RangeRate.java

  1. /* Copyright 2002-2020 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;

  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.hipparchus.analysis.differentiation.GradientField;
  25. import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  26. import org.orekit.frames.FieldTransform;
  27. import org.orekit.propagation.SpacecraftState;
  28. import org.orekit.time.AbsoluteDate;
  29. import org.orekit.time.FieldAbsoluteDate;
  30. import org.orekit.utils.ParameterDriver;
  31. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  32. import org.orekit.utils.TimeStampedPVCoordinates;

  34. /** Class modeling one-way or two-way range rate measurement between two vehicles.
  35.  * One-way range rate (or Doppler) measurements generally apply to specific satellites
  36.  * (e.g. GNSS, DORIS), where a signal is transmitted from a satellite to a
  37.  * measuring station.
  38.  * Two-way range rate measurements are applicable to any system. The signal is
  39.  * transmitted to the (non-spinning) satellite and returned by a transponder
  40.  * (or reflected back)to the same measuring station.
  41.  * The Doppler measurement can be obtained by multiplying the velocity by (fe/c), where
  42.  * fe is the emission frequency.
  43.  *
  44.  * @author Thierry Ceolin
  45.  * @author Joris Olympio
  46.  * @since 8.0
  47.  */
  48. public class RangeRate extends AbstractMeasurement<RangeRate> {

  50.     /** Ground station from which measurement is performed. */
  51.     private final GroundStation station;

  53.     /** Flag indicating whether it is a two-way measurement. */
  54.     private final boolean twoway;

  56.     /** Simple constructor.
  57.      * @param station ground station from which measurement is performed
  58.      * @param date date of the measurement
  59.      * @param rangeRate observed value, m/s
  60.      * @param sigma theoretical standard deviation
  61.      * @param baseWeight base weight
  62.      * @param twoway if true, this is a two-way measurement
  63.      * @param satellite satellite related to this measurement
  64.      * @since 9.3
  65.      */
  66.     public RangeRate(final GroundStation station, final AbsoluteDate date,
  67.                      final double rangeRate, final double sigma, final double baseWeight,
  68.                      final boolean twoway, final ObservableSatellite satellite) {
  69.         super(date, rangeRate, sigma, baseWeight, Arrays.asList(satellite));
  70.         addParameterDriver(station.getClockOffsetDriver());
  71.         addParameterDriver(station.getEastOffsetDriver());
  72.         addParameterDriver(station.getNorthOffsetDriver());
  73.         addParameterDriver(station.getZenithOffsetDriver());
  74.         addParameterDriver(station.getPrimeMeridianOffsetDriver());
  75.         addParameterDriver(station.getPrimeMeridianDriftDriver());
  76.         addParameterDriver(station.getPolarOffsetXDriver());
  77.         addParameterDriver(station.getPolarDriftXDriver());
  78.         addParameterDriver(station.getPolarOffsetYDriver());
  79.         addParameterDriver(station.getPolarDriftYDriver());
  80.         this.station = station;
  81.         this.twoway  = twoway;
  82.     }

  84.     /** Check if the instance represents a two-way measurement.
  85.      * @return true if the instance represents a two-way measurement
  86.      */
  87.     public boolean isTwoWay() {
  88.         return twoway;
  89.     }

  91.     /** Get the ground station from which measurement is performed.
  92.      * @return ground station from which measurement is performed
  93.      */
  94.     public GroundStation getStation() {
  95.         return station;
  96.     }

  98.     /** {@inheritDoc} */
  99.     @Override
  100.     protected EstimatedMeasurement<RangeRate> theoreticalEvaluation(final int iteration, final int evaluation,
  101.                                                                     final SpacecraftState[] states) {

  103.         final SpacecraftState state = states[0];

  105.         // Range-rate derivatives are computed with respect to spacecraft state in inertial frame
  106.         // and station position in station's offset frame
  107.         // -------
  108.         //
  109.         // Parameters:
  110.         //  - 0..2 - Position of the spacecraft in inertial frame
  111.         //  - 3..5 - Velocity of the spacecraft in inertial frame
  112.         //  - 6..n - station parameters (clock offset, station offsets, pole, prime meridian...)
  113.         int nbParams = 6;
  114.         final Map<String, Integer> indices = new HashMap<>();
  115.         for (ParameterDriver driver : getParametersDrivers()) {
  116.             if (driver.isSelected()) {
  117.                 indices.put(driver.getName(), nbParams++);
  118.             }
  119.         }
  120.         final FieldVector3D<Gradient> zero = FieldVector3D.getZero(GradientField.getField(nbParams));

