1 /* Copyright 2002-2021 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;
18
19 import java.util.Arrays;
20 import java.util.Collections;
21 import java.util.HashMap;
22 import java.util.Map;
23
24 import org.hipparchus.analysis.differentiation.Gradient;
25 import org.hipparchus.analysis.differentiation.GradientField;
26 import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
27 import org.orekit.frames.FieldTransform;
28 import org.orekit.propagation.SpacecraftState;
29 import org.orekit.time.AbsoluteDate;
30 import org.orekit.time.FieldAbsoluteDate;
31 import org.orekit.utils.Constants;
32 import org.orekit.utils.ParameterDriver;
33 import org.orekit.utils.TimeStampedFieldPVCoordinates;
34 import org.orekit.utils.TimeStampedPVCoordinates;
35
36 /** Class modeling a range measurement from a ground station.
37 * <p>
38 * For one-way measurements, a signal is emitted by the satellite
39 * and received by the ground station. The measurement value is the
40 * elapsed time between emission and reception multiplied by c where
41 * c is the speed of light.
42 * </p>
43 * <p>
44 * For two-way measurements, the measurement is considered to be a signal
45 * emitted from a ground station, reflected on spacecraft, and received
46 * on the same ground station. Its value is the elapsed time between
47 * emission and reception multiplied by c/2 where c is the speed of light.
48 * </p>
49 * <p>
50 * The motion of both the station and the spacecraft during the signal
51 * flight time are taken into account. The date of the measurement
52 * corresponds to the reception on ground of the emitted or reflected signal.
53 * </p>
54 * <p>
55 * The clock offsets of both the ground station and the satellite are taken
56 * into account. These offsets correspond to the values that must be subtracted
57 * from station (resp. satellite) reading of time to compute the real physical
58 * date. These offsets have two effects:
59 * </p>
60 * <ul>
61 * <li>as measurement date is evaluated at reception time, the real physical date
62 * of the measurement is the observed date to which the receiving ground station
63 * clock offset is subtracted</li>
64 * <li>as range is evaluated using the total signal time of flight, for one-way
65 * measurements the observed range is the real physical signal time of flight to
66 * which (Δtg - Δts) ⨉ c is added, where Δtg (resp. Δts) is the clock offset for the
67 * receiving ground station (resp. emitting satellite). A similar effect exists in
68 * two-way measurements but it is computed as (Δtg - Δtg) ⨉ c / 2 as the same ground
69 * station clock is used for initial emission and final reception and therefore it evaluates
70 * to zero.</li>
71 * </ul>
72 * <p>
73 * @author Thierry Ceolin
74 * @author Luc Maisonobe
75 * @author Maxime Journot
76 * @since 8.0
77 */
78 public class Range extends AbstractMeasurement<Range> {
79
80 /** Ground station from which measurement is performed. */
81 private final GroundStation station;
82
83 /** Flag indicating whether it is a two-way measurement. */
84 private final boolean twoway;
85
86 /** Simple constructor.
87 * @param station ground station from which measurement is performed
88 * @param twoWay flag indicating whether it is a two-way measurement
89 * @param date date of the measurement
90 * @param range observed value
91 * @param sigma theoretical standard deviation
92 * @param baseWeight base weight
93 * @param satellite satellite related to this measurement
94 * @since 9.3
95 */
96 public Range(final GroundStation station, final boolean twoWay, final AbsoluteDate date,
97 final double range, final double sigma, final double baseWeight,
98 final ObservableSatellite satellite) {
99 super(date, range, sigma, baseWeight, Collections.singletonList(satellite));
100 addParameterDriver(station.getClockOffsetDriver());
101 addParameterDriver(station.getEastOffsetDriver());
102 addParameterDriver(station.getNorthOffsetDriver());
103 addParameterDriver(station.getZenithOffsetDriver());
104 addParameterDriver(station.getPrimeMeridianOffsetDriver());
105 addParameterDriver(station.getPrimeMeridianDriftDriver());
106 addParameterDriver(station.getPolarOffsetXDriver());
107 addParameterDriver(station.getPolarDriftXDriver());
108 addParameterDriver(station.getPolarOffsetYDriver());
109 addParameterDriver(station.getPolarDriftYDriver());
110 if (!twoWay) {
111 // for one way measurements, the satellite clock offset affects the measurement
112 addParameterDriver(satellite.getClockOffsetDriver());
113 }
114 this.station = station;
115 this.twoway = twoWay;
116 }
117
118 /** Get the ground station from which measurement is performed.
119 * @return ground station from which measurement is performed
120 */
121 public GroundStation getStation() {
122 return station;
123 }
124
125 /** Check if the instance represents a two-way measurement.
