InterSatellitesRange.java

  1. /* Copyright 2002-2024 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;
  31. import org.orekit.utils.TimeSpanMap.Span;

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

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

  78.     /** Simple constructor.
  79.      * @param local satellite which receives the signal and performs the measurement
  80.      * @param remote satellite which simply emits the signal in the one-way case,
  81.      * 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.      * @since 9.3
  88.      */
  89.     public InterSatellitesRange(final ObservableSatellite local,
  90.                                 final ObservableSatellite remote,
  91.                                 final boolean twoWay,
  92.                                 final AbsoluteDate date, final double range,
  93.                                 final double sigma, final double baseWeight) {
  94.         super(date, range, sigma, baseWeight, Arrays.asList(local, remote));
  95.         // for one way and two ways measurements, the local satellite clock offsets affects the measurement
  96.         addParameterDriver(local.getClockOffsetDriver());
  97.         if (!twoWay) {
  98.             // for one way measurements, the remote satellite clock offsets also affects the measurement
  99.             addParameterDriver(remote.getClockOffsetDriver());
  100.         }
  101.         this.twoway = twoWay;
  102.     }

  104.     /** Check if the instance represents a two-way measurement.
  105.      * @return true if the instance represents a two-way measurement
  106.      */
  107.     public boolean isTwoWay() {
  108.         return twoway;
  109.     }

  111.     /** {@inheritDoc} */
  112.     @Override
  113.     protected EstimatedMeasurementBase<InterSatellitesRange> theoreticalEvaluationWithoutDerivatives(final int iteration,
  114.                                                                                                      final int evaluation,
  115.                                                                                                      final SpacecraftState[] states) {

  117.         // coordinates of both satellites
  118.         final SpacecraftState local = states[0];
  119.         final TimeStampedPVCoordinates pvaL = local.getPVCoordinates();
  120.         final SpacecraftState remote = states[1];
  121.         final TimeStampedPVCoordinates pvaR = remote.getPVCoordinates();

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

  127.         // downlink delay
  128.         final double dtl = getSatellites().get(0).getClockOffsetDriver().getValue(local.getDate());
  129.         final AbsoluteDate arrivalDate = getDate().shiftedBy(-dtl);

  131.         final TimeStampedPVCoordinates s1Downlink =
  132.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  133.         final double tauD = signalTimeOfFlight(pvaR, s1Downlink.getPosition(), arrivalDate);

  135.         // Transit state
  136.         final double delta      = getDate().durationFrom(remote.getDate());
  137.         final double deltaMTauD = delta - tauD;

  139.         // prepare the evaluation
  140.         final EstimatedMeasurementBase<InterSatellitesRange> estimated;

  142.         final double range;
  143.         if (twoway) {
  144.             // Transit state (re)computed with derivative structures
  145.             final TimeStampedPVCoordinates transitState = pvaR.shiftedBy(deltaMTauD);

  147.             // uplink delay
  148.             final double tauU = signalTimeOfFlight(pvaL,
  149.                                                    transitState.getPosition(),
  150.                                                    transitState.getDate());
  151.             estimated = new EstimatedMeasurementBase<>(this, iteration, evaluation,
  152.                                                        new SpacecraftState[] {
  153.                                                            local.shiftedBy(deltaMTauD),
  154.                                                            remote.shiftedBy(deltaMTauD)
  155.                                                        }, new TimeStampedPVCoordinates[] {
  156.                                                            local.shiftedBy(delta - tauD - tauU).getPVCoordinates(),
  157.                                                            remote.shiftedBy(delta - tauD).getPVCoordinates(),
  158.                                                            local.shiftedBy(delta).getPVCoordinates()
  159.                                                        });

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

  164.         } else {

  166.             estimated = new EstimatedMeasurementBase<>(this, iteration, evaluation,
  167.                                                        new SpacecraftState[] {
  168.                                                            local.shiftedBy(deltaMTauD),
  169.                                                            remote.shiftedBy(deltaMTauD)
  170.                                                        }, new TimeStampedPVCoordinates[] {
  171.                                                            remote.shiftedBy(delta - tauD).getPVCoordinates(),
  172.                                                            local.shiftedBy(delta).getPVCoordinates()
  173.                                                        });

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

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

  181.         }
  182.         estimated.setEstimatedValue(range);

  184.         return estimated;

  186.     }

  188.     /** {@inheritDoc} */
  189.     @Override
  190.     protected EstimatedMeasurement<InterSatellitesRange> theoreticalEvaluation(final int iteration,
  191.                                                                                final int evaluation,
  192.                                                                                final SpacecraftState[] states) {

