FieldAngularSeparationDetector.java

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package org.orekit.propagation.events;

import org.hipparchus.CalculusFieldElement;
import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
import org.hipparchus.ode.events.Action;
import org.orekit.bodies.CelestialBodies;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.events.handlers.FieldEventHandler;
import org.orekit.propagation.events.handlers.FieldStopOnEvent;
import org.orekit.utils.ExtendedPositionProvider;

/** 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
 * {@code #withHandler(EventHandler)} after construction.</p>
 * @see org.orekit.propagation.Propagator#addEventDetector(EventDetector)
 * @author Luc Maisonobe
 * @author Romain Serra
 * @see AngularSeparationDetector
 * @since 13.1
 */
public class FieldAngularSeparationDetector<T extends CalculusFieldElement<T>>
        extends FieldAbstractDetector<FieldAngularSeparationDetector<T>, T> {

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

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

    /** Proximity angle (rad). */
    private final T 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 FieldAngularSeparationDetector(final ExtendedPositionProvider beacon,
                                          final ExtendedPositionProvider observer,
                                          final T proximityAngle) {
        this(new FieldEventDetectionSettings<>(proximityAngle.getField(), AngularSeparationDetector.DEFAULT_SETTINGS),
                new FieldStopOnEvent<>(), 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 FieldAngularSeparationDetector(final FieldEventDetectionSettings<T> detectionSettings,
                                             final FieldEventHandler<T> handler,
                                             final ExtendedPositionProvider beacon,
                                             final ExtendedPositionProvider observer,
                                             final T proximityAngle) {
        super(detectionSettings, handler);
        this.beacon         = beacon;
        this.observer       = observer;
        this.proximityAngle = proximityAngle;
    }

    /** {@inheritDoc} */
    @Override
    protected FieldAngularSeparationDetector<T> create(final FieldEventDetectionSettings<T> detectionSettings,
                                                       final FieldEventHandler<T> newHandler) {
        return new FieldAngularSeparationDetector<>(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 ExtendedPositionProvider getBeacon() {
        return beacon;
    }

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

    /** Get the proximity angle (rad).
     * @return the proximity angle
     */
    public T 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 T g(final FieldSpacecraftState<T> s) {
        final FieldVector3D<T> sPos = s.getPosition();
        final FieldVector3D<T> bP = beacon.getPosition(s.getDate(), s.getFrame());
        final FieldVector3D<T> oP = observer.getPosition(s.getDate(), s.getFrame());
        final T separation = FieldVector3D.angle(sPos.subtract(oP), bP.subtract(oP));
        return separation.subtract(proximityAngle);
    }

}