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.models.earth.tessellation;
18  
19  import java.util.Collections;
20  import java.util.List;
21  
22  import org.hipparchus.geometry.euclidean.threed.Vector3D;
23  import org.hipparchus.geometry.spherical.twod.S2Point;
24  import org.hipparchus.geometry.spherical.twod.SphericalPolygonsSet;
25  import org.hipparchus.util.FastMath;
26  import org.orekit.bodies.GeodeticPoint;
27  
28  /** Class used to orient tiles such that there are no singularities within the zone of interest.
29   * <p>
30   * This class is mainly useful for {@link EllipsoidTessellator#sample(org.hipparchus.geometry.spherical.twod.SphericalPolygonsSet,
31   * double, double) sampling} a zone on ground when the grid directions is not really important
32   * and when the zone contains the pole, which is a singular point for both
33   * {@link ConstantAzimuthAiming} and {@link AlongTrackAiming}.
34   * </p>
35   * @see AlongTrackAiming
36   * @see ConstantAzimuthAiming
37   * @author Luc Maisonobe
38   */
39  public class DivertedSingularityAiming implements TileAiming {
40  
41      /** Singularity location. */
42      private final Vector3D singularity;
43  
44      /** Singularity location. */
45      private final GeodeticPoint singularityGP;
46  
47      /** Dipole moment. */
48      private final Vector3D moment;
49  
50      /** Simple constructor.
51       * @param forbiddenZone zone out of which singularity should be diverted
52       */
53      public DivertedSingularityAiming(final SphericalPolygonsSet forbiddenZone) {
54          final S2Point outside = forbiddenZone.getEnclosingCap().getCenter().negate();
55          this.singularity      = outside.getVector();
56          this.singularityGP    = new GeodeticPoint(0.5 * FastMath.PI - outside.getPhi(), outside.getTheta(), 0.0);
57          this.moment           = singularity.orthogonal();
58      }
59  
60      /** {@inheritDoc} */
61      @Override
62      public List<GeodeticPoint> getSingularPoints() {
63          return Collections.singletonList(singularityGP);
64      }
65  
66      /** {@inheritDoc} */
67      @Override
68      public Vector3D alongTileDirection(final Vector3D point, final GeodeticPoint gp) {
69  
70          // compute the dipole field at point
71          final Vector3D p     = new S2Point(gp.getLongitude(), 0.5 * FastMath.PI - gp.getLatitude()).getVector();
72          final Vector3D r     = p.subtract(singularity).normalize();
73          final Vector3D field = new Vector3D(3.0 * Vector3D.dotProduct(moment, r), r, -1.0, moment);
74  
75          // the aiming direction is the horizontal component of the field
76          final Vector3D horizontal = new Vector3D(1, field, -Vector3D.dotProduct(field, p), p);
77          return horizontal.normalize();
78  
79      }
80  
81  }