CelestialBodyPointed.java
/* Copyright 2002-2020 CS GROUP
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* 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
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*
* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.orekit.attitudes;
import org.hipparchus.Field;
import org.hipparchus.RealFieldElement;
import org.hipparchus.geometry.euclidean.threed.FieldRotation;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.geometry.euclidean.threed.Rotation;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.frames.FieldTransform;
import org.orekit.frames.Frame;
import org.orekit.frames.Transform;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.FieldPVCoordinates;
import org.orekit.utils.FieldPVCoordinatesProvider;
import org.orekit.utils.PVCoordinates;
import org.orekit.utils.PVCoordinatesProvider;
/**
* This class handles a celestial body pointed attitude provider.
* <p>The celestial body pointed law is defined by two main elements:
* <ul>
* <li>a celestial body towards which some satellite axis is exactly aimed</li>
* <li>a phasing reference defining the rotation around the pointing axis</li>
* </ul>
*
* <p>
* The celestial body implicitly defines two of the three degrees of freedom
* and the phasing reference defines the remaining degree of freedom. This definition
* can be represented as first aligning exactly the satellite pointing axis to
* the current direction of the celestial body, and then to find the rotation
* around this axis such that the satellite phasing axis is in the half-plane
* defined by a cut line on the pointing axis and containing the celestial
* phasing reference.
* </p>
* <p>
* In order for this definition to work, the user must ensure that the phasing
* reference is <strong>never</strong> aligned with the pointing reference.
* Since the pointed body moves as the date changes, this should be ensured
* regardless of the date. A simple way to do this for Sun, Moon or any planet
* pointing is to choose a phasing reference far from the ecliptic plane. Using
* <code>Vector3D.PLUS_K</code>, the equatorial pole, is perfect in these cases.
* </p>
* <p>Instances of this class are guaranteed to be immutable.</p>
* @author Luc Maisonobe
*/
public class CelestialBodyPointed implements AttitudeProvider {
/** Frame in which {@link #phasingCel} is defined. */
private final Frame celestialFrame;
/** Celestial body to point at. */
private final PVCoordinatesProvider pointedBody;
/** Phasing reference, in celestial frame. */
private final Vector3D phasingCel;
/** Satellite axis aiming at the pointed body, in satellite frame. */
private final Vector3D pointingSat;
/** Phasing reference, in satellite frame. */
private final Vector3D phasingSat;
/** Creates new instance.
* @param celestialFrame frame in which <code>phasingCel</code> is defined
* @param pointedBody celestial body to point at
* @param phasingCel phasing reference, in celestial frame
* @param pointingSat satellite vector defining the pointing direction
* @param phasingSat phasing reference, in satellite frame
*/
public CelestialBodyPointed(final Frame celestialFrame,
final PVCoordinatesProvider pointedBody,
final Vector3D phasingCel,
final Vector3D pointingSat,
final Vector3D phasingSat) {
this.celestialFrame = celestialFrame;
this.pointedBody = pointedBody;
this.phasingCel = phasingCel;
this.pointingSat = pointingSat;
this.phasingSat = phasingSat;
}
/** {@inheritDoc} */
public Attitude getAttitude(final PVCoordinatesProvider pvProv,
final AbsoluteDate date, final Frame frame) {
final PVCoordinates satPV = pvProv.getPVCoordinates(date, celestialFrame);
// compute celestial references at the specified date
final PVCoordinates bodyPV = pointedBody.getPVCoordinates(date, celestialFrame);
final PVCoordinates pointing = new PVCoordinates(satPV, bodyPV);
final Vector3D pointingP = pointing.getPosition();
final double r2 = Vector3D.dotProduct(pointingP, pointingP);
// evaluate instant rotation axis due to sat and body motion only (no phasing yet)
final Vector3D rotAxisCel =
new Vector3D(1 / r2, Vector3D.crossProduct(pointingP, pointing.getVelocity()));
// fix instant rotation to take phasing constraint into account
// (adding a rotation around pointing axis ensuring the motion of the phasing axis
// is constrained in the pointing-phasing plane)
final Vector3D v1 = Vector3D.crossProduct(rotAxisCel, phasingCel);
final Vector3D v2 = Vector3D.crossProduct(pointingP, phasingCel);
final double compensation = -Vector3D.dotProduct(v1, v2) / v2.getNormSq();
final Vector3D phasedRotAxisCel = new Vector3D(1.0, rotAxisCel, compensation, pointingP);
// compute transform from celestial frame to satellite frame
final Rotation celToSatRotation =
new Rotation(pointingP, phasingCel, pointingSat, phasingSat);
// build transform combining rotation and instant rotation axis
Transform transform = new Transform(date, celToSatRotation, celToSatRotation.applyTo(phasedRotAxisCel));
if (frame != celestialFrame) {
// prepend transform from specified frame to celestial frame
transform = new Transform(date, frame.getTransformTo(celestialFrame, date), transform);
}
// build the attitude
return new Attitude(date, frame, transform.getRotation(), transform.getRotationRate(), transform.getRotationAcceleration());
}
/** {@inheritDoc} */
public <T extends RealFieldElement<T>> FieldAttitude<T> getAttitude(final FieldPVCoordinatesProvider<T> pvProv,
final FieldAbsoluteDate<T> date,
final Frame frame) {
final Field<T> field = date.getField();
final FieldPVCoordinates<T> satPV = pvProv.getPVCoordinates(date, celestialFrame);
// compute celestial references at the specified date
final FieldPVCoordinates<T> bodyPV = new FieldPVCoordinates<>(field,
pointedBody.getPVCoordinates(date.toAbsoluteDate(),
celestialFrame));
final FieldPVCoordinates<T> pointing = new FieldPVCoordinates<>(satPV, bodyPV);
final FieldVector3D<T> pointingP = pointing.getPosition();
final T r2 = FieldVector3D.dotProduct(pointingP, pointingP);
// evaluate instant rotation axis due to sat and body motion only (no phasing yet)
final FieldVector3D<T> rotAxisCel =
new FieldVector3D<>(r2.reciprocal(), FieldVector3D.crossProduct(pointingP, pointing.getVelocity()));
// fix instant rotation to take phasing constraint into account
// (adding a rotation around pointing axis ensuring the motion of the phasing axis
// is constrained in the pointing-phasing plane)
final FieldVector3D<T> v1 = FieldVector3D.crossProduct(rotAxisCel, phasingCel);
final FieldVector3D<T> v2 = FieldVector3D.crossProduct(pointingP, phasingCel);
final T compensation = FieldVector3D.dotProduct(v1, v2).negate().divide(v2.getNormSq());
final FieldVector3D<T> phasedRotAxisCel = new FieldVector3D<>(field.getOne(), rotAxisCel, compensation, pointingP);
// compute transform from celestial frame to satellite frame
final FieldRotation<T> celToSatRotation =
new FieldRotation<>(pointingP, new FieldVector3D<>(field, phasingCel),
new FieldVector3D<>(field, pointingSat), new FieldVector3D<>(field, phasingSat));
// build transform combining rotation and instant rotation axis
FieldTransform<T> transform = new FieldTransform<>(date, celToSatRotation, celToSatRotation.applyTo(phasedRotAxisCel));
if (frame != celestialFrame) {
// prepend transform from specified frame to celestial frame
transform = new FieldTransform<>(date, frame.getTransformTo(celestialFrame, date), transform);
}
// build the attitude
return new FieldAttitude<>(date, frame,
transform.getRotation(), transform.getRotationRate(), transform.getRotationAcceleration());
}
}