1   /* Copyright 2002-2024 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.forces.gravity;
18  
19  import org.hipparchus.CalculusFieldElement;
20  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
21  import org.hipparchus.geometry.euclidean.threed.Vector3D;
22  import org.hipparchus.util.FastMath;
23  import org.orekit.bodies.CelestialBodies;
24  import org.orekit.bodies.CelestialBody;
25  import org.orekit.propagation.FieldSpacecraftState;
26  import org.orekit.propagation.SpacecraftState;
27  
28  /** Third body attraction force model.
29   *
30   * @author Fabien Maussion
31   * @author Véronique Pommier-Maurussane
32   */
33  public class ThirdBodyAttraction extends AbstractBodyAttraction {
34  
35      /** Simple constructor.
36       * @param body the third body to consider
37       * (ex: {@link CelestialBodies#getSun()} or
38       * {@link CelestialBodies#getMoon()})
39       */
40      public ThirdBodyAttraction(final CelestialBody body) {
41          super(body);
42      }
43  
44      /** {@inheritDoc} */
45      @Override
46      public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
47  
48          final double gm = parameters[0];
49  
50          // compute bodies separation vectors and squared norm
51          final Vector3D centralToBody = getBody().getPosition(s.getDate(), s.getFrame());
52          final double r2Central       = centralToBody.getNormSq();
53          final Vector3D satToBody     = centralToBody.subtract(s.getPosition());
54          final double r2Sat           = satToBody.getNormSq();
55  
56          // compute relative acceleration
57          return new Vector3D(gm / (r2Sat * FastMath.sqrt(r2Sat)), satToBody,
58                             -gm / (r2Central * FastMath.sqrt(r2Central)), centralToBody);
59  
60      }
61  
62      /** {@inheritDoc} */
63      @Override
64      public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
65                                                                               final T[] parameters) {
66  
67          final T gm = parameters[0];
68  
69          // compute bodies separation vectors and squared norm
70          final FieldVector3D<T> centralToBody = getBody().getPosition(s.getDate(), s.getFrame());
71          final T                r2Central     = centralToBody.getNormSq();
72          final FieldVector3D<T> satToBody     = centralToBody.subtract(s.getPosition());
73          final T                r2Sat         = satToBody.getNormSq();
74  
75          // compute relative acceleration
76          return new FieldVector3D<>(r2Sat.multiply(r2Sat.sqrt()).reciprocal().multiply(gm), satToBody,
77                                     r2Central.multiply(r2Central.sqrt()).reciprocal().multiply(gm).negate(), centralToBody);
78  
79      }
80  
81  }