/* Copyright 2002-2017 CS Systèmes d'Information
* Licensed to CS Systèmes d'Information (CS) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.orekit.propagation.events;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.MathUtils;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.bodies.GeodeticPoint;
import org.orekit.errors.OrekitException;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;
/** 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> {
/** Serializable UID. */
private static final long serialVersionUID = 20150824L;
/** Body on which the longitude is defined. */
private OneAxisEllipsoid body;
/** Fixed longitude to be crossed. */
private final double longitude;
/** Sign to apply for longitude difference. */
private double sign;
/** Previous longitude difference. */
private double previousDelta;
/** 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(maxCheck, threshold, DEFAULT_MAX_ITER, new StopOnIncreasing<LongitudeCrossingDetector>(),
body, longitude);
}
/** 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 body body on which the longitude is defined
* @param longitude longitude to be crossed
*/
private LongitudeCrossingDetector(final double maxCheck, final double threshold,
final int maxIter, final EventHandler<? super LongitudeCrossingDetector> handler,
final OneAxisEllipsoid body, final double longitude) {
super(maxCheck, threshold, maxIter, handler);
this.body = body;
this.longitude = longitude;
this.sign = +1.0;
this.previousDelta = Double.NaN;
}
/** {@inheritDoc} */
@Override
protected LongitudeCrossingDetector create(final double newMaxCheck, final double newThreshold,
final int newMaxIter,
final EventHandler<? super LongitudeCrossingDetector> 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;
}
/** 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
* @exception OrekitException if some specific error occurs
*/
public double g(final SpacecraftState s) throws OrekitException {
// convert state to geodetic coordinates
final GeodeticPoint gp = body.transform(s.getPVCoordinates().getPosition(),
s.getFrame(), s.getDate());
// longitude difference
double delta = MathUtils.normalizeAngle(sign * (gp.getLongitude() - longitude), 0.0);
// ensure continuity
if (FastMath.abs(delta - previousDelta) > FastMath.PI) {
sign = -sign;
delta = MathUtils.normalizeAngle(sign * (gp.getLongitude() - longitude), 0.0);
}
previousDelta = delta;
return delta;
}
}