Range.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.Field;
  24. import org.hipparchus.analysis.differentiation.DSFactory;
  25. import org.hipparchus.analysis.differentiation.DerivativeStructure;
  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;

  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> {

  80.     /** Ground station from which measurement is performed. */
  81.     private final GroundStation station;

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

  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, Arrays.asList(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.     }

  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.     }

  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.     }

  132.     /** {@inheritDoc} */
  133.     @Override
  134.     protected EstimatedMeasurement<Range> theoreticalEvaluation(final int iteration,
  135.                                                                 final int evaluation,
  136.                                                                 final SpacecraftState[] states) {

  138.         final SpacecraftState state = states[0];

  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 DSFactory                          factory = new DSFactory(nbParams, 1);
  156.         final Field<DerivativeStructure>         field   = factory.getDerivativeField();
  157.         final FieldVector3D<DerivativeStructure> zero    = FieldVector3D.getZero(field);

  159.         // Coordinates of the spacecraft expressed as a derivative structure
  160.         final TimeStampedFieldPVCoordinates<DerivativeStructure> pvaDS = getCoordinates(state, 0, factory);

  162.         // transform between station and inertial frame, expressed as a derivative structure
  163.         // The components of station's position in offset frame are the 3 last derivative parameters
  164.         final FieldTransform<DerivativeStructure> offsetToInertialDownlink =
  165.                         station.getOffsetToInertial(state.getFrame(), getDate(), factory, indices);
  166.         final FieldAbsoluteDate<DerivativeStructure> downlinkDateDS = offsetToInertialDownlink.getFieldDate();

  168.         // Station position in inertial frame at end of the downlink leg
  169.         final TimeStampedFieldPVCoordinates<DerivativeStructure> stationDownlink =
  170.                         offsetToInertialDownlink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDateDS,
  171.                                                                                                             zero, zero, zero));

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

  177.         // Downlink delay
  178.         final DerivativeStructure tauD = signalTimeOfFlight(pvaDS, stationDownlink.getPosition(), downlinkDateDS);

  180.         // Transit state & Transit state (re)computed with derivative structures
  181.         final DerivativeStructure   delta        = downlinkDateDS.durationFrom(state.getDate());
  182.         final DerivativeStructure   deltaMTauD   = tauD.negate().add(delta);
  183.         final SpacecraftState       transitState = state.shiftedBy(deltaMTauD.getValue());
  184.         final TimeStampedFieldPVCoordinates<DerivativeStructure> transitStateDS = pvaDS.shiftedBy(deltaMTauD);

  186.         // prepare the evaluation
  187.         final EstimatedMeasurement<Range> estimated;
  188.         final DerivativeStructure range;

  190.         if (twoway) {

  192.             // Station at transit state date (derivatives of tauD taken into account)
  193.             final TimeStampedFieldPVCoordinates<DerivativeStructure> stationAtTransitDate =
  194.                             stationDownlink.shiftedBy(tauD.negate());
  195.             // Uplink delay
  196.             final DerivativeStructure tauU =
  197.                             signalTimeOfFlight(stationAtTransitDate, transitStateDS.getPosition(), transitStateDS.getDate());
  198.             final TimeStampedFieldPVCoordinates<DerivativeStructure> stationUplink =
  199.                             stationDownlink.shiftedBy(-tauD.getValue() - tauU.getValue());

  201.             // Prepare the evaluation
  202.             estimated = new EstimatedMeasurement<Range>(this, iteration, evaluation,
  203.                                                             new SpacecraftState[] {
  204.                                                                 transitState
  205.                                                             }, new TimeStampedPVCoordinates[] {
  206.                                                                 stationUplink.toTimeStampedPVCoordinates(),
  207.                                                                 transitStateDS.toTimeStampedPVCoordinates(),
  208.                                                                 stationDownlink.toTimeStampedPVCoordinates()
  209.                                                             });

  211.             // Range value
  212.             final double              cOver2 = 0.5 * Constants.SPEED_OF_LIGHT;
  213.             final DerivativeStructure tau    = tauD.add(tauU);
  214.             range                            = tau.multiply(cOver2);

  216.         } else {

  218.             estimated = new EstimatedMeasurement<Range>(this, iteration, evaluation,
  219.                             new SpacecraftState[] {
  220.                                 transitState
  221.                             }, new TimeStampedPVCoordinates[] {
  222.                                 transitStateDS.toTimeStampedPVCoordinates(),
  223.                                 stationDownlink.toTimeStampedPVCoordinates()
  224.                             });

  226.             // Clock offsets
  227.             final ObservableSatellite satellite = getSatellites().get(0);
  228.             final DerivativeStructure dts       = satellite.getClockOffsetDriver().getValue(factory, indices);
  229.             final DerivativeStructure dtg       = station.getClockOffsetDriver().getValue(factory, indices);

  231.             // Range value
  232.             range = tauD.add(dtg).subtract(dts).multiply(Constants.SPEED_OF_LIGHT);

  234.         }

  236.         estimated.setEstimatedValue(range.getValue());

  238.         // Range partial derivatives with respect to state
  239.         final double[] derivatives = range.getAllDerivatives();
  240.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 1, 7));

  242.         // set partial derivatives with respect to parameters
  243.         // (beware element at index 0 is the value, not a derivative)
  244.         for (final ParameterDriver driver : getParametersDrivers()) {
  245.             final Integer index = indices.get(driver.getName());
  246.             if (index != null) {
  247.                 estimated.setParameterDerivatives(driver, derivatives[index + 1]);
  248.             }
  249.         }

  251.         return estimated;

  253.     }

  255. }
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