SingleBodyRelativeAttraction.java

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package org.orekit.forces.gravity;

import java.util.Collections;
import java.util.List;

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.forces.ForceModel;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.SpacecraftState;
import org.orekit.utils.FieldPVCoordinates;
import org.orekit.utils.PVCoordinates;
import org.orekit.utils.ParameterDriver;

/** Body attraction force model computed as relative acceleration towards frame center.
 * @author Luc Maisonabe
 * @author Julio Hernanz
 */
public class SingleBodyRelativeAttraction implements ForceModel {

    /** Suffix for parameter name for attraction coefficient enabling Jacobian processing. */
    public static final String ATTRACTION_COEFFICIENT_SUFFIX = " attraction coefficient";

    /** Central attraction scaling factor.
     * <p>
     * We use a power of 2 to avoid numeric noise introduction
     * in the multiplications/divisions sequences.
     * </p>
     */
    private static final double MU_SCALE = FastMath.scalb(1.0, 32);

    /** Drivers for body attraction coefficient. */
    private final ParameterDriver gmDriver;

    /** The body to consider. */
    private final CelestialBody body;

    /** Simple constructor.
     * @param body the body to consider
     * (ex: {@link CelestialBodies#getSun()} or
     * {@link CelestialBodies#getMoon()})
     */
    public SingleBodyRelativeAttraction(final CelestialBody body) {
        gmDriver = new ParameterDriver(body.getName() + ATTRACTION_COEFFICIENT_SUFFIX,
                                       body.getGM(), MU_SCALE,
                                       0.0, Double.POSITIVE_INFINITY);

        this.body = body;
    }

    /** {@inheritDoc} */
    @Override
    public boolean dependsOnPositionOnly() {
        return true;
    }

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

        // compute bodies separation vectors and squared norm
        final PVCoordinates bodyPV   = body.getPVCoordinates(s.getDate(), s.getFrame());
        final Vector3D satToBody     = bodyPV.getPosition().subtract(s.getPosition());
        final double r2Sat           = satToBody.getNormSq();

        // compute relative acceleration
        final double gm = parameters[0];
        final double a = gm / r2Sat;
        return new Vector3D(a, satToBody.normalize()).add(bodyPV.getAcceleration());

    }

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

        // compute bodies separation vectors and squared norm
        final FieldPVCoordinates<T> bodyPV = body.getPVCoordinates(s.getDate(), s.getFrame());
        final FieldVector3D<T> satToBody   = bodyPV.getPosition().subtract(s.getPosition());
        final T                r2Sat       = satToBody.getNormSq();

        // compute relative acceleration
        final T gm = parameters[0];
        final T a  = gm.divide(r2Sat);
        return new FieldVector3D<>(a, satToBody.normalize()).add(bodyPV.getAcceleration());

    }

    /** {@inheritDoc} */
    public List<ParameterDriver> getParametersDrivers() {
        return Collections.singletonList(gmDriver);
    }

}