PositionAngleDetector.java
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* CS licenses this file to You under the Apache License, Version 2.0
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package org.orekit.propagation.events;
import java.util.function.Function;
import org.hipparchus.analysis.UnivariateFunction;
import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
import org.hipparchus.util.FastMath;
import org.hipparchus.util.MathUtils;
import org.orekit.errors.OrekitIllegalArgumentException;
import org.orekit.errors.OrekitMessages;
import org.orekit.orbits.CircularOrbit;
import org.orekit.orbits.EquinoctialOrbit;
import org.orekit.orbits.KeplerianOrbit;
import org.orekit.orbits.Orbit;
import org.orekit.orbits.OrbitType;
import org.orekit.orbits.PositionAngleType;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnEvent;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.TimeSpanMap;
/** Detector for in-orbit position angle.
* <p>
* The detector is based on anomaly for {@link OrbitType#KEPLERIAN Keplerian}
* orbits, latitude argument for {@link OrbitType#CIRCULAR circular} orbits,
* or longitude argument for {@link OrbitType#EQUINOCTIAL equinoctial} orbits.
* It does not support {@link OrbitType#CARTESIAN Cartesian} orbits. The
* angles can be either {@link PositionAngleType#TRUE true}, {@link PositionAngleType#MEAN
* mean} or {@link PositionAngleType#ECCENTRIC eccentric} angles.
* </p>
* @author Luc Maisonobe
* @since 7.1
*/
public class PositionAngleDetector extends AbstractDetector<PositionAngleDetector> {
/** Orbit type defining the angle type. */
private final OrbitType orbitType;
/** Type of position angle. */
private final PositionAngleType positionAngleType;
/** Fixed angle to be crossed. */
private final double angle;
/** Position angle extraction function. */
private final Function<Orbit, Double> positionAngleExtractor;
/** Estimators for the offset angle, taking care of 2π wrapping and g function continuity. */
private TimeSpanMap<OffsetEstimator> offsetEstimators;
/** 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 orbitType orbit type defining the angle type
* @param positionAngleType type of position angle
* @param angle fixed angle to be crossed
* @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
*/
public PositionAngleDetector(final OrbitType orbitType, final PositionAngleType positionAngleType,
final double angle)
throws OrekitIllegalArgumentException {
this(DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, orbitType, positionAngleType, angle);
}
/** Build a detector.
* <p> This instance uses by default the {@link StopOnEvent} handler </p>
* @param maxCheck maximal checking interval (s)
* @param threshold convergence threshold (s)
* @param orbitType orbit type defining the angle type
* @param positionAngleType type of position angle
* @param angle fixed angle to be crossed
* @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
*/
public PositionAngleDetector(final double maxCheck, final double threshold,
final OrbitType orbitType, final PositionAngleType positionAngleType,
final double angle)
throws OrekitIllegalArgumentException {
this(new EventDetectionSettings(maxCheck, threshold, DEFAULT_MAX_ITER), new StopOnEvent(),
orbitType, positionAngleType, angle);
}
/** 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 orbitType orbit type defining the angle type
* @param positionAngleType type of position angle
* @param angle fixed angle to be crossed
* @exception OrekitIllegalArgumentException if orbit type is {@link OrbitType#CARTESIAN}
* @since 13.0
*/
protected PositionAngleDetector(final EventDetectionSettings detectionSettings, final EventHandler handler,
final OrbitType orbitType, final PositionAngleType positionAngleType,
final double angle)
throws OrekitIllegalArgumentException {
super(detectionSettings, handler);
this.orbitType = orbitType;
this.positionAngleType = positionAngleType;
this.angle = angle;
this.offsetEstimators = null;
switch (orbitType) {
case KEPLERIAN:
positionAngleExtractor = o -> ((KeplerianOrbit) orbitType.convertType(o)).getAnomaly(positionAngleType);
break;
case CIRCULAR:
positionAngleExtractor = o -> ((CircularOrbit) orbitType.convertType(o)).getAlpha(positionAngleType);
break;
case EQUINOCTIAL:
positionAngleExtractor = o -> ((EquinoctialOrbit) orbitType.convertType(o)).getL(positionAngleType);
break;
default:
final String sep = ", ";
throw new OrekitIllegalArgumentException(OrekitMessages.ORBIT_TYPE_NOT_ALLOWED,
orbitType,
OrbitType.KEPLERIAN + sep +
OrbitType.CIRCULAR + sep +
OrbitType.EQUINOCTIAL);
}
}
/** {@inheritDoc} */
@Override
protected PositionAngleDetector create(final EventDetectionSettings detectionSettings,
final EventHandler newHandler) {
return new PositionAngleDetector(detectionSettings, newHandler, orbitType, positionAngleType, angle);
}
/** Get the orbit type defining the angle type.
