FieldLongitudeCrossingDetector.java
/* Copyright 2023-2026 Alberto Ferrero
* Licensed to CS GROUP (CS) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* Alberto Ferrero 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.CalculusFieldElement;
import org.hipparchus.Field;
import org.hipparchus.util.FastMath;
import org.orekit.bodies.BodyShape;
import org.orekit.propagation.FieldSpacecraftState;
import org.orekit.propagation.events.functions.EventFunction;
import org.orekit.propagation.events.functions.LongitudeValueCrossingFunction;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.FieldContinueOnEvent;
import org.orekit.propagation.events.handlers.FieldEventHandler;
import org.orekit.propagation.events.handlers.FieldStopOnIncreasing;
import org.orekit.propagation.events.intervals.FieldAdaptableInterval;
import org.orekit.time.FieldAbsoluteDate;
/** Detector for geographic longitude crossing.
* <p>This detector identifies when a spacecraft crosses a fixed
* longitude with respect to a central body.</p>
* <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>
* @author Alberto Ferrero
* @since 12.0
* @param <T> type of the field elements
*/
public class FieldLongitudeCrossingDetector <T extends CalculusFieldElement<T>>
extends FieldAbstractGeographicalDetector<FieldLongitudeCrossingDetector<T>, T> implements FieldDetectorModifier<T> {
/**
* Fixed longitude to be crossed.
*/
private final double longitude;
/**
* Filtering detector.
*/
private final FieldEventEnablingPredicateFilter<T> filtering;
/**
* 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 field the type of numbers to use.
* @param body body on which the longitude is defined
* @param longitude longitude to be crossed
*/
public FieldLongitudeCrossingDetector(final Field<T> field, final BodyShape body, final double longitude) {
this(new FieldEventDetectionSettings<>(field, EventDetectionSettings.getDefaultEventDetectionSettings()),
new FieldStopOnIncreasing<>(), 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 FieldLongitudeCrossingDetector(final T maxCheck,
final T threshold,
final BodyShape body,
final double longitude) {
this(new FieldEventDetectionSettings<>(FieldAdaptableInterval.of(maxCheck.getReal()), threshold, DEFAULT_MAX_ITER),
new FieldStopOnIncreasing<>(), 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 detectionSettings event detection settings
* @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 FieldLongitudeCrossingDetector(
final FieldEventDetectionSettings<T> detectionSettings,
final FieldEventHandler<T> handler,
final BodyShape body,
final double longitude) {
super(detectionSettings, handler, body);
this.longitude = longitude;
// 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
final EventFunction eventFunction = new LongitudeValueCrossingFunction(getBodyShape(), longitude);
final FieldEventDetector<T> raw = FieldEventDetector.of(eventFunction, new FieldContinueOnEvent<>(), getDetectionSettings());
final FieldEnablingPredicate<T> predicate = new FieldEnablingPredicate<T>() {
@Override
public boolean eventIsEnabled(final FieldSpacecraftState<T> state, final FieldEventDetector<T> detector, final T g) {
return FastMath.abs(g).getReal() < 0.5 * FastMath.PI;
}
@Override
public boolean dependsOnMainVariablesOnly() {
return true;
}
};
this.filtering = new FieldEventEnablingPredicateFilter<>(raw, predicate);
}
/**
* {@inheritDoc}
*/
@Override
protected FieldLongitudeCrossingDetector<T> create(
final FieldEventDetectionSettings<T> detectionSettings, final FieldEventHandler<T> newHandler) {
return new FieldLongitudeCrossingDetector<>(detectionSettings, newHandler, getBodyShape(), longitude);
}
@Override
public EventFunction getEventFunction() {
return filtering.getEventFunction();
}
/**
* Get the fixed longitude to be crossed (radians).
*
* @return fixed longitude to be crossed (radians)
*/
public double getLongitude() {
return longitude;
}
/**
* {@inheritDoc}
*/
@Override
public void init(final FieldSpacecraftState<T> s0, final FieldAbsoluteDate<T> t) {
super.init(s0, t);
filtering.init(s0, t);
}
@Override
public FieldEventDetector<T> getDetector() {
return filtering;
}
@Override
public LongitudeCrossingDetector toEventDetector(final EventHandler eventHandler) {
return new LongitudeCrossingDetector(getDetectionSettings().toEventDetectionSettings(), eventHandler,
getBodyShape(), getLongitude());
}
}