  122.         // Coordinates of the spacecraft expressed as a gradient
  123.         final TimeStampedFieldPVCoordinates<Gradient> pvaDS = getCoordinates(state, 0, nbParams);

  125.         // transform between station and inertial frame, expressed as a gradient
  126.         // The components of station's position in offset frame are the 3 last derivative parameters
  127.         final FieldTransform<Gradient> offsetToInertialDownlink =
  128.                         station.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
  129.         final FieldAbsoluteDate<Gradient> downlinkDateDS =
  130.                         offsetToInertialDownlink.getFieldDate();

  132.         // Station position in inertial frame at end of the downlink leg
  133.         final TimeStampedFieldPVCoordinates<Gradient> stationDownlink =
  134.                         offsetToInertialDownlink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDateDS,
  135.                                                                                                             zero, zero, zero));

  137.         // Compute propagation times
  138.         // (if state has already been set up to pre-compensate propagation delay,
  139.         //  we will have delta == tauD and transitState will be the same as state)

  141.         // Downlink delay
  142.         final Gradient tauD = signalTimeOfFlight(pvaDS, stationDownlink.getPosition(), downlinkDateDS);

  144.         // Transit state
  145.         final Gradient        delta        = downlinkDateDS.durationFrom(state.getDate());
  146.         final Gradient        deltaMTauD   = tauD.negate().add(delta);
  147.         final SpacecraftState transitState = state.shiftedBy(deltaMTauD.getValue());

  149.         // Transit state (re)computed with gradients
  150.         final TimeStampedFieldPVCoordinates<Gradient> transitPV = pvaDS.shiftedBy(deltaMTauD);

  152.         // one-way (downlink) range-rate
  153.         final EstimatedMeasurement<RangeRate> evalOneWay1 =
  154.                         oneWayTheoreticalEvaluation(iteration, evaluation, true,
  155.                                                     stationDownlink, transitPV, transitState, indices);
  156.         final EstimatedMeasurement<RangeRate> estimated;
  157.         if (twoway) {
  158.             // one-way (uplink) light time correction
  159.             final FieldTransform<Gradient> offsetToInertialApproxUplink =
  160.                             station.getOffsetToInertial(state.getFrame(),
  161.                                                         downlinkDateDS.shiftedBy(tauD.multiply(-2)), nbParams, indices);
  162.             final FieldAbsoluteDate<Gradient> approxUplinkDateDS =
  163.                             offsetToInertialApproxUplink.getFieldDate();

  165.             final TimeStampedFieldPVCoordinates<Gradient> stationApproxUplink =
  166.                             offsetToInertialApproxUplink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(approxUplinkDateDS,
  167.                                                                                                                     zero, zero, zero));

  169.             final Gradient tauU = signalTimeOfFlight(stationApproxUplink, transitPV.getPosition(), transitPV.getDate());

  171.             final TimeStampedFieldPVCoordinates<Gradient> stationUplink =
  172.                             stationApproxUplink.shiftedBy(transitPV.getDate().durationFrom(approxUplinkDateDS).subtract(tauU));

  174.             final EstimatedMeasurement<RangeRate> evalOneWay2 =
  175.                             oneWayTheoreticalEvaluation(iteration, evaluation, false,
  176.                                                         stationUplink, transitPV, transitState, indices);

  178.             // combine uplink and downlink values
  179.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  180.                                                    evalOneWay1.getStates(),
  181.                                                    new TimeStampedPVCoordinates[] {
  182.                                                        evalOneWay2.getParticipants()[0],
  183.                                                        evalOneWay1.getParticipants()[0],
  184.                                                        evalOneWay1.getParticipants()[1]
  185.                                                    });
  186.             estimated.setEstimatedValue(0.5 * (evalOneWay1.getEstimatedValue()[0] + evalOneWay2.getEstimatedValue()[0]));

