1   /* Copyright 2020-2025 CS GROUP
2    * Licensed to CS GROUP (CS) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * CS licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *   http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.orekit.propagation.events;
18  
19  import org.hipparchus.geometry.euclidean.threed.Vector3D;
20  import org.hipparchus.ode.events.Action;
21  import org.orekit.bodies.CelestialBodies;
22  import org.orekit.propagation.SpacecraftState;
23  import org.orekit.propagation.events.handlers.EventHandler;
24  import org.orekit.propagation.events.handlers.StopOnDecreasing;
25  import org.orekit.utils.PVCoordinatesProvider;
26  
27  /** Detects when two moving objects come close to each other, as seen from spacecraft.
28   * <p>The main use case for this detector is when the primary object is in fact a ground
29   * station, modeled as a {@link org.orekit.frames.TopocentricFrame} and when the secondary
30   * is the {@link CelestialBodies#getSun() Sun}, for computing
31   * optical reflections.</p>
32   * <p>The default handler behavior is to {@link Action#STOP stop}
33   * propagation when objects enter the proximity zone. This can be changed by calling
34   * {@link #withHandler(EventHandler)} after construction.</p>
35   * @see org.orekit.propagation.Propagator#addEventDetector(EventDetector)
36   * @author Luc Maisonobe
37   * @author Thomas Paulet
38   * @since 11.0
39   */
40  public class AngularSeparationFromSatelliteDetector extends AbstractDetector<AngularSeparationFromSatelliteDetector> {
41  
42      /** Primary object, at the center of the proximity zone. */
43      private final PVCoordinatesProvider primaryObject;
44  
45      /** Secondary object, that may come close to the primary, as seen from the spacecraft . */
46      private final PVCoordinatesProvider secondaryObject;
47  
48      /** Proximity angle (rad). */
49      private final double proximityAngle;
50  
51      /** Build a new angular detachment detector.
52       * @param primaryObject primaryObject, at the center of the proximity zone
53       * @param secondaryObject secondaryObject, that may come close to
54       *        the primaryObject as seen from the spacecraft
55       * @param proximityAngle proximity angle as seen from spacecraft, at which events are triggered (rad)
56       */
57      public AngularSeparationFromSatelliteDetector(final PVCoordinatesProvider primaryObject,
58                                                    final PVCoordinatesProvider secondaryObject,
59                                                    final double proximityAngle) {
60          this(EventDetectionSettings.getDefaultEventDetectionSettings(), new StopOnDecreasing(),
61               primaryObject, secondaryObject, proximityAngle);
62      }
63  
64      /** Protected constructor with full parameters.
65       * <p>
66       * This constructor is not public as users are expected to use the builder
67       * API with the various {@code withXxx()} methods to set up the instance
68       * in a readable manner without using a huge amount of parameters.
69       * </p>
70       * @param detectionSettings detection Settings
71       * @param handler event handler to call at event occurrences
72       * @param primaryObject primaryObject at the center of the proximity zone
73       * @param secondaryObject secondaryObject, that may come close to
74       *        the primaryObject as seen from the spacecraft
75       * @param proximityAngle proximity angle as seen from secondaryObject, at which events are triggered (rad)
76       * @since 13.0
77       */
78      protected AngularSeparationFromSatelliteDetector(final EventDetectionSettings detectionSettings,
79                                                       final EventHandler handler,
80                                                       final PVCoordinatesProvider primaryObject,
81                                                       final PVCoordinatesProvider secondaryObject,
82                                                       final double proximityAngle) {
83          super(detectionSettings, handler);
84          this.primaryObject         = primaryObject;
85          this.secondaryObject       = secondaryObject;
86          this.proximityAngle = proximityAngle;
87      }
88  
89      /** {@inheritDoc} */
90      @Override
91      protected AngularSeparationFromSatelliteDetector create(final EventDetectionSettings detectionSettings,
92                                                              final EventHandler newHandler) {
93          return new AngularSeparationFromSatelliteDetector(detectionSettings, newHandler,
94                                               primaryObject, secondaryObject, proximityAngle);
95      }
96  
97      /** Get the primaryObject, at the center of the proximity zone.
98       * @return primaryObject
99       */
100     public PVCoordinatesProvider getPrimaryObject() {
101         return primaryObject;
102     }
103 
104     /** Get the secondaryObject.
105      * @return secondaryObject
106      */
107     public PVCoordinatesProvider getSecondaryObject() {
108         return secondaryObject;
109     }
110 
111     /** Get the proximity angle (rad).
112      * @return the proximity angle
113      */
114     public double getProximityAngle() {
115         return proximityAngle;
116     }
117 
118     /** Compute the value of the switching function.
119      * <p>
120      * This function measures the angular separation between primary and secondary objects
121      * as seen from the spacecraft minus the proximity angle. It therefore triggers
122      * decreasing events when the secondary object enters the proximity zone and increasing
123      * events when it leaves the proximity zone.
124      * </p>
125      * <p>
126      * No shadowing effect is taken into account, so this method is computed and
127      * may trigger events even when the secondary object is behind the primary.
128      * If such effects must be taken into account the
129      * detector must be associated with a {@link EventEnablingPredicateFilter predicate
130      * filter} where the {@link EnablingPredicate predicate function} is based on eclipse conditions.
131      * </p>
132      * @param s the current state information: date, kinematics, attitude
133      * @return value of the switching function
134      */
135     public double g(final SpacecraftState s) {
136         final Vector3D sPosition = s.getPosition();
137         final Vector3D primaryPos   = primaryObject  .getPosition(s.getDate(), s.getFrame());
138         final Vector3D secondaryPos = secondaryObject.getPosition(s.getDate(), s.getFrame());
139         final double separation = Vector3D.angle(primaryPos.subtract(sPosition),
140                                                  secondaryPos.subtract(sPosition));
141         return separation - proximityAngle;
142     }
143 
144 }