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  /** Body attraction force model computed as absolute acceleration towards a body.
29   * <p>
30   * This force model represents the same physical principles as {@link NewtonianAttraction},
31   * but has several major differences:
32   * </p>
33   * <ul>
34   *   <li>the attracting body can be <em>away</em> from the integration frame center,</li>
35   *   <li>several instances of this force model can be added when several bodies are involved,</li>
36   *   <li>this force model is <em>never</em> automatically added by the numerical propagator</li>
37   * </ul>
38   * <p>
39   * The possibility for the attracting body to be away from the frame center allows to use this force
40   * model when integrating for example an interplanetary trajectory propagated in an Earth centered
41   * frame (in which case an instance of {@link org.orekit.forces.inertia.InertialForces} must also be
42   * added to take into account the coupling effect of relative frames motion).
43   * </p>
44   * <p>
45   * The possibility to add several instances allows to use this in interplanetary trajectories or
46   * in trajectories about Lagrangian points
47   * </p>
48   * <p>
49   * The fact this force model is <em>never</em> automatically added by the numerical propagator differs
50   * from {@link NewtonianAttraction} as {@link NewtonianAttraction} may be added automatically when
51   * propagating a trajectory represented as an {@link org.orekit.orbits.Orbit}, which must always refer
52   * to a central body, if user did not add the {@link NewtonianAttraction} or set the central attraction
53   * coefficient by himself.
54   * </p>
55   * @see org.orekit.forces.inertia.InertialForces
56   * @author Luc Maisonobe
57   * @author Julio Hernanz
58   */
59  public class SingleBodyAbsoluteAttraction extends AbstractBodyAttraction {
60  
61      /** Simple constructor.
62       * @param body the body to consider
63       * (ex: {@link CelestialBodies#getSun()} or
64       * {@link CelestialBodies#getMoon()})
65       */
66      public SingleBodyAbsoluteAttraction(final CelestialBody body) {
67          super(body);
68      }
69  
70      /** {@inheritDoc} */
71      @Override
72      public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
73  
74          // compute bodies separation vectors and squared norm
75          final Vector3D bodyPosition = getBody().getPosition(s.getDate(), s.getFrame());
76          final Vector3D satToBody     = bodyPosition.subtract(s.getPosition());
77          final double r2Sat           = satToBody.getNormSq();
78  
79          // compute absolute acceleration
80          return new Vector3D(parameters[0] / (r2Sat * FastMath.sqrt(r2Sat)), satToBody);
81  
82      }
83  
84      /** {@inheritDoc} */
85      @Override
86      public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
87                                                                               final T[] parameters) {
88           // compute bodies separation vectors and squared norm
89          final FieldVector3D<T> centralToBody = getBody().getPosition(s.getDate(), s.getFrame());
90          final FieldVector3D<T> satToBody     = centralToBody.subtract(s.getPosition());
91          final T                r2Sat         = satToBody.getNormSq();
92  
93          // compute absolute acceleration
94          return new FieldVector3D<>(parameters[0].divide(r2Sat.multiply(r2Sat.sqrt())), satToBody);
95  
96      }
97  
98  }