InterSatellitesRange.java

  1. /* Copyright 2002-2025 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.frames.Frame;
  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;
  32. import org.orekit.utils.TimeSpanMap.Span;

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

  73.     /** Type of the measurement. */
  74.     public static final String MEASUREMENT_TYPE = "InterSatellitesRange";

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

  79.     /** Simple constructor.
  80.      * @param local satellite which receives the signal and performs the measurement
  81.      * @param remote satellite which simply emits the signal in the one-way case,
  82.      * or reflects the signal in the two-way case
  83.      * @param twoWay flag indicating whether it is a two-way measurement
  84.      * @param date date of the measurement
  85.      * @param range observed value
  86.      * @param sigma theoretical standard deviation
  87.      * @param baseWeight base weight
  88.      * @since 9.3
  89.      */
  90.     public InterSatellitesRange(final ObservableSatellite local,
  91.                                 final ObservableSatellite remote,
  92.                                 final boolean twoWay,
  93.                                 final AbsoluteDate date, final double range,
  94.                                 final double sigma, final double baseWeight) {
  95.         super(date, range, sigma, baseWeight, Arrays.asList(local, remote));
  96.         // for one way and two ways measurements, the local satellite clock offsets affects the measurement
  97.         addParameterDriver(local.getClockOffsetDriver());
  98.         addParameterDriver(local.getClockDriftDriver());
  99.         addParameterDriver(local.getClockAccelerationDriver());
  100.         if (!twoWay) {
  101.             // for one way measurements, the remote satellite clock offsets also affects the measurement
  102.             addParameterDriver(remote.getClockOffsetDriver());
  103.             addParameterDriver(remote.getClockDriftDriver());
  104.             addParameterDriver(remote.getClockAccelerationDriver());
  105.         }
  106.         this.twoway = twoWay;
  107.     }

  109.     /** Check if the instance represents a two-way measurement.
  110.      * @return true if the instance represents a two-way measurement
  111.      */
  112.     public boolean isTwoWay() {
  113.         return twoway;
  114.     }

  116.     /** {@inheritDoc} */
  117.     @Override
  118.     protected EstimatedMeasurementBase<InterSatellitesRange> theoreticalEvaluationWithoutDerivatives(final int iteration,
  119.                                                                                                      final int evaluation,
  120.                                                                                                      final SpacecraftState[] states) {

  122.         // coordinates of both satellites
  123.         final Frame           frame = states[0].getFrame();
  124.         final SpacecraftState local = states[0];
  125.         final TimeStampedPVCoordinates pvaL = local.getPVCoordinates(frame);
  126.         final SpacecraftState remote = states[1];
  127.         final TimeStampedPVCoordinates pvaR = remote.getPVCoordinates(frame);

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

  133.         // downlink delay
  134.         final double dtl = getSatellites().get(0).getClockOffsetDriver().getValue(local.getDate());
  135.         final AbsoluteDate arrivalDate = getDate().shiftedBy(-dtl);

  137.         final TimeStampedPVCoordinates s1Downlink =
  138.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  139.         final double tauD = signalTimeOfFlightAdjustableEmitter(pvaR, s1Downlink.getPosition(), arrivalDate, frame);

  141.         // Transit state
  142.         final double delta      = getDate().durationFrom(remote.getDate());
  143.         final double deltaMTauD = delta - tauD;

  145.         // prepare the evaluation
  146.         final EstimatedMeasurementBase<InterSatellitesRange> estimated;

  148.         final double range;
  149.         if (twoway) {
  150.             // Transit state (re)computed with derivative structures
  151.             final TimeStampedPVCoordinates transitState = pvaR.shiftedBy(deltaMTauD);

  153.             // uplink delay
  154.             final double tauU = signalTimeOfFlightAdjustableEmitter(pvaL,
  155.                                                                     transitState.getPosition(),
  156.                                                                     transitState.getDate(),
  157.                                                                     frame);
  158.             estimated = new EstimatedMeasurementBase<>(this, iteration, evaluation,
  159.                                                        new SpacecraftState[] {
  160.                                                            local.shiftedBy(deltaMTauD),
  161.                                                            remote.shiftedBy(deltaMTauD)
  162.                                                        }, new TimeStampedPVCoordinates[] {
  163.                                                            local.shiftedBy(delta - tauD - tauU).getPVCoordinates(frame),
  164.                                                            remote.shiftedBy(delta - tauD).getPVCoordinates(frame),
  165.                                                            local.shiftedBy(delta).getPVCoordinates(frame)
  166.                                                        });

  168.             // Range value
  169.             range  = (tauD + tauU) * (0.5 * Constants.SPEED_OF_LIGHT);

  171.         } else {

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

  182.             // Clock offsets
  183.             final double dtr = getSatellites().get(1).getClockOffsetDriver().getValue(remote.getDate());

  185.             // Range value
  186.             range  = (tauD + dtl - dtr) * Constants.SPEED_OF_LIGHT;

  188.         }
  189.         estimated.setEstimatedValue(range);

