite sign than the so-called unnormalized C20 coefficient.
* </p>
*
* <p>
* This class should not be used in combination of {@link HolmesFeatherstoneAttractionModel},
* otherwise the J2 term would be taken into account twice.
* </p>
*
* @author Romain Serra
*/
public class J2OnlyPerturbation implements ForceModel {
/** Central body's gravitational constant. */
private final double mu;
/** Central body's equatorial radius. */
private final double rEq;
/** Central body's J2 coefficient as a function of time. */
private final TimeScalarFunction j2OverTime;
/** Frame where J2 applies. */
private final Frame frame;
/** Constructor with {@link TimeScalarFunction}.
* It is the user's responsibility to make sure the Field and double versions are consistent with each other.
* @param mu central body's gravitational constant
* @param rEq central body's equatorial radius
* @param j2OverTime J2 coefficient as a function of time.
* @param frame frame where J2 applies
*/
public J2OnlyPerturbation(final double mu, final double rEq, final TimeScalarFunction j2OverTime,
final Frame frame) {
this.mu = mu;
this.rEq = rEq;
this.j2OverTime = j2OverTime;
this.frame = frame;
}
/** Constructor with constant J2.
* @param mu central body gravitational constant
* @param rEq central body's equatorial radius
* @param constantJ2 constant J2 coefficient
* @param frame frame where J2 applies
*/
public J2OnlyPerturbation(final double mu, final double rEq, final double constantJ2, final Frame frame) {
this.mu = mu;
this.rEq = rEq;
this.frame = frame;
this.j2OverTime = new TimeScalarFunction() {
@Override
public double value(final AbsoluteDate date) {
return constantJ2;
}
@Override
public <T extends CalculusFieldElement<T>> T value(final FieldAbsoluteDate<T> date) {
return date.getField().getZero().newInstance(constantJ2);
}
};
}
/** Constructor with spherical harmonics provider.
* @param harmonicsProvider spherical harmonics provider of unnormalized coefficients
* @param frame frame where J2 applies
*/
public J2OnlyPerturbation(final UnnormalizedSphericalHarmonicsProvider harmonicsProvider, final Frame frame) {
this.mu = harmonicsProvider.getMu();
this.rEq = harmonicsProvider.getAe();
this.frame = frame;
this.j2OverTime = new TimeScalarFunction() {
@Override
public double value(final AbsoluteDate date) {
return -harmonicsProvider.getUnnormalizedC20(date);
}
@Override
public <T extends CalculusFieldElement<T>> T value(final FieldAbsoluteDate<T> date) {
return date.getField().getZero().newInstance(value(date.toAbsoluteDate()));
}
};
}
/** Getter for mu.
* @return mu
*/
public double getMu() {
return mu;
}
/** Getter for equatorial radius.
* @return equatorial radius
*/
public double getrEq() {
return rEq;
}
/** Getter for frame.
* @return frame
*/
public Frame getFrame() {
return frame;
}
/** Return J2 at requested date.
* @param date epoch at which J2 coefficient should be retrieved
* @return J2 coefficient
*/
public double getJ2(final AbsoluteDate date) {
return j2OverTime.value(date);
}
/** Return J2 at requested date (Field version).
* @param <T> field
* @param date epoch at which J2 coefficient should be retrieved
* @return J2 coefficient
*/
public <T extends CalculusFieldElement<T>> T getJ2(final FieldAbsoluteDate<T> date) {
return j2OverTime.value(date);
}
/** {@inheritDoc} */
@Override
public boolean dependsOnPositionOnly() {
return true;
}
/** {@inheritDoc} */
@Override
public Vector3D acceleration(final SpacecraftState state, final double[] parameters) {
final Vector3D positionInJ2Frame = state.getPosition(frame);
final double squaredRadius = positionInJ2Frame.getNormSq();
final double squaredZ = positionInJ2Frame.getZ() * positionInJ2Frame.getZ();
final double ratioTimesFive = 5. * squaredZ / squaredRadius;
final double ratioTimesFiveMinusOne = ratioTimesFive - 1.;
final double accelerationX = positionInJ2Frame.getX() * ratioTimesFiveMinusOne;
final double accelerationY = positionInJ2Frame.getY() * ratioTimesFiveMinusOne;
final double accelerationZ = positionInJ2Frame.getZ() * (ratioTimesFive - 3);
final Vector3D accelerationInJ2Frame = new Vector3D(accelerationX, accelerationY, accelerationZ);
final AbsoluteDate date = state.getDate();
final StaticTransform fromJ2FrameToPropagationOne = frame.getTransformTo(state.getFrame(), date);
final Vector3D transformedAcceleration = fromJ2FrameToPropagationOne.transformVector(accelerationInJ2Frame);
final double j2 = j2OverTime.value(date);
final double squaredRadiiRatio = rEq * rEq / squaredRadius;
final double cubedRadius = squaredRadius * FastMath.sqrt(squaredRadius);
final double factor = 3 * j2 * mu * squaredRadiiRatio / (2 * cubedRadius);
return transformedAcceleration.scalarMultiply(factor);
}
/** {@inheritDoc} */
@Override
public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> state,
final T[] parameters) {
final FieldVector3D<T> positionInJ2Frame = state.getPosition(frame);
final T squaredRadius = positionInJ2Frame.getNormSq();
final T squaredZ = positionInJ2Frame.getZ().square();
final T ratioTimesFive = squaredZ.multiply(5.).divide(squaredRadius);
final T ratioTimesFiveMinusOne = ratioTimesFive.subtract(1.);
final T accelerationX = positionInJ2Frame.getX().multiply(ratioTimesFiveMinusOne);
final T accelerationY = positionInJ2Frame.getY().multiply(ratioTimesFiveMinusOne);
final T accelerationZ = positionInJ2Frame.getZ().multiply(ratioTimesFive.subtract(3.));
final FieldVector3D<T> accelerationInJ2Frame = new FieldVector3D<>(accelerationX, accelerationY, accelerationZ);
final FieldAbsoluteDate<T> date = state.getDate();
final FieldStaticTransform<T> fromJ2FrameToPropagationOne = frame.getTransformTo(state.getFrame(), date);
final FieldVector3D<T> transformedAcceleration = fromJ2FrameToPropagationOne.transformVector(accelerationInJ2Frame);
final T j2 = j2OverTime.value(date);
final T squaredRadiiRatio = squaredRadius.reciprocal().multiply(rEq * rEq);
final T cubedRadius = squaredRadius.multiply(FastMath.sqrt(squaredRadius));
final T factor = j2.multiply(mu).multiply(3.).multiply(squaredRadiiRatio).divide(cubedRadius.multiply(2));
return transformedAcceleration.scalarMultiply(factor);
}
/** {@inheritDoc} */
@Override
public List<ParameterDriver> getParametersDrivers() {
return Collections.emptyList();
}
}