TurnAroundRange.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.Gradient;
  25. import org.hipparchus.analysis.differentiation.GradientField;
  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.FieldPVCoordinates;
  33. import org.orekit.utils.PVCoordinates;
  34. import org.orekit.utils.ParameterDriver;
  35. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  36. import org.orekit.utils.TimeStampedPVCoordinates;

  38. /** Class modeling a turn-around range measurement using a master ground station and a slave ground station.
  39.  * <p>
  40.  * The measurement is considered to be a signal:
  41.  * - Emitted from the master ground station
  42.  * - Reflected on the spacecraft
  43.  * - Reflected on the slave ground station
  44.  * - Reflected on the spacecraft again
  45.  * - Received on the master ground station
  46.  * Its value is the elapsed time between emission and reception
  47.  * divided by 2c were c is the speed of light.
  48.  * The motion of the stations and the spacecraft
  49.  * during the signal flight time are taken into account.
  50.  * The date of the measurement corresponds to the
  51.  * reception on ground of the reflected signal.
  52.  * </p>
  53.  * @author Thierry Ceolin
  54.  * @author Luc Maisonobe
  55.  * @author Maxime Journot
  56.  *
  57.  * @since 9.0
  58.  */
  59. public class TurnAroundRange extends AbstractMeasurement<TurnAroundRange> {

  61.     /** Master ground station from which measurement is performed. */
  62.     private final GroundStation masterStation;

  64.     /** Slave ground station reflecting the signal. */
  65.     private final GroundStation slaveStation;

  67.     /** Simple constructor.
  68.      * @param masterStation ground station from which measurement is performed
  69.      * @param slaveStation ground station reflecting the signal
  70.      * @param date date of the measurement
  71.      * @param turnAroundRange observed value
  72.      * @param sigma theoretical standard deviation
  73.      * @param baseWeight base weight
  74.      * @param satellite satellite related to this measurement
  75.      * @since 9.3
  76.      */
  77.     public TurnAroundRange(final GroundStation masterStation, final GroundStation slaveStation,
  78.                            final AbsoluteDate date, final double turnAroundRange,
  79.                            final double sigma, final double baseWeight,
  80.                            final ObservableSatellite satellite) {
  81.         super(date, turnAroundRange, sigma, baseWeight, Arrays.asList(satellite));
  82.         addParameterDriver(masterStation.getClockOffsetDriver());
  83.         addParameterDriver(masterStation.getEastOffsetDriver());
  84.         addParameterDriver(masterStation.getNorthOffsetDriver());
  85.         addParameterDriver(masterStation.getZenithOffsetDriver());
  86.         addParameterDriver(masterStation.getPrimeMeridianOffsetDriver());
  87.         addParameterDriver(masterStation.getPrimeMeridianDriftDriver());
  88.         addParameterDriver(masterStation.getPolarOffsetXDriver());
  89.         addParameterDriver(masterStation.getPolarDriftXDriver());
  90.         addParameterDriver(masterStation.getPolarOffsetYDriver());
  91.         addParameterDriver(masterStation.getPolarDriftYDriver());
  92.         // the slave station clock is not used at all, we ignore the corresponding parameter driver
  93.         addParameterDriver(slaveStation.getEastOffsetDriver());
  94.         addParameterDriver(slaveStation.getNorthOffsetDriver());
  95.         addParameterDriver(slaveStation.getZenithOffsetDriver());
  96.         addParameterDriver(slaveStation.getPrimeMeridianOffsetDriver());
  97.         addParameterDriver(slaveStation.getPrimeMeridianDriftDriver());
  98.         addParameterDriver(slaveStation.getPolarOffsetXDriver());
  99.         addParameterDriver(slaveStation.getPolarDriftXDriver());
  100.         addParameterDriver(slaveStation.getPolarOffsetYDriver());
  101.         addParameterDriver(slaveStation.getPolarDriftYDriver());
  102.         this.masterStation = masterStation;
  103.         this.slaveStation = slaveStation;
  104.     }

  106.     /** Get the master ground station from which measurement is performed.
  107.      * @return master ground station from which measurement is performed
  108.      */
  109.     public GroundStation getMasterStation() {
  110.         return masterStation;
  111.     }

  113.     /** Get the slave ground station reflecting the signal.
  114.      * @return slave ground station reflecting the signal
  115.      */
  116.     public GroundStation getSlaveStation() {
  117.         return slaveStation;
  118.     }

  120.     /** {@inheritDoc} */
  121.     @Override
  122.     protected EstimatedMeasurement<TurnAroundRange> theoreticalEvaluation(final int iteration, final int evaluation,
  123.                                                                           final SpacecraftState[] states) {

  125.         final SpacecraftState state = states[0];

