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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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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
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17  package org.orekit.estimation.measurements.modifiers;
18  
19  import java.util.List;
20  
21  import org.hipparchus.geometry.euclidean.threed.Vector3D;
22  import org.hipparchus.util.MathUtils;
23  import org.orekit.estimation.measurements.AngularAzEl;
24  import org.orekit.estimation.measurements.EstimatedMeasurementBase;
25  import org.orekit.estimation.measurements.EstimationModifier;
26  import org.orekit.estimation.measurements.GroundStation;
27  import org.orekit.frames.Frame;
28  import org.orekit.frames.TopocentricFrame;
29  import org.orekit.models.earth.ionosphere.IonosphericModel;
30  import org.orekit.propagation.SpacecraftState;
31  import org.orekit.time.AbsoluteDate;
32  import org.orekit.utils.Constants;
33  import org.orekit.utils.ParameterDriver;
34  import org.orekit.utils.TrackingCoordinates;
35  
36  /** Class modifying theoretical angular measurement with ionospheric delay.
37   * <p>
38   * The effect of ionospheric correction on the angular measurement is computed
39   * through the computation of the ionospheric delay. The spacecraft state
40   * is shifted by the computed delay time and elevation and azimuth are computed
41   * again with the new spacecraft state.
42   * </p>
43   * <p>
44   * The ionospheric delay depends on the frequency of the signal (GNSS, VLBI, ...).
45   * For optical measurements (e.g. SLR), the ray is not affected by ionosphere charged particles.
46   * </p>
47   * <p>
48   * Since 10.0, state derivatives and ionospheric parameters derivates are computed
49   * using automatic differentiation.
50   * </p>
51   * @author Thierry Ceolin
52   * @since 8.0
53   */
54  public class AngularIonosphericDelayModifier implements EstimationModifier<AngularAzEl> {
55  
56      /** Ionospheric delay model. */
57      private final IonosphericModel ionoModel;
58  
59      /** Frequency [Hz]. */
60      private final double frequency;
61  
62      /** Constructor.
63       *
64       * @param model  Ionospheric delay model appropriate for the current angular measurement method.
65       * @param freq frequency of the signal in Hz
66       */
67      public AngularIonosphericDelayModifier(final IonosphericModel model,
68                                             final double freq) {
69          ionoModel = model;
70          frequency = freq;
71      }
72  
73      /** Compute the measurement error due to ionosphere.
74       * @param station station
75       * @param state spacecraft state
76       * @return the measurement error due to ionosphere
77       */
78      private double angularErrorIonosphericModel(final GroundStation station,
79                                                  final SpacecraftState state) {
80          // Base frame associated with the station
81          final TopocentricFrame baseFrame = station.getBaseFrame();
82          // delay in meters
83          return ionoModel.pathDelay(state, baseFrame, frequency, ionoModel.getParameters(state.getDate()));
84      }
85  
86      /** {@inheritDoc} */
87      @Override
88      public List<ParameterDriver> getParametersDrivers() {
89          return ionoModel.getParametersDrivers();
90      }
91  
92      @Override
93      public void modifyWithoutDerivatives(final EstimatedMeasurementBase<AngularAzEl> estimated) {
94          final AngularAzEl     measure = estimated.getObservedMeasurement();
95          final GroundStation   station = measure.getStation();
96          final SpacecraftState state   = estimated.getStates()[0];
97  
98          final double delay = angularErrorIonosphericModel(station, state);
99          // Delay is taken into account to shift the spacecraft position
100         final double dt = delay / Constants.SPEED_OF_LIGHT;
101 
102         // Position of the spacecraft shifted of dt
103         final SpacecraftState transitState = state.shiftedBy(-dt);
104 
105         // Update estimated value taking into account the ionospheric delay.
106         final AbsoluteDate date     = transitState.getDate();
107         final Vector3D     position = transitState.getPosition();
108         final Frame        inertial = transitState.getFrame();
109 
110         // Elevation and azimuth in radians
111         final TrackingCoordinates tc = station.getBaseFrame().getTrackingCoordinates(position, inertial, date);
112         final double twoPiWrap   = MathUtils.normalizeAngle(tc.getAzimuth(), measure.getObservedValue()[0]) - tc.getAzimuth();
113         final double azimuth     = tc.getAzimuth() + twoPiWrap;
114 
115         // Update estimated value taking into account the ionospheric delay.
116         // Azimuth - elevation values
117         estimated.modifyEstimatedValue(this, azimuth, tc.getElevation());
118     }
119 
120 }