MagneticFieldDetector

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package org.orekit.propagation.events;

import org.orekit.annotation.DefaultDataContext;
import org.orekit.bodies.GeodeticPoint;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.data.DataContext;
import org.orekit.errors.OrekitIllegalArgumentException;
import org.orekit.models.earth.GeoMagneticField;
import org.orekit.models.earth.GeoMagneticFieldFactory.FieldModel;
import org.orekit.orbits.OrbitType;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.TimeScale;

/** Detector for South-Atlantic anomaly frontier crossing.
 * <p>
 * The detector is based on the value of the earth magnetic field at see level at the satellite latitude and longitude.
 * </p>
 * @author Romaric Her
 */
public class MagneticFieldDetector extends AbstractDetector<MagneticFieldDetector> {

    /** Fixed threshold value of Magnetic field to be crossed. */
    private final double limit;

    /** Fixed altitude of computed magnetic field value. */
    private final boolean seaLevel;

    /** earth geomagnetic field. */
    private GeoMagneticField field;

    /** year of the current state. */
    private double currentYear;

    /** the geomagnetic field model enum. */
    private final FieldModel type;

    /** the body. */
    private final OneAxisEllipsoid body;

    /** the timescale. */
    private final DataContext dataContext;


    /** Build a new detector.
     * <p>The new instance uses default values for maximal checking interval
     * ({@link #DEFAULT_MAXCHECK}) and convergence threshold ({@link
     * #DEFAULT_THRESHOLD}).</p>
     *
     * <p>This method uses the {@link DataContext#getDefault() default data context}.
     *
     * @param limit the threshold value of magnetic field at see level
     * @param type the magnetic field model
     * @param body the body
     * @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
     * @see #MagneticFieldDetector(double, double, double, GeoMagneticFieldFactory.FieldModel, OneAxisEllipsoid, boolean, DataContext)
     */
    @DefaultDataContext
    public MagneticFieldDetector(final double limit, final FieldModel type, final OneAxisEllipsoid body)
        throws OrekitIllegalArgumentException {
        this(DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, limit, type, body, false);
    }

    /** Build a new detector.
     * <p>The new instance uses default values for maximal checking interval
     * ({@link #DEFAULT_MAXCHECK}) and convergence threshold ({@link
     * #DEFAULT_THRESHOLD}).</p>
     *
     * <p>This method uses the {@link DataContext#getDefault() default data context}.
     *
     * @param limit the threshold value of magnetic field at see level
     * @param type the magnetic field model
     * @param body the body
     * @param seaLevel true if the magnetic field intensity is computed at the sea level, false if it is computed at satellite altitude
     * @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
     * @see #MagneticFieldDetector(double, double, double, GeoMagneticFieldFactory.FieldModel, OneAxisEllipsoid, boolean, DataContext)
     */
    @DefaultDataContext
    public MagneticFieldDetector(final double limit, final FieldModel type, final OneAxisEllipsoid body, final boolean seaLevel)
        throws OrekitIllegalArgumentException {
        this(DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, limit, type, body, seaLevel);
    }

    /** Build a detector.
     *
     * <p>This method uses the {@link DataContext#getDefault() default data context}.
     *
     * @param maxCheck maximal checking interval (s)
     * @param threshold convergence threshold (s)
     * @param limit the threshold value of magnetic field at see level
     * @param type the magnetic field model
     * @param body the body
     * @param seaLevel true if the magnetic field intensity is computed at the sea level, false if it is computed at satellite altitude
     * @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
     * @see #MagneticFieldDetector(double, double, double, GeoMagneticFieldFactory.FieldModel, OneAxisEllipsoid, boolean, DataContext)
     */
    @DefaultDataContext
    public MagneticFieldDetector(final double maxCheck, final double threshold, final double limit,
                                 final FieldModel type, final OneAxisEllipsoid body, final boolean seaLevel)
        throws OrekitIllegalArgumentException {
        this(maxCheck, threshold, limit, type, body, seaLevel,
                DataContext.getDefault());
    }

