1   /* Copyright 2002-2025 CS GROUP
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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
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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  import org.orekit.utils.ExtendedPositionProvider;
28  
29  /** Body attraction force model computed as absolute acceleration towards a body.
30   * <p>
31   * This force model represents the same physical principles as {@link NewtonianAttraction},
32   * but has several major differences:
33   * </p>
34   * <ul>
35   *   <li>the attracting body can be <em>away</em> from the integration frame center,</li>
36   *   <li>several instances of this force model can be added when several bodies are involved,</li>
37   *   <li>this force model is <em>never</em> automatically added by the numerical propagator</li>
38   * </ul>
39   * <p>
40   * The possibility for the attracting body to be away from the frame center allows to use this force
41   * model when integrating for example an interplanetary trajectory propagated in an Earth centered
42   * frame (in which case an instance of {@link org.orekit.forces.inertia.InertialForces} must also be
43   * added to take into account the coupling effect of relative frames motion).
44   * </p>
45   * <p>
46   * The possibility to add several instances allows to use this in interplanetary trajectories or
47   * in trajectories about Lagrangian points
48   * </p>
49   * <p>
50   * The fact this force model is <em>never</em> automatically added by the numerical propagator differs
51   * from {@link NewtonianAttraction} as {@link NewtonianAttraction} may be added automatically when
52   * propagating a trajectory represented as an {@link org.orekit.orbits.Orbit}, which must always refer
53   * to a central body, if user did not add the {@link NewtonianAttraction} or set the central attraction
54   * coefficient by himself.
55   * </p>
56   * @see org.orekit.forces.inertia.InertialForces
57   * @author Luc Maisonobe
58   * @author Julio Hernanz
59   */
60  public class SingleBodyAbsoluteAttraction extends AbstractBodyAttraction {
61  
62      /** Simple constructor.
63       * @param positionProvider extended position provider for the body to consider
64       * @param name name of the body
65       * @param mu body gravitational constant
66       * @since 13.0
67       */
68      public SingleBodyAbsoluteAttraction(final ExtendedPositionProvider positionProvider,
69                                          final String name, final double mu) {
70          super(positionProvider, name, mu);
71      }
72  
73      /** Constructor.
74       * @param body the body to consider
75       * (ex: {@link CelestialBodies#getSun()} or
76       * {@link CelestialBodies#getMoon()})
77       */
78      public SingleBodyAbsoluteAttraction(final CelestialBody body) {
79          this(body, body.getName(), body.getGM());
80      }
81  
82      /** {@inheritDoc} */
83      @Override
84      public Vector3D acceleration(final SpacecraftState s, final double[] parameters) {
85  
86          // compute bodies separation vectors and squared norm
87          final Vector3D bodyPosition = getBodyPosition(s.getDate(), s.getFrame());
88          final Vector3D satToBody     = bodyPosition.subtract(s.getPosition());
89          final double r2Sat           = satToBody.getNormSq();
90  
91          // compute absolute acceleration
92          return new Vector3D(parameters[0] / (r2Sat * FastMath.sqrt(r2Sat)), satToBody);
93  
94      }
95  
96      /** {@inheritDoc} */
97      @Override
98      public <T extends CalculusFieldElement<T>> FieldVector3D<T> acceleration(final FieldSpacecraftState<T> s,
99                                                                               final T[] parameters) {
100          // compute bodies separation vectors and squared norm
101         final FieldVector3D<T> centralToBody = getBodyPosition(s.getDate(), s.getFrame());
102         final FieldVector3D<T> satToBody     = centralToBody.subtract(s.getPosition());
103         final T                r2Sat         = satToBody.getNormSq();
104 
105         // compute absolute acceleration
106         return new FieldVector3D<>(parameters[0].divide(r2Sat.multiply(r2Sat.sqrt())), satToBody);
107 
108     }
109 
110 }