  127.         // Turn around range derivatives are computed with respect to:
  128.         // - Spacecraft state in inertial frame
  129.         // - Master station parameters
  130.         // - Slave station parameters
  131.         // --------------------------
  132.         //
  133.         //  - 0..2 - Position of the spacecraft in inertial frame
  134.         //  - 3..5 - Velocity of the spacecraft in inertial frame
  135.         //  - 6..n - stations' parameters (clock offset, station offsets, pole, prime meridian...)
  136.         int nbParams = 6;
  137.         final Map<String, Integer> indices = new HashMap<>();
  138.         for (ParameterDriver driver : getParametersDrivers()) {
  139.             // we have to check for duplicate keys because master and slave station share
  140.             // pole and prime meridian parameters names that must be considered
  141.             // as one set only (they are combined together by the estimation engine)
  142.             if (driver.isSelected() && !indices.containsKey(driver.getName())) {
  143.                 indices.put(driver.getName(), nbParams++);
  144.             }
  145.         }
  146.         final Field<Gradient>         field   = GradientField.getField(nbParams);
  147.         final FieldVector3D<Gradient> zero    = FieldVector3D.getZero(field);

  149.         // Place the gradient in a time-stamped PV
  150.         final TimeStampedFieldPVCoordinates<Gradient> pvaDS = getCoordinates(state, 0, nbParams);

  152.         // The path of the signal is divided in two legs.
  153.         // Leg1: Emission from master station to satellite in masterTauU seconds
  154.         //     + Reflection from satellite to slave station in slaveTauD seconds
  155.         // Leg2: Reflection from slave station to satellite in slaveTauU seconds
  156.         //     + Reflection from satellite to master station in masterTaudD seconds
  157.         // The measurement is considered to be time stamped at reception on ground
  158.         // by the master station. All times are therefore computed as backward offsets
  159.         // with respect to this reception time.
  160.         //
  161.         // Two intermediate spacecraft states are defined:
  162.         //  - transitStateLeg2: State of the satellite when it bounced back the signal
  163.         //                      from slave station to master station during the 2nd leg
  164.         //  - transitStateLeg1: State of the satellite when it bounced back the signal
  165.         //                      from master station to slave station during the 1st leg

  167.         // Compute propagation time for the 2nd leg of the signal path
  168.         // --

  170.         // Time difference between t (date of the measurement) and t' (date tagged in spacecraft state)
  171.         // (if state has already been set up to pre-compensate propagation delay,
  172.         // we will have delta = masterTauD + slaveTauU)
  173.         final double delta = getDate().durationFrom(state.getDate());

  175.         // transform between master station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
  176.         final FieldTransform<Gradient> masterToInert =
  177.                         masterStation.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
  178.         final FieldAbsoluteDate<Gradient> measurementDateDS =
  179.                         masterToInert.getFieldDate();

  181.         // Master station PV in inertial frame at measurement date
  182.         final TimeStampedFieldPVCoordinates<Gradient> masterArrival =
  183.                         masterToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(measurementDateDS,
  184.                                                                                                  zero, zero, zero));

  186.         // Compute propagation times
  187.         final Gradient masterTauD = signalTimeOfFlight(pvaDS, masterArrival.getPosition(), measurementDateDS);

  189.         // Elapsed time between state date t' and signal arrival to the transit state of the 2nd leg
  190.         final Gradient dtLeg2 = masterTauD.negate().add(delta);

  192.         // Transit state where the satellite reflected the signal from slave to master station
  193.         final SpacecraftState transitStateLeg2 = state.shiftedBy(dtLeg2.getValue());

  195.         // Transit state pv of leg2 (re)computed with gradient
  196.         final TimeStampedFieldPVCoordinates<Gradient> transitStateLeg2PV = pvaDS.shiftedBy(dtLeg2);

  198.         // transform between slave station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
  199.         // The components of slave station's position in offset frame are the 3 last derivative parameters
  200.         final FieldAbsoluteDate<Gradient> approxReboundDate = measurementDateDS.shiftedBy(-delta);
  201.         final FieldTransform<Gradient> slaveToInertApprox =
  202.                         slaveStation.getOffsetToInertial(state.getFrame(), approxReboundDate, nbParams, indices);

  204.         // Slave station PV in inertial frame at approximate rebound date on slave station
  205.         final TimeStampedFieldPVCoordinates<Gradient> QSlaveApprox =
  206.                         slaveToInertApprox.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(approxReboundDate,
  207.                                                                                                       zero, zero, zero));

  209.         // Uplink time of flight from slave station to transit state of leg2
  210.         final Gradient slaveTauU = signalTimeOfFlight(QSlaveApprox,
  211.                                                       transitStateLeg2PV.getPosition(),
  212.                                                       transitStateLeg2PV.getDate());

  214.         // Total time of flight for leg 2
  215.         final Gradient tauLeg2 = masterTauD.add(slaveTauU);

  217.         // Compute propagation time for the 1st leg of the signal path
  218.         // --

  220.         // Absolute date of rebound of the signal to slave station
  221.         final FieldAbsoluteDate<Gradient> reboundDateDS = measurementDateDS.shiftedBy(tauLeg2.negate());
  222.         final FieldTransform<Gradient> slaveToInert =
  223.                         slaveStation.getOffsetToInertial(state.getFrame(), reboundDateDS, nbParams, indices);