  188.             // combine uplink and downlink partial derivatives with respect to state
  189.             final double[][] sd1 = evalOneWay1.getStateDerivatives(0);
  190.             final double[][] sd2 = evalOneWay2.getStateDerivatives(0);
  191.             final double[][] sd = new double[sd1.length][sd1[0].length];
  192.             for (int i = 0; i < sd.length; ++i) {
  193.                 for (int j = 0; j < sd[0].length; ++j) {
  194.                     sd[i][j] = 0.5 * (sd1[i][j] + sd2[i][j]);
  195.                 }
  196.             }
  197.             estimated.setStateDerivatives(0, sd);

  199.             // combine uplink and downlink partial derivatives with respect to parameters
  200.             evalOneWay1.getDerivativesDrivers().forEach(driver -> {
  201.                 final double[] pd1 = evalOneWay1.getParameterDerivatives(driver);
  202.                 final double[] pd2 = evalOneWay2.getParameterDerivatives(driver);
  203.                 final double[] pd = new double[pd1.length];
  204.                 for (int i = 0; i < pd.length; ++i) {
  205.                     pd[i] = 0.5 * (pd1[i] + pd2[i]);
  206.                 }
  207.                 estimated.setParameterDerivatives(driver, pd);
  208.             });

  210.         } else {
  211.             estimated = evalOneWay1;
  212.         }

  214.         return estimated;

  216.     }

  218.     /** Evaluate measurement in one-way.
  219.      * @param iteration iteration number
  220.      * @param evaluation evaluations counter
  221.      * @param downlink indicator for downlink leg
  222.      * @param stationPV station coordinates when signal is at station
  223.      * @param transitPV spacecraft coordinates at onboard signal transit
  224.      * @param transitState orbital state at onboard signal transit
  225.      * @param indices indices of the estimated parameters in derivatives computations
  226.      * @return theoretical value
  227.      * @see #evaluate(SpacecraftStatet)
  228.      */
  229.     private EstimatedMeasurement<RangeRate> oneWayTheoreticalEvaluation(final int iteration, final int evaluation, final boolean downlink,
  230.                                                                         final TimeStampedFieldPVCoordinates<Gradient> stationPV,
  231.                                                                         final TimeStampedFieldPVCoordinates<Gradient> transitPV,
  232.                                                                         final SpacecraftState transitState,
  233.                                                                         final Map<String, Integer> indices) {

  235.         // prepare the evaluation
  236.         final EstimatedMeasurement<RangeRate> estimated =
  237.                         new EstimatedMeasurement<RangeRate>(this, iteration, evaluation,
  238.                                                             new SpacecraftState[] {
  239.                                                                 transitState
  240.                                                             }, new TimeStampedPVCoordinates[] {
  241.                                                                 (downlink ? transitPV : stationPV).toTimeStampedPVCoordinates(),
  242.                                                                 (downlink ? stationPV : transitPV).toTimeStampedPVCoordinates()
  243.                                                             });

  245.         // range rate value
  246.         final FieldVector3D<Gradient> stationPosition  = stationPV.getPosition();
  247.         final FieldVector3D<Gradient> relativePosition = stationPosition.subtract(transitPV.getPosition());

  249.         final FieldVector3D<Gradient> stationVelocity  = stationPV.getVelocity();
  250.         final FieldVector3D<Gradient> relativeVelocity = stationVelocity.subtract(transitPV.getVelocity());

  252.         // radial direction
  253.         final FieldVector3D<Gradient> lineOfSight      = relativePosition.normalize();

  255.         // range rate
  256.         final Gradient rangeRate = FieldVector3D.dotProduct(relativeVelocity, lineOfSight);

  258.         estimated.setEstimatedValue(rangeRate.getValue());

  260.         // compute partial derivatives of (rr) with respect to spacecraft state Cartesian coordinates
  261.         final double[] derivatives = rangeRate.getGradient();
  262.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));

  264.         // set partial derivatives with respect to parameters
  265.         // (beware element at index 0 is the value, not a derivative)
  266.         for (final ParameterDriver driver : getParametersDrivers()) {
  267.             final Integer index = indices.get(driver.getName());
  268.             if (index != null) {
  269.                 estimated.setParameterDerivatives(driver, derivatives[index]);
  270.             }
  271.         }

  273.         return estimated;

  275.     }

  277. }
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