LongitudeCrossingDetector.java
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package org.orekit.propagation.events;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.MathUtils;
import org.orekit.bodies.GeodeticPoint;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.ContinueOnEvent;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;
import org.orekit.time.AbsoluteDate;
/** Detector for geographic longitude crossing.
* <p>This detector identifies when a spacecraft crosses a fixed
* longitude with respect to a central body.</p>
* @author Luc Maisonobe
* @since 7.1
*/
public class LongitudeCrossingDetector extends AbstractDetector<LongitudeCrossingDetector> {
/** Body on which the longitude is defined. */
private OneAxisEllipsoid body;
/** Fixed longitude to be crossed. */
private final double longitude;
/** Filtering detector. */
private final EventEnablingPredicateFilter filtering;
/** 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>
* @param body body on which the longitude is defined
* @param longitude longitude to be crossed
*/
public LongitudeCrossingDetector(final OneAxisEllipsoid body, final double longitude) {
this(DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, body, longitude);
}
/** Build a detector.
* @param maxCheck maximal checking interval (s)
* @param threshold convergence threshold (s)
* @param body body on which the longitude is defined
* @param longitude longitude to be crossed
*/
public LongitudeCrossingDetector(final double maxCheck, final double threshold,
final OneAxisEllipsoid body, final double longitude) {
this(s -> maxCheck, threshold, DEFAULT_MAX_ITER, new StopOnIncreasing(),
body, longitude);
}
/** 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 maxCheck maximum checking interval
* @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 body body on which the longitude is defined
* @param longitude longitude to be crossed
*/
protected LongitudeCrossingDetector(final AdaptableInterval maxCheck, final double threshold,
final int maxIter, final EventHandler handler,
final OneAxisEllipsoid body, final double longitude) {
super(maxCheck, threshold, maxIter, handler);
this.body = body;
this.longitude = longitude;
// we filter out spurious longitude crossings occurring at the antimeridian
final RawLongitudeCrossingDetector raw = new RawLongitudeCrossingDetector(maxCheck, threshold, maxIter,
new ContinueOnEvent());
final EnablingPredicate predicate =
(state, detector, g) -> FastMath.abs(g) < 0.5 * FastMath.PI;
this.filtering = new EventEnablingPredicateFilter(raw, predicate);
}
/** {@inheritDoc} */
@Override
protected LongitudeCrossingDetector create(final AdaptableInterval newMaxCheck, final double newThreshold, final int newMaxIter,
final EventHandler newHandler) {
return new LongitudeCrossingDetector(newMaxCheck, newThreshold, newMaxIter, newHandler,
body, longitude);
}
/** Get the body on which the geographic zone is defined.
* @return body on which the geographic zone is defined
*/
public OneAxisEllipsoid getBody() {
return body;
}
/** Get the fixed longitude to be crossed (radians).
* @return fixed longitude to be crossed (radians)
*/
public double getLongitude() {
return longitude;
}
/** {@inheritDoc} */
public void init(final SpacecraftState s0, final AbsoluteDate t) {
filtering.init(s0, t);
}
/** Compute the value of the detection function.
* <p>
* The value is the longitude difference between the spacecraft and the fixed
* longitude 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 longitude of a prograde spacecraft
* will always increase, 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. If eastwards and westwards crossing have to
* be distinguished, the velocity direction has to be checked instead of looking
* at the {@code increasing} flag.
* </p>
* @param s the current state information: date, kinematics, attitude
* @return longitude difference between the spacecraft and the fixed
* longitude, with some sign tweaks to ensure continuity
*/
public double g(final SpacecraftState s) {
return filtering.g(s);
}
private class RawLongitudeCrossingDetector extends AbstractDetector<RawLongitudeCrossingDetector> {
/** 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 maxCheck maximum checking interval
* @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
*/
protected RawLongitudeCrossingDetector(final AdaptableInterval maxCheck, final double threshold, final int maxIter,
final EventHandler handler) {
super(maxCheck, threshold, maxIter, handler);
}
/** {@inheritDoc} */
@Override
protected RawLongitudeCrossingDetector create(final AdaptableInterval newMaxCheck, final double newThreshold,
final int newMaxIter,
final EventHandler newHandler) {
return new RawLongitudeCrossingDetector(newMaxCheck, newThreshold, newMaxIter, newHandler);
}
/** Compute the value of the detection function.
* <p>
* The value is the longitude difference between the spacecraft and the fixed
* longitude to be crossed, and it <em>does</em> change sign twice around
* the central body: once at expected longitude and once at antimeridian.
* The second sign change is a spurious one and is filtered out by the
* outer class.
* </p>
* @param s the current state information: date, kinematics, attitude
* @return longitude difference between the spacecraft and the fixed
* longitude
*/
public double g(final SpacecraftState s) {
// convert state to geodetic coordinates
final GeodeticPoint gp = body.transform(s.getPosition(),
s.getFrame(), s.getDate());
// longitude difference
return MathUtils.normalizeAngle(gp.getLongitude() - longitude, 0.0);
}
}
}