* @return orbit type defining the angle type
*/
public OrbitType getOrbitType() {
return orbitType;
}
/** Get the type of position angle.
* @return type of position angle
*/
public PositionAngleType getPositionAngleType() {
return positionAngleType;
}
/** Get the fixed angle to be crossed (radians).
* @return fixed angle to be crossed (radians)
*/
public double getAngle() {
return angle;
}
/** {@inheritDoc} */
@Override
public void init(final SpacecraftState s0, final AbsoluteDate t) {
super.init(s0, t);
offsetEstimators = new TimeSpanMap<>(new OffsetEstimator(s0.getOrbit(), +1.0));
}
/** 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 Orbit orbit = s.getOrbit();
// angle difference
OffsetEstimator estimator = offsetEstimators.get(s.getDate());
double delta = estimator.delta(orbit);
// we use a value greater than π for handover in order to avoid
// several switches to be estimated as the calling propagator
// and Orbit.shiftedBy have different accuracy. It is sufficient
// to have a handover roughly opposite to the detected position angle
while (FastMath.abs(delta) >= 3.5) {
// we are too far away from the current estimator, we need to set up a new one
// ensuring that we do have a crossing event in the current orbit
// and we ensure sign continuity with the current estimator
// find when the previous estimator becomes invalid
final AbsoluteDate handover = estimator.dateForOffset(FastMath.copySign(FastMath.PI, delta), orbit);
// perform handover to a new estimator at this date
estimator = new OffsetEstimator(orbit, delta);
delta = estimator.delta(orbit);
if (isForward()) {
offsetEstimators.addValidAfter(estimator, handover.getDate(), false);
} else {
offsetEstimators.addValidBefore(estimator, handover.getDate(), false);
}
}
return delta;
}
/** Local class for estimating offset angle, handling 2π wrap-up and sign continuity. */
private class OffsetEstimator {
/** Target angle. */
private final double target;
/** Sign correction to offset. */
private final double sign;
/** Reference angle. */
private final double r0;
/** Slope of the linearized model. */
private final double r1;
/** Reference date. */
private final AbsoluteDate t0;
/** Simple constructor.
* @param orbit current orbit
* @param currentSign desired sign of the offset at current orbit time (magnitude is ignored)
*/
OffsetEstimator(final Orbit orbit, final double currentSign) {
r0 = positionAngleExtractor.apply(orbit);
target = MathUtils.normalizeAngle(angle, r0);
sign = FastMath.copySign(1.0, (r0 - target) * currentSign);
r1 = orbit.getKeplerianMeanMotion();
t0 = orbit.getDate();
}
/** Compute offset from reference angle.
* @param orbit current orbit
* @return offset between current angle and reference angle
*/
public double delta(final Orbit orbit) {
final double rawAngle = positionAngleExtractor.apply(orbit);
final double linearReference = r0 + r1 * orbit.getDate().durationFrom(t0);
final double linearizedAngle = MathUtils.normalizeAngle(rawAngle, linearReference);
return sign * (linearizedAngle - target);
}
/** Find date at which offset reaches specified value.
* <p>
* This computation is an approximation because it relies on
* {@link Orbit#shiftedBy(double)} only.
* </p>
* @param offset target value for offset angle
* @param orbit current orbit
* @return approximate date at which offset reached specified value
*/
public AbsoluteDate dateForOffset(final double offset, final Orbit orbit) {
// bracket the search
final double period = orbit.getKeplerianPeriod();
final double delta0 = delta(orbit);
final double searchInf;
final double searchSup;
if ((delta0 - offset) * sign >= 0) {
// the date is before current orbit
searchInf = -period;
searchSup = 0;
} else {
// the date is after current orbit
searchInf = 0;
searchSup = +period;
}
// find the date as an offset from current orbit
final BracketingNthOrderBrentSolver solver = new BracketingNthOrderBrentSolver(getThreshold(), 5);
final UnivariateFunction f = dt -> delta(orbit.shiftedBy(dt)) - offset;
final double root = solver.solve(getMaxIterationCount(), f, searchInf, searchSup);
return orbit.getDate().shiftedBy(root);
}
}
}