ThirdBodyAttraction.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;

/** Third body attraction force model.
 *
 * @author Fabien Maussion
 * @author Véronique Pommier-Maurussane
 */
public class ThirdBodyAttraction extends AbstractBodyAttraction {

    /** Simple constructor.
     * @param body the third body to consider
     * (ex: {@link CelestialBodies#getSun()} or
     * {@link CelestialBodies#getMoon()})
     */
    public ThirdBodyAttraction(final CelestialBody body) {
        super(body);
    }

    /** {@inheritDoc} */
    @Override
    public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {

        final double gm = parameters[0];

        // compute bodies separation vectors and squared norm
        final Vector3D centralToBody = getBody().getPosition(s.getDate(), s.getFrame());
        final double r2Central       = centralToBody.getNormSq();
        final Vector3D satToBody     = centralToBody.subtract(s.getPosition());
        final double r2Sat           = satToBody.getNormSq();

        // compute relative acceleration
        return new Vector3D(gm / (r2Sat * FastMath.sqrt(r2Sat)), satToBody,
                           -gm / (r2Central * FastMath.sqrt(r2Central)), centralToBody);

    }

    /** {@inheritDoc} */
    @Override
    public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
                                                                             final T[] parameters) {

        final T gm = parameters[0];

        // compute bodies separation vectors and squared norm
        final FieldVector3D<T> centralToBody = getBody().getPosition(s.getDate(), s.getFrame());
        final T                r2Central     = centralToBody.getNormSq();
        final FieldVector3D<T> satToBody     = centralToBody.subtract(s.getPosition());
        final T                r2Sat         = satToBody.getNormSq();

        // compute relative acceleration
        return new FieldVector3D<>(r2Sat.multiply(r2Sat.sqrt()).reciprocal().multiply(gm), satToBody,
                                   r2Central.multiply(r2Central.sqrt()).reciprocal().multiply(gm).negate(), centralToBody);

    }

}