EclipseDetector

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package org.orekit.propagation.events;

import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.ode.events.Action;
import org.hipparchus.util.FastMath;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;
import org.orekit.utils.PVCoordinatesProvider;

/** Finder for satellite eclipse related events.
 * <p>This class finds eclipse events, i.e. satellite within umbra (total
 * eclipse) or penumbra (partial eclipse).</p>
 * <p>The occulted body is given through a {@link PVCoordinatesProvider} and its radius in meters. It is modeled as a sphere.
 * </p>
 * <p>Since v10.0 the occulting body is a {@link OneAxisEllipsoid}, before it was modeled as a  sphere.
 * <br>It was changed to precisely model Solar eclipses by the Earth, especially for Low Earth Orbits.
 * <br>If you want eclipses by a spherical occulting body, set its flattening to 0. when defining its OneAxisEllipsoid model..
 * </p>
 * <p>The {@link #withUmbra} or {@link #withPenumbra} methods will tell you if the event is triggered when complete umbra/lighting
 * is achieved or when entering/living the penumbra zone.
 * <br>The default behavior is detecting complete umbra/lighting events.
 * <br>If you want to have both, you'll need to set up two distinct detectors.
 * </p>
 * <p>The default implementation behavior is to {@link Action#CONTINUE continue}
 * propagation when entering the eclipse and to {@link Action#STOP stop} propagation
 * when exiting the eclipse.
 * <br>This can be changed by calling {@link #withHandler(EventHandler)} after construction.
 * </p>
 * @see org.orekit.propagation.Propagator#addEventDetector(EventDetector)
 * @author Pascal Parraud
 * @author Luc Maisonobe
 */
public class EclipseDetector extends AbstractDetector<EclipseDetector> {

    /** Occulting body. */
    private final OneAxisEllipsoid occulting;

    /** Occulted body. */
    private final PVCoordinatesProvider occulted;

    /** Occulted body radius (m). */
    private final double occultedRadius;

    /** Umbra, if true, or penumbra, if false, detection flag. */
    private final boolean totalEclipse;

    /** Build a new eclipse detector.
     * <p>The new instance is a total eclipse (umbra) detector with default
     * values for maximal checking interval ({@link #DEFAULT_MAXCHECK})
     * and convergence threshold ({@link #DEFAULT_THRESHOLD}).</p>
     * @param occulted the body to be occulted
     * @param occultedRadius the radius of the body to be occulted (m)
     * @param occulting the occulting body
     * @since 10.0
     */
    public EclipseDetector(final PVCoordinatesProvider occulted,  final double occultedRadius,
                           final OneAxisEllipsoid occulting) {
        this(DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, DEFAULT_MAX_ITER,
             new StopOnIncreasing<EclipseDetector>(),
             occulted, occultedRadius, occulting, true);
    }

    /** Private constructor with full parameters.
     * <p>
     * This constructor is private as users are expected to use the builder
     * API with the various {@code withXxx()} methods to set up the instance
     * in a readable manner without using a huge amount of parameters.
     * </p>
     * @param maxCheck maximum checking interval (s)
     * @param threshold convergence threshold (s)
     * @param maxIter maximum number of iterations in the event time search
     * @param handler event handler to call at event occurrences
     * @param occulted the body to be occulted
     * @param occultedRadius the radius of the body to be occulted in meters
     * @param occulting the occulting body
     * @param totalEclipse umbra (true) or penumbra (false) detection flag
     * @since 10.0
     */
    private EclipseDetector(final double maxCheck, final double threshold,
                            final int maxIter, final EventHandler<? super EclipseDetector> handler,
                            final PVCoordinatesProvider occulted,  final double occultedRadius,
                            final OneAxisEllipsoid occulting, final boolean totalEclipse) {
        super(maxCheck, threshold, maxIter, handler);
        this.occulted       = occulted;
        this.occultedRadius = FastMath.abs(occultedRadius);
        this.occulting      = occulting;
        this.totalEclipse   = totalEclipse;
    }

    /** {@inheritDoc} */
    @Override
    protected EclipseDetector create(final double newMaxCheck, final double newThreshold,
                                     final int nawMaxIter, final EventHandler<? super EclipseDetector> newHandler) {
        return new EclipseDetector(newMaxCheck, newThreshold, nawMaxIter, newHandler,
                                   occulted, occultedRadius, occulting, totalEclipse);
    }

    /**
     * Setup the detector to full umbra detection.
     * <p>
     * This will override a penumbra/umbra flag if it has been configured previously.
     * </p>
     * @return a new detector with updated configuration (the instance is not changed)
     * @see #withPenumbra()
     * @since 6.1
     */
    public EclipseDetector withUmbra() {
        return new EclipseDetector(getMaxCheckInterval(), getThreshold(), getMaxIterationCount(), getHandler(),
                                   occulted, occultedRadius, occulting, true);
    }

    /**
     * Setup the detector to penumbra detection.
     * <p>
     * This will override a penumbra/umbra flag if it has been configured previously.
     * </p>
     * @return a new detector with updated configuration (the instance is not changed)
     * @see #withUmbra()
     * @since 6.1
     */
    public EclipseDetector withPenumbra() {
        return new EclipseDetector(getMaxCheckInterval(), getThreshold(), getMaxIterationCount(), getHandler(),
                                   occulted, occultedRadius, occulting, false);
    }

    /** Getter for the occulting body.
     * @return the occulting body
     */
    public OneAxisEllipsoid getOcculting() {
        return occulting;
    }

    /** Getter for the occulted body.
     * @return the occulted body
     */
    public PVCoordinatesProvider getOcculted() {
        return occulted;
    }

    /** Getter for the occultedRadius.
     * @return the occultedRadius
     */
    public double getOccultedRadius() {
        return occultedRadius;
    }

    /** Get the total eclipse detection flag.
     * @return the total eclipse detection flag (true for umbra events detection,
     * false for penumbra events detection)
     */
    public boolean getTotalEclipse() {
        return totalEclipse;
    }

    /** Compute the value of the switching function.
     * This function becomes negative when entering the region of shadow
     * and positive when exiting.
     * @param s the current state information: date, kinematics, attitude
     * @return value of the switching function
     */
    public double g(final SpacecraftState s) {
        final Vector3D pted  = occulted.getPVCoordinates(s.getDate(), occulting.getBodyFrame()).getPosition();
        final Vector3D psat  = s.getPVCoordinates(occulting.getBodyFrame()).getPosition();
        final Vector3D plimb = occulting.pointOnLimb(psat, pted);
        final Vector3D ps    = psat.subtract(pted);
        final Vector3D pi    = psat.subtract(plimb);
        final double angle   = Vector3D.angle(ps, psat);
        final double rs      = FastMath.asin(occultedRadius / ps.getNorm());
        if (Double.isNaN(rs)) {
            return FastMath.PI;
        }
        final double ro = Vector3D.angle(pi, psat);
        return totalEclipse ? (angle - ro + rs) : (angle - ro - rs);
    }
}