    /**
     * Build a detector.
     *
     * @param maxCheck    maximal checking interval (s)
     * @param threshold   convergence threshold (s)
     * @param limit       the threshold value of magnetic field at see level
     * @param type        the magnetic field model
     * @param body        the body
     * @param seaLevel    true if the magnetic field intensity is computed at the sea
     *                    level, false if it is computed at satellite altitude
     * @param dataContext used to look up the magnetic field model.
     * @throws OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
     * @since 10.1
     */
    public MagneticFieldDetector(final double maxCheck,
                                 final double threshold,
                                 final double limit,
                                 final FieldModel type,
                                 final OneAxisEllipsoid body,
                                 final boolean seaLevel,
                                 final DataContext dataContext)
        throws OrekitIllegalArgumentException {
        this(maxCheck, threshold, DEFAULT_MAX_ITER, new StopOnIncreasing<>(),
             limit, type, body, seaLevel, dataContext);
    }

    /** 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 limit the threshold value of magnetic field at see level
     * @param type the magnetic field model
     * @param body the body
     * @param seaLevel true if the magnetic field intensity is computed at the sea level, false if it is computed at satellite altitude
     * @param dataContext used to look up the magnetic field model.
     * @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
     */
    private MagneticFieldDetector(final double maxCheck, final double threshold,
                                  final int maxIter, final EventHandler<? super MagneticFieldDetector> handler,
                                  final double limit, final FieldModel type, final OneAxisEllipsoid body, final boolean seaLevel,
                                  final DataContext dataContext)
        throws OrekitIllegalArgumentException {

        super(maxCheck, threshold, maxIter, handler);

        this.limit = limit;
        this.type = type;
        this.body = body;
        this.seaLevel = seaLevel;
        this.dataContext = dataContext;
    }

    /** {@inheritDoc} */
    @Override
    protected MagneticFieldDetector create(final double newMaxCheck, final double newThreshold,
                                           final int newMaxIter, final EventHandler<? super MagneticFieldDetector> newHandler) {
        return new MagneticFieldDetector(newMaxCheck, newThreshold, newMaxIter, newHandler,
                                         limit, type, body, seaLevel, dataContext);
    }

    /** {@inheritDoc} */
    public void init(final SpacecraftState s0, final AbsoluteDate t) {
        super.init(s0, t);
        final TimeScale utc = dataContext.getTimeScales().getUTC();
        this.currentYear = s0.getDate().getComponents(utc).getDate().getYear();
        this.field = dataContext.getGeoMagneticFields().getField(type, currentYear);
    }

    /** Compute the value of the detection function.
     * <p>
     * The value is the angle difference between the spacecraft and the fixed
     * angle to be crossed, with some sign tweaks to ensure continuity.
     * These tweaks imply the {@code increasing} flag in events detection becomes
     * irrelevant here! As an example, the angle always increase in a Keplerian
     * orbit, but this g function will increase and decrease so it
     * will cross the zero value once per orbit, in increasing and decreasing
     * directions on alternate orbits..
     * </p>
     * @param s the current state information: date, kinematics, attitude
     * @return angle difference between the spacecraft and the fixed
     * angle, with some sign tweaks to ensure continuity
     */
    public double g(final SpacecraftState s) {
        final TimeScale utc = dataContext.getTimeScales().getUTC();
        if (s.getDate().getComponents(utc).getDate().getYear() != currentYear) {
            this.currentYear = s.getDate().getComponents(utc).getDate().getYear();
            this.field = dataContext.getGeoMagneticFields().getField(type, currentYear);
        }
        final GeodeticPoint geoPoint = body.transform(s.getPVCoordinates().getPosition(), s.getFrame(), s.getDate());
        final double altitude;
        if (seaLevel) {
            altitude = 0;
        }
        else {
            altitude = geoPoint.getAltitude();
        }
        final double value = field.calculateField(geoPoint.getLatitude(), geoPoint.getLongitude(), altitude).getTotalIntensity();
        return value - limit;
    }

}