InterSatellitesPhase.java

  1. /* Copyright 2002-2022 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;

  19. import java.util.Arrays;
  20. import java.util.HashMap;
  21. import java.util.Map;

  23. import org.hipparchus.analysis.differentiation.Gradient;
  24. import org.orekit.estimation.measurements.AbstractMeasurement;
  25. import org.orekit.estimation.measurements.EstimatedMeasurement;
  26. import org.orekit.estimation.measurements.ObservableSatellite;
  27. import org.orekit.propagation.SpacecraftState;
  28. import org.orekit.time.AbsoluteDate;
  29. import org.orekit.time.FieldAbsoluteDate;
  30. import org.orekit.utils.Constants;
  31. import org.orekit.utils.ParameterDriver;
  32. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  33. import org.orekit.utils.TimeStampedPVCoordinates;

  35. /** Phase measurement between two satellites.
  36.  * <p>
  37.  * The measurement is considered to be a signal emitted from
  38.  * a remote satellite and received by a local satellite.
  39.  * Its value is the number of cycles between emission and reception.
  40.  * The motion of both spacecrafts during the signal flight time
  41.  * are taken into account. The date of the measurement corresponds to the
  42.  * reception on ground of the emitted signal.
  43.  * </p>
  44.  * @author Bryan Cazabonne
  45.  * @since 10.3
  46.  */
  47. public class InterSatellitesPhase extends AbstractMeasurement<InterSatellitesPhase> {

  49.     /** Name for ambiguity driver. */
  50.     public static final String AMBIGUITY_NAME = "ambiguity";

  52.     /** Driver for ambiguity. */
  53.     private final ParameterDriver ambiguityDriver;

  55.     /** Wavelength of the phase observed value [m]. */
  56.     private final double wavelength;

  58.     /** Constructor.
  59.      * @param local satellite which receives the signal and performs the measurement
  60.      * @param remote emote satellite which simply emits the signal
  61.      * @param date date of the measurement
  62.      * @param phase observed value (cycles)
  63.      * @param wavelength phase observed value wavelength (m)
  64.      * @param sigma theoretical standard deviation
  65.      * @param baseWeight base weight
  66.      */
  67.     public InterSatellitesPhase(final ObservableSatellite local,
  68.                                 final ObservableSatellite remote,
  69.                                 final AbsoluteDate date, final double phase,
  70.                                 final double wavelength, final double sigma,
  71.                                 final double baseWeight) {
  72.         // Call to super constructor
  73.         super(date, phase, sigma, baseWeight, Arrays.asList(local, remote));

  75.         // Initialize phase ambiguity driver
  76.         ambiguityDriver = new ParameterDriver(AMBIGUITY_NAME, 0.0, 1.0,
  77.                                               Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY);

  79.         // Add parameter drivers
  80.         addParameterDriver(ambiguityDriver);
  81.         addParameterDriver(local.getClockOffsetDriver());
  82.         addParameterDriver(remote.getClockOffsetDriver());

  84.         // Initialize fields
  85.         this.wavelength = wavelength;
  86.     }

  88.     /** Get the wavelength.
  89.      * @return wavelength (m)
  90.      */
  91.     public double getWavelength() {
  92.         return wavelength;
  93.     }

  95.     /** Get the driver for phase ambiguity.
  96.      * @return the driver for phase ambiguity
  97.      */
  98.     public ParameterDriver getAmbiguityDriver() {
  99.         return ambiguityDriver;
  100.     }

  102.     /** {@inheritDoc} */
  103.     @Override
  104.     protected EstimatedMeasurement<InterSatellitesPhase> theoreticalEvaluation(final int iteration,
  105.                                                                                final int evaluation,
  106.                                                                                final SpacecraftState[] states) {

  108.         // Phase derivatives are computed with respect to spacecrafts states in inertial frame
  109.         // ----------------------
  110.         //
  111.         // Parameters:
  112.         //  - 0..2  - Position of the receiver satellite in inertial frame
  113.         //  - 3..5  - Velocity of the receiver satellite in inertial frame
  114.         //  - 6..8  - Position of the remote satellite in inertial frame
  115.         //  - 9..11 - Velocity of the remote satellite in inertial frame
  116.         //  - 12..  - Measurement parameters: ambiguity, local clock offset, remote clock offset...
  117.         int nbParams = 12;
  118.         final Map<String, Integer> indices = new HashMap<>();
  119.         for (ParameterDriver phaseMeasurementDriver : getParametersDrivers()) {
  120.             if (phaseMeasurementDriver.isSelected()) {
  121.                 indices.put(phaseMeasurementDriver.getName(), nbParams++);
  122.             }
  123.         }

  125.         // Coordinates of both satellites
  126.         final SpacecraftState local = states[0];
  127.         final TimeStampedFieldPVCoordinates<Gradient> pvaL = getCoordinates(local, 0, nbParams);
  128.         final SpacecraftState remote = states[1];
  129.         final TimeStampedFieldPVCoordinates<Gradient> pvaR = getCoordinates(remote, 6, nbParams);

  131.         // Compute propagation times
  132.         // Downlink delay
  133.         final Gradient dtl = getSatellites().get(0).getClockOffsetDriver().getValue(nbParams, indices);
  134.         final FieldAbsoluteDate<Gradient> arrivalDate = new FieldAbsoluteDate<>(getDate(), dtl.negate());

  136.         final TimeStampedFieldPVCoordinates<Gradient> s1Downlink =
  137.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  138.         final Gradient tauD = signalTimeOfFlight(pvaR, s1Downlink.getPosition(), arrivalDate);

  140.         // Transit state
  141.         final double   delta      = getDate().durationFrom(remote.getDate());
  142.         final Gradient deltaMTauD = tauD.negate().add(delta);

  144.         // prepare the evaluation
  145.         final EstimatedMeasurement<InterSatellitesPhase> estimatedPhase =
  146.                         new EstimatedMeasurement<>(this, iteration, evaluation,
  147.                                                    new SpacecraftState[] {
  148.                                                        local.shiftedBy(deltaMTauD.getValue()),
  149.                                                        remote.shiftedBy(deltaMTauD.getValue())
  150.                                                    }, new TimeStampedPVCoordinates[] {
  151.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  152.                                                        local.shiftedBy(delta).getPVCoordinates()
  153.                                                    });

  155.         // Clock offsets
  156.         final Gradient dtr = getSatellites().get(1).getClockOffsetDriver().getValue(nbParams, indices);

  158.         // Phase value
  159.         final double   cOverLambda = Constants.SPEED_OF_LIGHT / wavelength;
  160.         final Gradient ambiguity   = ambiguityDriver.getValue(nbParams, indices);
  161.         final Gradient phase       = tauD.add(dtl).subtract(dtr).multiply(cOverLambda).add(ambiguity);

  163.         estimatedPhase.setEstimatedValue(phase.getValue());

  165.         // Range partial derivatives with respect to states
  166.         final double[] derivatives = phase.getGradient();
  167.         estimatedPhase.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0,  6));
  168.         estimatedPhase.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 6, 12));

  170.         // Set partial derivatives with respect to parameters
  171.         for (final ParameterDriver driver : getParametersDrivers()) {
  172.             final Integer index = indices.get(driver.getName());
  173.             if (index != null) {
  174.                 estimatedPhase.setParameterDerivatives(driver, derivatives[index]);
  175.             }
  176.         }

  178.         // Return the estimated measurement
  179.         return estimatedPhase;

  181.     }

  183. }
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