  225.         // Slave station PV in inertial frame at rebound date on slave station
  226.         final TimeStampedFieldPVCoordinates<Gradient> slaveRebound =
  227.                         slaveToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(reboundDateDS,
  228.                                                                                                 FieldPVCoordinates.getZero(field)));

  230.         // Downlink time of flight from transitStateLeg1 to slave station at rebound date
  231.         final Gradient slaveTauD = signalTimeOfFlight(transitStateLeg2PV,
  232.                                                       slaveRebound.getPosition(),
  233.                                                       reboundDateDS);


  236.         // Elapsed time between state date t' and signal arrival to the transit state of the 1st leg
  237.         final Gradient dtLeg1 = dtLeg2.subtract(slaveTauU).subtract(slaveTauD);

  239.         // Transit state pv of leg2 (re)computed with gradients
  240.         final TimeStampedFieldPVCoordinates<Gradient> transitStateLeg1PV = pvaDS.shiftedBy(dtLeg1);

  242.         // transform between master station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
  243.         // The components of master station's position in offset frame are the 3 third derivative parameters
  244.         final FieldAbsoluteDate<Gradient> approxEmissionDate =
  245.                         measurementDateDS.shiftedBy(-2 * (slaveTauU.getValue() + masterTauD.getValue()));
  246.         final FieldTransform<Gradient> masterToInertApprox =
  247.                         masterStation.getOffsetToInertial(state.getFrame(), approxEmissionDate, nbParams, indices);

  249.         // Master station PV in inertial frame at approximate emission date
  250.         final TimeStampedFieldPVCoordinates<Gradient> QMasterApprox =
  251.                         masterToInertApprox.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(approxEmissionDate,
  252.                                                                                                        zero, zero, zero));

  254.         // Uplink time of flight from master station to transit state of leg1
  255.         final Gradient masterTauU = signalTimeOfFlight(QMasterApprox,
  256.                                                        transitStateLeg1PV.getPosition(),
  257.                                                        transitStateLeg1PV.getDate());

  259.         // Master station PV in inertial frame at exact emission date
  260.         final AbsoluteDate emissionDate = transitStateLeg1PV.getDate().toAbsoluteDate().shiftedBy(-masterTauU.getValue());
  261.         final TimeStampedPVCoordinates masterDeparture =
  262.                         masterToInertApprox.shiftedBy(emissionDate.durationFrom(masterToInertApprox.getDate())).
  263.                         transformPVCoordinates(new TimeStampedPVCoordinates(emissionDate, PVCoordinates.ZERO)).
  264.                         toTimeStampedPVCoordinates();

  266.         // Total time of flight for leg 1
  267.         final Gradient tauLeg1 = slaveTauD.add(masterTauU);


  270.         // --
  271.         // Evaluate the turn-around range value and its derivatives
  272.         // --------------------------------------------------------

  274.         // The state we use to define the estimated measurement is a middle ground between the two transit states
  275.         // This is done to avoid calling "SpacecraftState.shiftedBy" function on long duration
  276.         // Thus we define the state at the date t" = date of rebound of the signal at the slave station
  277.         // Or t" = t -masterTauD -slaveTauU
  278.         // The iterative process in the estimation ensures that, after several iterations, the date stamped in the
  279.         // state S in input of this function will be close to t"
  280.         // Therefore we will shift state S by:
  281.         //  - +slaveTauU to get transitStateLeg2
  282.         //  - -slaveTauD to get transitStateLeg1
  283.         final EstimatedMeasurement<TurnAroundRange> estimated =
  284.                         new EstimatedMeasurement<>(this, iteration, evaluation,
  285.                                                    new SpacecraftState[] {
  286.                                                        transitStateLeg2.shiftedBy(-slaveTauU.getValue())
  287.                                                    },
  288.                                                    new TimeStampedPVCoordinates[] {
  289.                                                        masterDeparture,
  290.                                                        transitStateLeg1PV.toTimeStampedPVCoordinates(),
  291.                                                        slaveRebound.toTimeStampedPVCoordinates(),
  292.                                                        transitStateLeg2.getPVCoordinates(),
  293.                                                        masterArrival.toTimeStampedPVCoordinates()
  294.                                                    });

  296.         // Turn-around range value = Total time of flight for the 2 legs divided by 2 and multiplied by c
  297.         final double cOver2 = 0.5 * Constants.SPEED_OF_LIGHT;
  298.         final Gradient turnAroundRange = (tauLeg2.add(tauLeg1)).multiply(cOver2);
  299.         estimated.setEstimatedValue(turnAroundRange.getValue());

  301.         // Turn-around range partial derivatives with respect to state
  302.         final double[] derivatives = turnAroundRange.getGradient();
  303.         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));

  305.         // set partial derivatives with respect to parameters
  306.         // (beware element at index 0 is the value, not a derivative)
  307.         for (final ParameterDriver driver : getParametersDrivers()) {
  308.             final Integer index = indices.get(driver.getName());
  309.             if (index != null) {
  310.                 estimated.setParameterDerivatives(driver, derivatives[index]);
  311.             }
  312.         }

  314.         return estimated;

  316.     }

  318. }
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