126 * @return true if the instance represents a two-way measurement
127 */
128 public boolean isTwoWay() {
129 return twoway;
130 }
131
132 /** {@inheritDoc} */
133 @Override
134 protected EstimatedMeasurement<Range> theoreticalEvaluation(final int iteration,
135 final int evaluation,
136 final SpacecraftState[] states) {
137
138 final SpacecraftState state = states[0];
139
140 // Range derivatives are computed with respect to spacecraft state in inertial frame
141 // and station parameters
142 // ----------------------
143 //
144 // Parameters:
145 // - 0..2 - Position of the spacecraft in inertial frame
146 // - 3..5 - Velocity of the spacecraft in inertial frame
147 // - 6..n - measurements parameters (clock offset, station offsets, pole, prime meridian, sat clock offset...)
148 int nbParams = 6;
149 final Map<String, Integer> indices = new HashMap<>();
150 for (ParameterDriver driver : getParametersDrivers()) {
151 if (driver.isSelected()) {
152 indices.put(driver.getName(), nbParams++);
153 }
154 }
155 final FieldVector3D<Gradient> zero = FieldVector3D.getZero(GradientField.getField(nbParams));
156
157 // Coordinates of the spacecraft expressed as a gradient
158 final TimeStampedFieldPVCoordinates<Gradient> pvaDS = getCoordinates(state, 0, nbParams);
159
160 // transform between station and inertial frame, expressed as a gradient
161 // The components of station's position in offset frame are the 3 last derivative parameters
162 final FieldTransform<Gradient> offsetToInertialDownlink =
163 station.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
164 final FieldAbsoluteDate<Gradient> downlinkDateDS = offsetToInertialDownlink.getFieldDate();
165
166 // Station position in inertial frame at end of the downlink leg
167 final TimeStampedFieldPVCoordinates<Gradient> stationDownlink =
168 offsetToInertialDownlink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDateDS,
169 zero, zero, zero));
170
171 // Compute propagation times
172 // (if state has already been set up to pre-compensate propagation delay,
173 // we will have delta == tauD and transitState will be the same as state)
174
175 // Downlink delay
176 final Gradient tauD = signalTimeOfFlight(pvaDS, stationDownlink.getPosition(), downlinkDateDS);
177
178 // Transit state & Transit state (re)computed with gradients
179 final Gradient delta = downlinkDateDS.durationFrom(state.getDate());
180 final Gradient deltaMTauD = tauD.negate().add(delta);
181 final SpacecraftState transitState = state.shiftedBy(deltaMTauD.getValue());
182 final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaDS.shiftedBy(deltaMTauD);
183
184 // prepare the evaluation
185 final EstimatedMeasurement<Range> estimated;
186 final Gradient range;
187
188 if (twoway) {
189
190 // Station at transit state date (derivatives of tauD taken into account)
191 final TimeStampedFieldPVCoordinates<Gradient> stationAtTransitDate =
192 stationDownlink.shiftedBy(tauD.negate());
193 // Uplink delay
194 final Gradient tauU =
195 signalTimeOfFlight(stationAtTransitDate, transitStateDS.getPosition(), transitStateDS.getDate());
196 final TimeStampedFieldPVCoordinates<Gradient> stationUplink =
197 stationDownlink.shiftedBy(-tauD.getValue() - tauU.getValue());
198
199 // Prepare the evaluation
200 estimated = new EstimatedMeasurement<Range>(this, iteration, evaluation,
201 new SpacecraftState[] {
202 transitState
203 }, new TimeStampedPVCoordinates[] {
204 stationUplink.toTimeStampedPVCoordinates(),
205 transitStateDS.toTimeStampedPVCoordinates(),
206 stationDownlink.toTimeStampedPVCoordinates()
207 });
208
209 // Range value
210 final double cOver2 = 0.5 * Constants.SPEED_OF_LIGHT;
211 final Gradient tau = tauD.add(tauU);
212 range = tau.multiply(cOver2);
213
214 } else {
215
216 estimated = new EstimatedMeasurement<Range>(this, iteration, evaluation,
217 new SpacecraftState[] {
218 transitState
219 }, new TimeStampedPVCoordinates[] {
220 transitStateDS.toTimeStampedPVCoordinates(),
221 stationDownlink.toTimeStampedPVCoordinates()
222 });
223
224 // Clock offsets
225 final ObservableSatellite satellite = getSatellites().get(0);
226 final Gradient dts = satellite.getClockOffsetDriver().getValue(nbParams, indices);
227 final Gradient dtg = station.getClockOffsetDriver().getValue(nbParams, indices);
228
229 // Range value
230 range = tauD.add(dtg).subtract(dts).multiply(Constants.SPEED_OF_LIGHT);
231
232 }
233
234 estimated.setEstimatedValue(range.getValue());
235
236 // Range partial derivatives with respect to state
237 final double[] derivatives = range.getGradient();
238 estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
239
240 // set partial derivatives with respect to parameters
241 // (beware element at index 0 is the value, not a derivative)
242 for (final ParameterDriver driver : getParametersDrivers()) {
243 final Integer index = indices.get(driver.getName());
244 if (index != null) {
245 estimated.setParameterDerivatives(driver, derivatives[index]);
246 }
247 }
248
249 return estimated;
250
251 }
252
253 }