LatitudeExtremumDetector.java

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package org.orekit.propagation.events;

import org.hipparchus.analysis.differentiation.UnivariateDerivative2;
import org.orekit.bodies.BodyShape;
import org.orekit.bodies.FieldGeodeticPoint;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;

/** Detector for geographic latitude extremum.
 * <p>This detector identifies when a spacecraft reaches its
 * extremum latitudes with respect to a central body.</p>
 * @author Luc Maisonobe
 * @since 7.1
 */
public class LatitudeExtremumDetector extends AbstractDetector<LatitudeExtremumDetector> {

    /** Body on which the latitude is defined. */
    private OneAxisEllipsoid body;

    /** Build a new detector.
     * <p>The new instance uses default values for maximal checking interval
     * ({@link #DEFAULT_MAX_CHECK}) and convergence threshold ({@link
     * #DEFAULT_THRESHOLD}).</p>
     * @param body body on which the latitude is defined
     */
    public LatitudeExtremumDetector(final OneAxisEllipsoid body) {
        this(DEFAULT_MAX_CHECK, DEFAULT_THRESHOLD, body);
    }

    /** Build a detector.
     * @param maxCheck maximal checking interval (s)
     * @param threshold convergence threshold (s)
     * @param body body on which the latitude is defined
     */
    public LatitudeExtremumDetector(final double maxCheck, final double threshold,
                                    final OneAxisEllipsoid body) {
        this(new EventDetectionSettings(maxCheck, threshold, DEFAULT_MAX_ITER), new StopOnIncreasing(), body);
    }

    /** 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 event detection settings
     * @param handler event handler to call at event occurrences
     * @param body body on which the latitude is defined
     */
    protected LatitudeExtremumDetector(final EventDetectionSettings detectionSettings, final EventHandler handler,
                                       final OneAxisEllipsoid body) {
        super(detectionSettings, handler);
        this.body = body;
    }

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

    /** Get the body on which the geographic zone is defined.
     * @return body on which the geographic zone is defined
     */
    public BodyShape getBody() {
        return body;
    }

    /** Compute the value of the detection function.
     * <p>
     * The value is the spacecraft latitude time derivative.
     * </p>
     * @param s the current state information: date, kinematics, attitude
     * @return spacecraft latitude time derivative
     */
    public double g(final SpacecraftState s) {

        // convert state to geodetic coordinates
        final FieldGeodeticPoint<UnivariateDerivative2> gp =
                        body.transform(s.getPVCoordinates(), s.getFrame(), s.getDate());

        // latitude time derivative
        return gp.getLatitude().getFirstDerivative();

    }

}