  191.         return estimated;

  193.     }

  195.     /** {@inheritDoc} */
  196.     @Override
  197.     protected EstimatedMeasurement<InterSatellitesRange> theoreticalEvaluation(final int iteration,
  198.                                                                                final int evaluation,
  199.                                                                                final SpacecraftState[] states) {

  201.         final Frame frame = states[0].getFrame();

  203.         // Range derivatives are computed with respect to spacecraft states in inertial frame
  204.         // ----------------------
  205.         //
  206.         // Parameters:
  207.         //  - 0..2  - Position of the receiver satellite in inertial frame
  208.         //  - 3..5  - Velocity of the receiver satellite in inertial frame
  209.         //  - 6..8  - Position of the remote satellite in inertial frame
  210.         //  - 9..11 - Velocity of the remote satellite in inertial frame
  211.         //  - 12..  - Measurement parameters: local clock offset, remote clock offset...
  212.         int nbParams = 12;
  213.         final Map<String, Integer> indices = new HashMap<>();
  214.         for (ParameterDriver driver : getParametersDrivers()) {
  215.             if (driver.isSelected()) {
  216.                 for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {

  218.                     if (!indices.containsKey(span.getData())) {
  219.                         indices.put(span.getData(), nbParams++);
  220.                     }
  221.                 }
  222.             }
  223.         }

  225.         // coordinates of both satellites
  226.         final SpacecraftState local = states[0];
  227.         final TimeStampedFieldPVCoordinates<Gradient> pvaL = getCoordinates(local, 0, nbParams);
  228.         final SpacecraftState remote = states[1];
  229.         final TimeStampedFieldPVCoordinates<Gradient> pvaR = states[1].
  230.                                                              getFrame().
  231.                                                              getTransformTo(frame, states[1].getDate()).
  232.                                                              transformPVCoordinates(getCoordinates(remote, 6, nbParams));

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

  238.         // downlink delay
  239.         final Gradient dtl = getSatellites().get(0).getClockOffsetDriver().getValue(nbParams, indices, local.getDate());
  240.         final FieldAbsoluteDate<Gradient> arrivalDate =
  241.                         new FieldAbsoluteDate<>(getDate(), dtl.negate());

  243.         final TimeStampedFieldPVCoordinates<Gradient> s1Downlink =
  244.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  245.         final Gradient tauD = signalTimeOfFlightAdjustableEmitter(pvaR, s1Downlink.getPosition(),
  246.                                                                   arrivalDate, frame);

  248.         // Transit state
  249.         final double              delta      = getDate().durationFrom(remote.getDate());
  250.         final Gradient deltaMTauD = tauD.negate().add(delta);

  252.         // prepare the evaluation
  253.         final EstimatedMeasurement<InterSatellitesRange> estimated;

  255.         final Gradient range;
  256.         if (twoway) {
  257.             // Transit state (re)computed with derivative structures
  258.             final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaR.shiftedBy(deltaMTauD);

  260.             // uplink delay
  261.             final Gradient tauU = signalTimeOfFlightAdjustableEmitter(pvaL,
  262.                                                                       transitStateDS.getPosition(),
  263.                                                                       transitStateDS.getDate(),
  264.                                                                       frame);
  265.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  266.                                                    new SpacecraftState[] {
  267.                                                        local.shiftedBy(deltaMTauD.getValue()),
  268.                                                        remote.shiftedBy(deltaMTauD.getValue())
  269.                                                    }, new TimeStampedPVCoordinates[] {
  270.                                                        local.shiftedBy(delta - tauD.getValue() - tauU.getValue()).getPVCoordinates(frame),
  271.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(frame),
  272.                                                        local.shiftedBy(delta).getPVCoordinates(frame)
  273.                                                    });

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

  278.         } else {

  280.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  281.                                                    new SpacecraftState[] {
  282.                                                        local.shiftedBy(deltaMTauD.getValue()),
  283.                                                        remote.shiftedBy(deltaMTauD.getValue())
  284.                                                    }, new TimeStampedPVCoordinates[] {
  285.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(frame),
  286.                                                        local.shiftedBy(delta).getPVCoordinates()
  287.                                                    });

  289.             // Clock offsets
  290.             final Gradient dtr = getSatellites().get(1).getClockOffsetDriver().getValue(nbParams, indices, remote.getDate());

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

  295.         }
  296.         estimated.setEstimatedValue(range.getValue());

  298.         // Range first order derivatives with respect to states
  299.         final double[] derivatives = range.getGradient();
  300.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0,  6));
  301.         estimated.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 6, 12));

  303.         // Set first order derivatives with respect to parameters
  304.         for (final ParameterDriver driver : getParametersDrivers()) {
  305.             for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {
  306.                 final Integer index = indices.get(span.getData());
  307.                 if (index != null) {
  308.                     estimated.setParameterDerivatives(driver, span.getStart(), derivatives[index]);
  309.                 }
  310.             }
  311.         }

  313.         return estimated;

  315.     }

  317. }
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