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.estimation.measurements.gnss;
18  
19  import org.hipparchus.util.FastMath;
20  import org.orekit.gnss.GnssSignal;
21  import org.orekit.gnss.MeasurementType;
22  import org.orekit.gnss.SatelliteSystem;
23  
24  /**
25   * Geometry-free combination.
26   * <p>
27   * This combination removes the geometry part of the measurement.
28   * It can be used to estimate the ionospheric electron content or to detect
29   * cycle slips in the carrier phase, as well.
30   * </p>
31   * <pre>
32   *    mGF =  m2 - m1
33   * </pre>
34   * With:
35   * <ul>
36   * <li>mGF: Geometry-free measurement.</li>
37   * <li>m1 : First measurement.</li>
38   * <li>m2 : Second measurement.</li>
39   * </ul>
40   * <p>
41   * Geometry-Free combination is a dual frequency combination.
42   * The two measurements shall have different frequencies but they must have the same {@link MeasurementType}.
43   * </p>
44   * @author Bryan Cazabonne
45   * @since 10.1
46   */
47  public class GeometryFreeCombination extends AbstractDualFrequencyCombination {
48  
49      /**
50       * Package private constructor for the factory.
51       * @param system satellite system for which the combination is applied
52       */
53      GeometryFreeCombination(final SatelliteSystem system) {
54          super(CombinationType.GEOMETRY_FREE, system);
55      }
56  
57      /** {@inheritDoc} */
58      @Override
59      protected double getCombinedValue(final double obs1, final GnssSignal s1,
60                                        final double obs2, final GnssSignal s2) {
61          // Combined observed value does not depend on frequency for the Geometry-Free combination
62          return FastMath.abs(obs2 - obs1);
63      }
64  
65      /** {@inheritDoc} */
66      @Override
67      protected double getCombinedFrequency(final GnssSignal s1, final GnssSignal s2) {
68          // There is not combined frequency for the Geometry-Free combination
69          return Double.NaN;
70      }
71  
72  }