InterSatellitesRange.java

  1. /* Copyright 2002-2019 CS Systèmes d'Information
  2.  * Licensed to CS Systèmes d'Information (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.DSFactory;
  24. import org.hipparchus.analysis.differentiation.DerivativeStructure;
  25. import org.orekit.propagation.SpacecraftState;
  26. import org.orekit.time.AbsoluteDate;
  27. import org.orekit.time.FieldAbsoluteDate;
  28. import org.orekit.utils.Constants;
  29. import org.orekit.utils.ParameterDriver;
  30. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  31. import org.orekit.utils.TimeStampedPVCoordinates;

  33. /** One-way or two-way range measurements between two satellites.
  34.  * <p>
  35.  * For one-way measurements, a signal is emitted by a remote satellite and received
  36.  * by local satellite. The measurement value is the elapsed time between emission
  37.  * and reception multiplied by c where c is the speed of light.
  38.  * </p>
  39.  * <p>
  40.  * For two-way measurements, a signal is emitted by local satellite, reflected on
  41.  * remote satellite, and received back by local satellite. The measurement value
  42.  * is the elapsed time between emission and reception multiplied by c/2 where c
  43.  * is the speed of light.
  44.  * </p>
  45.  * <p>
  46.  * Since 9.3, this class also uses the clock offsets of both satellites,
  47.  * which manage the value that must be added to each satellite reading of time to
  48.  * compute the real physical date. In this measurement, these offsets have two effects:
  49.  * </p>
  50.  * <ul>
  51.  *   <li>as measurement date is evaluated at reception time, the real physical date
  52.  *   of the measurement is the observed date to which the local satellite clock
  53.  *   offset is subtracted</li>
  54.  *   <li>as range is evaluated using the total signal time of flight, for one-way
  55.  *   measurements the observed range is the real physical signal time of flight to
  56.  *   which (Δtl - Δtr) ⨉ c is added, where Δtl (resp. Δtr) is the clock offset for the
  57.  *   local satellite (resp. remote satellite). A similar effect exists in
  58.  *   two-way measurements but it is computed as (Δtl - Δtl) ⨉ c / 2 as the local satellite
  59.  *   clock is used for both initial emission and final reception and therefore it evaluates
  60.  *   to zero.</li>
  61.  * </ul>
  62.  * <p>
  63.  * The motion of both satellites during the signal flight time is
  64.  * taken into account. The date of the measurement corresponds to
  65.  * the reception of the signal by satellite 1.
  66.  * </p>
  67.  * @author Luc Maisonobe
  68.  * @since 9.0
  69.  */
  70. public class InterSatellitesRange extends AbstractMeasurement<InterSatellitesRange> {

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

  75.     /** Simple constructor.
  76.      * @param localIndex index of local satellite propagator
  77.      * (i.e. the satellite which receives the signal and performs
  78.      * the measurement)
  79.      * @param remoteIndex index of remote satellite propagator
  80.      * (i.e. the satellite which simply emits the signal in the one-way
  81.      * case, or reflects the signal in the two-way case)
  82.      * @param twoWay flag indicating whether it is a two-way measurement
  83.      * @param date date of the measurement
  84.      * @param range observed value
  85.      * @param sigma theoretical standard deviation
  86.      * @param baseWeight base weight
  87.      * @deprecated as of 9.3, replaced by {@link #InterSatellitesRange(ObservableSatellite, ObservableSatellite,
  88.      * boolean, AbsoluteDate, double, double, double)}
  89.      */
  90.     @Deprecated
  91.     public InterSatellitesRange(final int localIndex, final int remoteIndex,
  92.                                 final boolean twoWay,
  93.                                 final AbsoluteDate date, final double range,
  94.                                 final double sigma, final double baseWeight) {
  95.         this(new ObservableSatellite(localIndex), new ObservableSatellite(remoteIndex),
  96.              twoWay, date, range, sigma, baseWeight);
  97.     }

  99.     /** Simple constructor.
  100.      * @param local satellite which receives the signal and performs the measurement
  101.      * @param remote satellite which simply emits the signal in the one-way case,
  102.      * or reflects the signal in the two-way case
  103.      * @param twoWay flag indicating whether it is a two-way measurement
  104.      * @param date date of the measurement
  105.      * @param range observed value
  106.      * @param sigma theoretical standard deviation
  107.      * @param baseWeight base weight
  108.      * @since 9.3
  109.      */
  110.     public InterSatellitesRange(final ObservableSatellite local,
  111.                                 final ObservableSatellite remote,
  112.                                 final boolean twoWay,
  113.                                 final AbsoluteDate date, final double range,
  114.                                 final double sigma, final double baseWeight) {
  115.         super(date, range, sigma, baseWeight, Arrays.asList(local, remote));
  116.         this.twoway = twoWay;
  117.     }

  119.     /** Check if the instance represents a two-way measurement.
  120.      * @return true if the instance represents a two-way measurement
  121.      */
  122.     public boolean isTwoWay() {
  123.         return twoway;
  124.     }

