SingleBodyAbsoluteAttraction.java
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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.bodies.CelestialBodies;
import org.orekit.bodies.CelestialBody;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.SpacecraftState;
/** Body attraction force model computed as absolute acceleration towards a body.
* <p>
* This force model represents the same physical principles as {@link NewtonianAttraction},
* but has several major differences:
* </p>
* <ul>
* <li>the attracting body can be <em>away</em> from the integration frame center,</li>
* <li>several instances of this force model can be added when several bodies are involved,</li>
* <li>this force model is <em>never</em> automatically added by the numerical propagator</li>
* </ul>
* <p>
* The possibility for the attracting body to be away from the frame center allows to use this force
* model when integrating for example an interplanetary trajectory propagated in an Earth centered
* frame (in which case an instance of {@link org.orekit.forces.inertia.InertialForces} must also be
* added to take into account the coupling effect of relative frames motion).
* </p>
* <p>
* The possibility to add several instances allows to use this in interplanetary trajectories or
* in trajectories about Lagrangian points
* </p>
* <p>
* The fact this force model is <em>never</em> automatically added by the numerical propagator differs
* from {@link NewtonianAttraction} as {@link NewtonianAttraction} may be added automatically when
* propagating a trajectory represented as an {@link org.orekit.orbits.Orbit}, which must always refer
* to a central body, if user did not add the {@link NewtonianAttraction} or set the central attraction
* coefficient by himself.
* </p>
* @see org.orekit.forces.inertia.InertialForces
* @author Luc Maisonobe
* @author Julio Hernanz
*/
public class SingleBodyAbsoluteAttraction extends AbstractBodyAttraction {
/** Simple constructor.
* @param body the body to consider
* (ex: {@link CelestialBodies#getSun()} or
* {@link CelestialBodies#getMoon()})
*/
public SingleBodyAbsoluteAttraction(final CelestialBody body) {
super(body);
}
/** {@inheritDoc} */
@Override
public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
// compute bodies separation vectors and squared norm
final Vector3D bodyPosition = getBody().getPosition(s.getDate(), s.getFrame());
final Vector3D satToBody = bodyPosition.subtract(s.getPosition());
final double r2Sat = satToBody.getNormSq();
// compute absolute acceleration
return new Vector3D(parameters[0] / (r2Sat * FastMath.sqrt(r2Sat)), satToBody);
}
/** {@inheritDoc} */
@Override
public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
final T[] parameters) {
// compute bodies separation vectors and squared norm
final FieldVector3D<T> centralToBody = getBody().getPosition(s.getDate(), s.getFrame());
final FieldVector3D<T> satToBody = centralToBody.subtract(s.getPosition());
final T r2Sat = satToBody.getNormSq();
// compute absolute acceleration
return new FieldVector3D<>(parameters[0].divide(r2Sat.multiply(r2Sat.sqrt())), satToBody);
}
}