AngularSeparationDetector.java

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 * contributor license agreements.  See the NOTICE file distributed with
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 * CS licenses this file to You under the Apache License, Version 2.0
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 *
 *   http://www.apache.org/licenses/LICENSE-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.orekit.bodies.CelestialBodies;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnDecreasing;
import org.orekit.utils.PVCoordinatesProvider;

/** Detects when spacecraft comes close to a moving beacon, as seen from a moving observer.
 * <p>The main use case for this detector is when the observer is in fact a ground
 * station, modeled as a {@link org.orekit.frames.TopocentricFrame} and when the beacon
 * is the {@link CelestialBodies#getSun() Sun}, for computing
 * interferences for the telemetry link. Another similar case is when the beacon is
 * another spacecraft, for interferences computation.</p>
 * <p>The default handler behavior is to {@link Action#STOP stop}
 * propagation when spacecraft enters the proximity zone. This can be changed by calling
 * {@link #withHandler(EventHandler)} after construction.</p>
 * @see org.orekit.propagation.Propagator#addEventDetector(EventDetector)
 * @author Luc Maisonobe
 * @since 8.0
 */
public class AngularSeparationDetector extends AbstractDetector<AngularSeparationDetector> {

    /** Default detection settings. */
    public static final EventDetectionSettings DEFAULT_SETTINGS = new EventDetectionSettings(60., 1e-3, EventDetectionSettings.DEFAULT_MAX_ITER);

    /** Beacon at the center of the proximity zone. */
    private final PVCoordinatesProvider beacon;

    /** Observer for the spacecraft, that may also see the beacon at the same time if they are too close. */
    private final PVCoordinatesProvider observer;

    /** Proximity angle (rad). */
    private final double proximityAngle;

    /** Build a new angular separation detector.
     * @param beacon beacon at the center of the proximity zone
     * @param observer observer for the spacecraft, that may also see
     * the beacon at the same time if they are too close to each other
     * @param proximityAngle proximity angle as seen from observer, at which events are triggered (rad)
     */
    public AngularSeparationDetector(final PVCoordinatesProvider beacon,
                                     final PVCoordinatesProvider observer,
                                     final double proximityAngle) {
        this(DEFAULT_SETTINGS, new StopOnDecreasing(), beacon, observer, proximityAngle);
    }

    /** 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 beacon beacon at the center of the proximity zone
     * @param observer observer for the spacecraft, that may also see
     * the beacon at the same time if they are too close to each other
     * @param proximityAngle proximity angle as seen from observer, at which events are triggered (rad)
     * @since 13.0
     */
    protected AngularSeparationDetector(final EventDetectionSettings detectionSettings,
                                        final EventHandler handler,
                                        final PVCoordinatesProvider beacon,
                                        final PVCoordinatesProvider observer,
                                        final double proximityAngle) {
        super(detectionSettings, handler);
        this.beacon         = beacon;
        this.observer       = observer;
        this.proximityAngle = proximityAngle;
    }

    /** {@inheritDoc} */
    @Override
    protected AngularSeparationDetector create(final EventDetectionSettings detectionSettings, final EventHandler newHandler) {
        return new AngularSeparationDetector(detectionSettings, newHandler,
                                             beacon, observer, proximityAngle);
    }

    /** Get the beacon at the center of the proximity zone.
     * @return beacon at the center of the proximity zone
     */
    public PVCoordinatesProvider getBeacon() {
        return beacon;
    }

    /** Get the observer for the spacecraft.
     * @return observer for the spacecraft
     */
    public PVCoordinatesProvider getObserver() {
        return observer;
    }

    /** Get the proximity angle (rad).
     * @return the proximity angle
     */
    public double getProximityAngle() {
        return proximityAngle;
    }

    /** Compute the value of the switching function.
     * <p>
     * This function measures the angular separation between beacon and spacecraft
     * as seen from the observer minus the proximity angle. It therefore triggers
     * decreasing events when the spacecraft enters the proximity zone and increasing
     * events when it leaves the proximity zone.
     * </p>
     * <p>
     * No shadowing effect is taken into account, so this method is computed and
     * may trigger events even when the spacecraft is below horizon for an observer
     * which is a ground station. If such effects must be taken into account the
     * detector must be associated with a {@link EventEnablingPredicateFilter predicate
     * filter} where the {@link EnablingPredicate predicate function} is based on elevation.
     * </p>
     * @param s the current state information: date, kinematics, attitude
     * @return value of the switching function
     */
    public double g(final SpacecraftState s) {
        final Vector3D sPosition = s.getPosition();
        final Vector3D bP = beacon.getPosition(s.getDate(), s.getFrame());
        final Vector3D oP = observer.getPosition(s.getDate(), s.getFrame());
        final double separation = Vector3D.angle(sPosition.subtract(oP), bP.subtract(oP));
        return separation - proximityAngle;
    }

}