1   /* Copyright 2002-2025 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.orekit.bodies.OneAxisEllipsoid;
20  import org.orekit.bodies.GeodeticPoint;
21  import org.orekit.propagation.SpacecraftState;
22  import org.orekit.propagation.events.handlers.EventHandler;
23  import org.orekit.propagation.events.handlers.StopOnIncreasing;
24  
25  /** Detector for geographic latitude crossing.
26   * <p>This detector identifies when a spacecraft crosses a fixed
27   * latitude with respect to a central body.</p>
28   * @author Luc Maisonobe
29   * @since 7.1
30   */
31  public class LatitudeCrossingDetector extends AbstractDetector<LatitudeCrossingDetector> {
32  
33      /** Body on which the latitude is defined. */
34      private final OneAxisEllipsoid body;
35  
36      /** Fixed latitude to be crossed. */
37      private final double latitude;
38  
39      /** Build a new detector.
40       * <p>The new instance uses default values for maximal checking interval
41       * ({@link #DEFAULT_MAX_CHECK}) and convergence threshold ({@link
42       * #DEFAULT_THRESHOLD}).</p>
43       * @param body body on which the latitude is defined
44       * @param latitude latitude to be crossed
45       */
46      public LatitudeCrossingDetector(final OneAxisEllipsoid body, final double latitude) {
47          this(DEFAULT_MAX_CHECK, DEFAULT_THRESHOLD, body, latitude);
48      }
49  
50      /** Build a detector.
51       * @param maxCheck maximal checking interval (s)
52       * @param threshold convergence threshold (s)
53       * @param body body on which the latitude is defined
54       * @param latitude latitude to be crossed
55       */
56      public LatitudeCrossingDetector(final double maxCheck, final double threshold,
57                                      final OneAxisEllipsoid body, final double latitude) {
58          this(new EventDetectionSettings(maxCheck, threshold, DEFAULT_MAX_ITER), new StopOnIncreasing(),
59               body, latitude);
60      }
61  
62      /** Protected constructor with full parameters.
63       * <p>
64       * This constructor is not public as users are expected to use the builder
65       * API with the various {@code withXxx()} methods to set up the instance
66       * in a readable manner without using a huge amount of parameters.
67       * </p>
68       * @param detectionSettings event detection settings
69       * @param handler event handler to call at event occurrences
70       * @param body body on which the latitude is defined
71       * @param latitude latitude to be crossed
72       * @since 13.0
73       */
74      protected LatitudeCrossingDetector(final EventDetectionSettings detectionSettings, final EventHandler handler,
75                                         final OneAxisEllipsoid body, final double latitude) {
76          super(detectionSettings, handler);
77          this.body     = body;
78          this.latitude = latitude;
79      }
80  
81      /** {@inheritDoc} */
82      @Override
83      protected LatitudeCrossingDetector create(final EventDetectionSettings detectionSettings,
84                                                final EventHandler newHandler) {
85          return new LatitudeCrossingDetector(detectionSettings, newHandler, body, latitude);
86      }
87  
88      /** Get the body on which the geographic zone is defined.
89       * @return body on which the geographic zone is defined
90       */
91      public OneAxisEllipsoid getBody() {
92          return body;
93      }
94  
95      /** Get the fixed latitude to be crossed (radians).
96       * @return fixed latitude to be crossed (radians)
97       */
98      public double getLatitude() {
99          return latitude;
100     }
101 
102     /** Compute the value of the detection function.
103      * <p>
104      * The value is the spacecraft latitude minus the fixed latitude to be crossed.
105      * It is positive if the spacecraft is northward and negative if it is southward
106      * with respect to the fixed latitude.
107      * </p>
108      * @param s the current state information: date, kinematics, attitude
109      * @return spacecraft latitude minus the fixed latitude to be crossed
110      */
111     public double g(final SpacecraftState s) {
112 
113         // convert state to geodetic coordinates
114         final GeodeticPoint gp = body.transform(s.getPosition(),
115                                                 s.getFrame(), s.getDate());
116 
117         // latitude difference
118         return gp.getLatitude() - latitude;
119 
120     }
121 
122 }