  126.     /** {@inheritDoc} */
  127.     @Override
  128.     protected EstimatedMeasurement<InterSatellitesRange> theoreticalEvaluation(final int iteration,
  129.                                                                                final int evaluation,
  130.                                                                                final SpacecraftState[] states) {

  132.         // Range derivatives are computed with respect to spacecrafts states in inertial frame
  133.         // ----------------------
  134.         //
  135.         // Parameters:
  136.         //  - 0..2  - Position of the receiver satellite in inertial frame
  137.         //  - 3..5  - Velocity of the receiver satellite in inertial frame
  138.         //  - 6..8  - Position of the remote satellite in inertial frame
  139.         //  - 9..11 - Velocity of the remote satellite in inertial frame
  140.         //  - 12..  - Measurement parameters: local clock offset, remote clock offset...
  141.         int nbParams = 12;
  142.         final Map<String, Integer> indices = new HashMap<>();
  143.         for (ParameterDriver driver : getParametersDrivers()) {
  144.             if (driver.isSelected()) {
  145.                 indices.put(driver.getName(), nbParams++);
  146.             }
  147.         }
  148.         final DSFactory factory = new DSFactory(nbParams, 1);

  150.         // coordinates of both satellites
  151.         final ObservableSatellite local = getSatellites().get(0);
  152.         final SpacecraftState stateL = states[local.getPropagatorIndex()];
  153.         final TimeStampedFieldPVCoordinates<DerivativeStructure> pvaL = getCoordinates(stateL, 0, factory);
  154.         final ObservableSatellite remote = getSatellites().get(1);
  155.         final SpacecraftState stateR = states[remote.getPropagatorIndex()];
  156.         final TimeStampedFieldPVCoordinates<DerivativeStructure> pvaR = getCoordinates(stateR, 6, factory);

  158.         // compute propagation times
  159.         // (if state has already been set up to pre-compensate propagation delay,
  160.         //  we will have delta == tauD and transitState will be the same as state)

  162.         // downlink delay
  163.         final DerivativeStructure dtl = local.getClockOffsetDriver().getValue(factory, indices);
  164.         final FieldAbsoluteDate<DerivativeStructure> arrivalDate =
  165.                         new FieldAbsoluteDate<DerivativeStructure>(getDate(), dtl.negate());

  167.         final TimeStampedFieldPVCoordinates<DerivativeStructure> s1Downlink =
  168.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  169.         final DerivativeStructure tauD = signalTimeOfFlight(pvaR, s1Downlink.getPosition(), arrivalDate);

  171.         // Transit state
  172.         final double              delta      = getDate().durationFrom(stateR.getDate());
  173.         final DerivativeStructure deltaMTauD = tauD.negate().add(delta);

  175.         // prepare the evaluation
  176.         final EstimatedMeasurement<InterSatellitesRange> estimated;

  178.         final DerivativeStructure range;
  179.         if (twoway) {
  180.             // Transit state (re)computed with derivative structures
  181.             final TimeStampedFieldPVCoordinates<DerivativeStructure> transitStateDS = pvaR.shiftedBy(deltaMTauD);

  183.             // uplink delay
  184.             final DerivativeStructure tauU = signalTimeOfFlight(pvaL,
  185.                                                                 transitStateDS.getPosition(),
  186.                                                                 transitStateDS.getDate());
  187.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  188.                                                    new SpacecraftState[] {
  189.                                                        stateL.shiftedBy(deltaMTauD.getValue()),
  190.                                                        stateR.shiftedBy(deltaMTauD.getValue())
  191.                                                    }, new TimeStampedPVCoordinates[] {
  192.                                                        stateL.shiftedBy(delta - tauD.getValue() - tauU.getValue()).getPVCoordinates(),
  193.                                                        stateR.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  194.                                                        stateL.shiftedBy(delta).getPVCoordinates()
  195.                                                    });

  197.             // Range value
  198.             range  = tauD.add(tauU).multiply(0.5 * Constants.SPEED_OF_LIGHT);

  200.         } else {

  202.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  203.                                                    new SpacecraftState[] {
  204.                                                        stateL.shiftedBy(deltaMTauD.getValue()),
  205.                                                        stateR.shiftedBy(deltaMTauD.getValue())
  206.                                                    }, new TimeStampedPVCoordinates[] {
  207.                                                        stateR.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  208.                                                        stateL.shiftedBy(delta).getPVCoordinates()
  209.                                                    });

  211.             // Clock offsets
  212.             final DerivativeStructure dtr = remote.getClockOffsetDriver().getValue(factory, indices);

  214.             // Range value
  215.             range  = tauD.add(dtl).subtract(dtr).multiply(Constants.SPEED_OF_LIGHT);

  217.         }
  218.         estimated.setEstimatedValue(range.getValue());

  220.         // Range partial derivatives with respect to states
  221.         final double[] derivatives = range.getAllDerivatives();
  222.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 1,  7));
  223.         estimated.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 7, 13));

  225.         return estimated;

  227.     }

  229. }
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