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.orekit.bodies.CelestialBodies;
23  import org.orekit.bodies.CelestialBody;
24  import org.orekit.propagation.FieldSpacecraftState;
25  import org.orekit.propagation.SpacecraftState;
26  import org.orekit.utils.FieldPVCoordinates;
27  import org.orekit.utils.PVCoordinates;
28  
29  /** Body attraction force model computed as relative acceleration towards frame center.
30   * @author Luc Maisonabe
31   * @author Julio Hernanz
32   */
33  public class SingleBodyRelativeAttraction extends AbstractBodyAttraction {
34  
35      /** Simple constructor.
36       * @param body the body to consider
37       * (ex: {@link CelestialBodies#getSun()} or
38       * {@link CelestialBodies#getMoon()})
39       */
40      public SingleBodyRelativeAttraction(final CelestialBody body) {
41          super(body);
42      }
43  
44      /** {@inheritDoc} */
45      public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
46  
47          // compute bodies separation vectors and squared norm
48          final PVCoordinates bodyPV   = getBody().getPVCoordinates(s.getDate(), s.getFrame());
49          final Vector3D satToBody     = bodyPV.getPosition().subtract(s.getPosition());
50          final double r2Sat           = satToBody.getNormSq();
51  
52          // compute relative acceleration
53          final double gm = parameters[0];
54          final double a = gm / r2Sat;
55          return new Vector3D(a, satToBody.normalize()).add(bodyPV.getAcceleration());
56  
57      }
58  
59      /** {@inheritDoc} */
60      public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
61                                                                           final T[] parameters) {
62  
63          // compute bodies separation vectors and squared norm
64          final FieldPVCoordinates<T> bodyPV = getBody().getPVCoordinates(s.getDate(), s.getFrame());
65          final FieldVector3D<T> satToBody   = bodyPV.getPosition().subtract(s.getPosition());
66          final T                r2Sat       = satToBody.getNormSq();
67  
68          // compute relative acceleration
69          final T gm = parameters[0];
70          final T a  = gm.divide(r2Sat);
71          return new FieldVector3D<>(a, satToBody.normalize()).add(bodyPV.getAcceleration());
72  
73      }
74  
75  }