  194.         // Range derivatives are computed with respect to spacecrafts states in inertial frame
  195.         // ----------------------
  196.         //
  197.         // Parameters:
  198.         //  - 0..2  - Position of the receiver satellite in inertial frame
  199.         //  - 3..5  - Velocity of the receiver satellite in inertial frame
  200.         //  - 6..8  - Position of the remote satellite in inertial frame
  201.         //  - 9..11 - Velocity of the remote satellite in inertial frame
  202.         //  - 12..  - Measurement parameters: local clock offset, remote clock offset...
  203.         int nbParams = 12;
  204.         final Map<String, Integer> indices = new HashMap<>();
  205.         for (ParameterDriver driver : getParametersDrivers()) {
  206.             if (driver.isSelected()) {
  207.                 for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {

  209.                     if (!indices.containsKey(span.getData())) {
  210.                         indices.put(span.getData(), nbParams++);
  211.                     }
  212.                 }
  213.             }
  214.         }

  216.         // coordinates of both satellites
  217.         final SpacecraftState local = states[0];
  218.         final TimeStampedFieldPVCoordinates<Gradient> pvaL = getCoordinates(local, 0, nbParams);
  219.         final SpacecraftState remote = states[1];
  220.         final TimeStampedFieldPVCoordinates<Gradient> pvaR = getCoordinates(remote, 6, nbParams);

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

  226.         // downlink delay
  227.         final Gradient dtl = getSatellites().get(0).getClockOffsetDriver().getValue(nbParams, indices, local.getDate());
  228.         final FieldAbsoluteDate<Gradient> arrivalDate =
  229.                         new FieldAbsoluteDate<>(getDate(), dtl.negate());

  231.         final TimeStampedFieldPVCoordinates<Gradient> s1Downlink =
  232.                         pvaL.shiftedBy(arrivalDate.durationFrom(pvaL.getDate()));
  233.         final Gradient tauD = signalTimeOfFlight(pvaR, s1Downlink.getPosition(), arrivalDate);

  235.         // Transit state
  236.         final double              delta      = getDate().durationFrom(remote.getDate());
  237.         final Gradient deltaMTauD = tauD.negate().add(delta);

  239.         // prepare the evaluation
  240.         final EstimatedMeasurement<InterSatellitesRange> estimated;

  242.         final Gradient range;
  243.         if (twoway) {
  244.             // Transit state (re)computed with derivative structures
  245.             final TimeStampedFieldPVCoordinates<Gradient> transitStateDS = pvaR.shiftedBy(deltaMTauD);

  247.             // uplink delay
  248.             final Gradient tauU = signalTimeOfFlight(pvaL,
  249.                                                      transitStateDS.getPosition(),
  250.                                                      transitStateDS.getDate());
  251.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  252.                                                    new SpacecraftState[] {
  253.                                                        local.shiftedBy(deltaMTauD.getValue()),
  254.                                                        remote.shiftedBy(deltaMTauD.getValue())
  255.                                                    }, new TimeStampedPVCoordinates[] {
  256.                                                        local.shiftedBy(delta - tauD.getValue() - tauU.getValue()).getPVCoordinates(),
  257.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  258.                                                        local.shiftedBy(delta).getPVCoordinates()
  259.                                                    });

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

  264.         } else {

  266.             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
  267.                                                    new SpacecraftState[] {
  268.                                                        local.shiftedBy(deltaMTauD.getValue()),
  269.                                                        remote.shiftedBy(deltaMTauD.getValue())
  270.                                                    }, new TimeStampedPVCoordinates[] {
  271.                                                        remote.shiftedBy(delta - tauD.getValue()).getPVCoordinates(),
  272.                                                        local.shiftedBy(delta).getPVCoordinates()
  273.                                                    });

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

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

  281.         }
  282.         estimated.setEstimatedValue(range.getValue());

  284.         // Range partial derivatives with respect to states
  285.         final double[] derivatives = range.getGradient();
  286.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0,  6));
  287.         estimated.setStateDerivatives(1, Arrays.copyOfRange(derivatives, 6, 12));

  289.         // Set partial derivatives with respect to parameters
  290.         for (final ParameterDriver driver : getParametersDrivers()) {
  291.             for (Span<String> span = driver.getNamesSpanMap().getFirstSpan(); span != null; span = span.next()) {
  292.                 final Integer index = indices.get(span.getData());
  293.                 if (index != null) {
  294.                     estimated.setParameterDerivatives(driver, span.getStart(), derivatives[index]);
  295.                 }
  296.             }
  297.         }

  299.         return estimated;

  301.     }

  303. }
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