AlignmentDetector.java

/* Copyright 2002-2025 CS GROUP
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 * CS licenses this file to You under the Apache License, Version 2.0
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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.hipparchus.util.SinCos;
import org.orekit.orbits.Orbit;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;
import org.orekit.utils.PVCoordinates;
import org.orekit.utils.PVCoordinatesProvider;

/** Finder for satellite/body alignment events in orbital plane.
 * <p>This class finds alignment events.</p>
 * <p>Alignment means the conjunction, with some threshold angle, between the satellite
 * position and the projection in the orbital plane of some body position.</p>
 * <p>The default handler behavior is to {@link Action#STOP stop}
 * propagation when alignment is reached. This can be changed by calling
 * {@link #withHandler(EventHandler)} after construction.</p>
 * @see org.orekit.propagation.Propagator#addEventDetector(EventDetector)
 * @author Pascal Parraud
 */
public class AlignmentDetector extends AbstractDetector<AlignmentDetector> {

    /** Body to align. */
    private final PVCoordinatesProvider body;

    /** Alignment angle (rad). */
    private final double alignAngle;

    /** Cosinus of alignment angle. */
    private final double cosAlignAngle;

    /** Sinus of alignment angle. */
    private final double sinAlignAngle;

    /** Build a new alignment detector.
     * <p>The orbit is used only to set an upper bound for the max check interval
     * to period/3 and to set the convergence threshold according to orbit size.</p>
     * @param orbit initial orbit
     * @param body the body to align
     * @param alignAngle the alignment angle (rad)
     */
    public AlignmentDetector(final Orbit orbit,
                             final PVCoordinatesProvider body,
                             final double alignAngle) {
        this(1.0e-13 * orbit.getKeplerianPeriod(), orbit, body, alignAngle);
    }

    /** Build a new alignment detector.
     * @param maxCheck maximum checking interval (s)
     * @param threshold convergence threshold (s)
     * @param body the body to align
     * @param alignAngle the alignment angle (rad)
     */
    public AlignmentDetector(final double maxCheck, final double threshold,
                             final PVCoordinatesProvider body,
                             final double alignAngle) {
        this(new EventDetectionSettings(maxCheck, threshold, EventDetectionSettings.DEFAULT_MAX_ITER),
             new StopOnIncreasing(), body, alignAngle);
    }

    /** Build a new alignment detector.
     * @param detectionSettings detection settings
     * @param body the body to align
     * @param alignAngle the alignment angle (rad)
     * @since 13.0
     */
    public AlignmentDetector(final EventDetectionSettings detectionSettings,
                             final PVCoordinatesProvider body,
                             final double alignAngle) {
        this(detectionSettings, new StopOnIncreasing(), body, alignAngle);
    }

    /** Build a new alignment detector.
     * <p>The orbit is used only to set an upper bound for the max check interval
     * to period/3.</p>
     * @param threshold convergence threshold (s)
     * @param orbit initial orbit
     * @param body the body to align
     * @param alignAngle the alignment angle (rad)
     */
    public AlignmentDetector(final double threshold,
                             final Orbit orbit,
                             final PVCoordinatesProvider body,
                             final double alignAngle) {
        this(orbit.getKeplerianPeriod() / 3, threshold, body, alignAngle);
    }

    /** Protected constructor with full parameters.
     * <p>
     * This constructor is not public 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 detectionSettings detection settings
     * @param handler event handler to call at event occurrences
     * @param body the body to align
     * @param alignAngle the alignment angle (rad)
     * @since 13.0
     */
    protected AlignmentDetector(final EventDetectionSettings detectionSettings, final EventHandler handler,
                                final PVCoordinatesProvider body,
                                final double alignAngle) {
        super(detectionSettings, handler);
        final SinCos sc    = FastMath.sinCos(alignAngle);
        this.body          = body;
        this.alignAngle    = alignAngle;
        this.cosAlignAngle = sc.cos();
        this.sinAlignAngle = sc.sin();
    }

    /** {@inheritDoc} */
    @Override
    protected AlignmentDetector create(final EventDetectionSettings detectionSettings, final EventHandler newHandler) {
        return new AlignmentDetector(detectionSettings, newHandler, body, alignAngle);
    }

    /** Get the body to align.
     * @return the body to align
     */
    public PVCoordinatesProvider getPVCoordinatesProvider() {
        return body;
    }

    /** Get the alignment angle (rad).
     * @return the alignment angle
     */
    public double getAlignAngle() {
        return alignAngle;
    }

    /** Compute the value of the switching function.
     * This function measures the difference between the alignment angle and the
     * angle between the satellite position and the body position projection in the
     * orbital plane.
     * @param s the current state information: date, kinematics, attitude
     * @return value of the switching function
     */
    public double g(final SpacecraftState s) {
        final PVCoordinates pv = s.getPVCoordinates();
        final Vector3D a  = pv.getPosition().normalize();
        final Vector3D b  = Vector3D.crossProduct(pv.getMomentum(), a).normalize();
        final Vector3D x  = new Vector3D(cosAlignAngle, a,  sinAlignAngle, b);
        final Vector3D y  = new Vector3D(sinAlignAngle, a, -cosAlignAngle, b);
        final Vector3D pb = body.getPosition(s.getDate(), s.getFrame());
        final double beta = FastMath.atan2(Vector3D.dotProduct(pb, y), Vector3D.dotProduct(pb, x));
        final double betm = -FastMath.PI - beta;
        final double betp =  FastMath.PI - beta;
        if (beta < betm) {
            return betm;
        } else if (beta < betp) {
            return beta;
        } else {
            return betp;
        }
    }

}