InterSatellitesRange.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.orekit.propagation.SpacecraftState;
  25. import org.orekit.time.AbsoluteDate;
  26. import org.orekit.time.FieldAbsoluteDate;
  27. import org.orekit.utils.Constants;
  28. import org.orekit.utils.ParameterDriver;
  29. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  30. import org.orekit.utils.TimeStampedPVCoordinates;

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

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

  74.     /** Simple constructor.
  75.      * @param local satellite which receives the signal and performs the measurement
  76.      * @param remote satellite which simply emits the signal in the one-way case,
  77.      * or reflects the signal in the two-way case
  78.      * @param twoWay flag indicating whether it is a two-way measurement
  79.      * @param date date of the measurement
  80.      * @param range observed value
  81.      * @param sigma theoretical standard deviation
  82.      * @param baseWeight base weight
  83.      * @since 9.3
  84.      */
  85.     public InterSatellitesRange(final ObservableSatellite local,
  86.                                 final ObservableSatellite remote,
  87.                                 final boolean twoWay,
  88.                                 final AbsoluteDate date, final double range,
  89.                                 final double sigma, final double baseWeight) {
  90.         super(date, range, sigma, baseWeight, Arrays.asList(local, remote));
  91.         this.twoway = twoWay;
  92.     }

  94.     /** Check if the instance represents a two-way measurement.
  95.      * @return true if the instance represents a two-way measurement
  96.      */
  97.     public boolean isTwoWay() {
  98.         return twoway;
  99.     }

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

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

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

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

  134.         // downlink delay
  135.         final Gradient dtl = getSatellites().get(0).getClockOffsetDriver().getValue(nbParams, indices);
  136.         final FieldAbsoluteDate<Gradient> arrivalDate =
  137.                         new FieldAbsoluteDate<>(getDate(), dtl.negate());

  139.         final TimeStampedFieldPVCoordinates<Gradient> s1Downlink =
  140.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  141.         final Gradient tauD = signalTimeOfFlight(pvaR, s1Downlink.getPosition(), arrivalDate);

  143.         // Transit state
  144.         final double              delta      = getDate().durationFrom(remote.getDate());
  145.         final Gradient deltaMTauD = tauD.negate().add(delta);

  147.         // prepare the evaluation
  148.         final EstimatedMeasurement<InterSatellitesRange> estimated;

  150.         final Gradient range;
  151.         if (twoway) {
  152.             // Transit state (re)computed with derivative structures
  153.             final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaR.shiftedBy(deltaMTauD);

  155.             // uplink delay
  156.             final Gradient tauU = signalTimeOfFlight(pvaL,
  157.                                                      transitStateDS.getPosition(),
  158.                                                      transitStateDS.getDate());
  159.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  160.                                                    new SpacecraftState[] {
  161.                                                        local.shiftedBy(deltaMTauD.getValue()),
  162.                                                        remote.shiftedBy(deltaMTauD.getValue())
  163.                                                    }, new TimeStampedPVCoordinates[] {
  164.                                                        local.shiftedBy(delta - tauD.getValue() - tauU.getValue()).getPVCoordinates(),
  165.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  166.                                                        local.shiftedBy(delta).getPVCoordinates()
  167.                                                    });

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

  172.         } else {

  174.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  175.                                                    new SpacecraftState[] {
  176.                                                        local.shiftedBy(deltaMTauD.getValue()),
  177.                                                        remote.shiftedBy(deltaMTauD.getValue())
  178.                                                    }, new TimeStampedPVCoordinates[] {
  179.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  180.                                                        local.shiftedBy(delta).getPVCoordinates()
  181.                                                    });

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

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

  189.         }
  190.         estimated.setEstimatedValue(range.getValue());

  192.         // Range partial derivatives with respect to states
  193.         final double[] derivatives = range.getGradient();
  194.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0,  6));
  195.         estimated.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 6, 12));

  197.         return estimated;

  199.     }

  201. }
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