TurnAroundRange.java

  1. /* Copyright 2002-2022 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.Collections;
  21. import java.util.HashMap;
  22. import java.util.Map;

  24. import org.hipparchus.Field;
  25. import org.hipparchus.analysis.differentiation.Gradient;
  26. import org.hipparchus.analysis.differentiation.GradientField;
  27. import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  28. import org.orekit.frames.FieldTransform;
  29. import org.orekit.propagation.SpacecraftState;
  30. import org.orekit.time.AbsoluteDate;
  31. import org.orekit.time.FieldAbsoluteDate;
  32. import org.orekit.utils.Constants;
  33. import org.orekit.utils.FieldPVCoordinates;
  34. import org.orekit.utils.PVCoordinates;
  35. import org.orekit.utils.ParameterDriver;
  36. import org.orekit.utils.TimeStampedFieldPVCoordinates;
  37. import org.orekit.utils.TimeStampedPVCoordinates;

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

  62.     /** Type of the measurement. */
  63.     public static final String MEASUREMENT_TYPE = "TurnAroundRange";

  65.     /** Primary ground station from which measurement is performed. */
  66.     private final GroundStation primaryStation;

  68.     /** Secondary ground station reflecting the signal. */
  69.     private final GroundStation secondaryStation;

  71.     /** Simple constructor.
  72.      * @param primaryStation ground station from which measurement is performed
  73.      * @param secondaryStation ground station reflecting the signal
  74.      * @param date date of the measurement
  75.      * @param turnAroundRange observed value
  76.      * @param sigma theoretical standard deviation
  77.      * @param baseWeight base weight
  78.      * @param satellite satellite related to this measurement
  79.      * @since 9.3
  80.      */
  81.     public TurnAroundRange(final GroundStation primaryStation, final GroundStation secondaryStation,
  82.                            final AbsoluteDate date, final double turnAroundRange,
  83.                            final double sigma, final double baseWeight,
  84.                            final ObservableSatellite satellite) {
  85.         super(date, turnAroundRange, sigma, baseWeight, Collections.singletonList(satellite));
  86.         addParameterDriver(primaryStation.getClockOffsetDriver());
  87.         addParameterDriver(primaryStation.getEastOffsetDriver());
  88.         addParameterDriver(primaryStation.getNorthOffsetDriver());
  89.         addParameterDriver(primaryStation.getZenithOffsetDriver());
  90.         addParameterDriver(primaryStation.getPrimeMeridianOffsetDriver());
  91.         addParameterDriver(primaryStation.getPrimeMeridianDriftDriver());
  92.         addParameterDriver(primaryStation.getPolarOffsetXDriver());
  93.         addParameterDriver(primaryStation.getPolarDriftXDriver());
  94.         addParameterDriver(primaryStation.getPolarOffsetYDriver());
  95.         addParameterDriver(primaryStation.getPolarDriftYDriver());
  96.         // the secondary station clock is not used at all, we ignore the corresponding parameter driver
  97.         addParameterDriver(secondaryStation.getEastOffsetDriver());
  98.         addParameterDriver(secondaryStation.getNorthOffsetDriver());
  99.         addParameterDriver(secondaryStation.getZenithOffsetDriver());
  100.         addParameterDriver(secondaryStation.getPrimeMeridianOffsetDriver());
  101.         addParameterDriver(secondaryStation.getPrimeMeridianDriftDriver());
  102.         addParameterDriver(secondaryStation.getPolarOffsetXDriver());
  103.         addParameterDriver(secondaryStation.getPolarDriftXDriver());
  104.         addParameterDriver(secondaryStation.getPolarOffsetYDriver());
  105.         addParameterDriver(secondaryStation.getPolarDriftYDriver());
  106.         this.primaryStation   = primaryStation;
  107.         this.secondaryStation = secondaryStation;
  108.     }

  110.     /** Get the primary ground station from which measurement is performed.
  111.      * @return primary ground station from which measurement is performed
  112.      */
  113.     public GroundStation getPrimaryStation() {
  114.         return primaryStation;
  115.     }

  117.     /** Get the secondary ground station reflecting the signal.
  118.      * @return secondary ground station reflecting the signal
  119.      */
  120.     public GroundStation getSecondaryStation() {
  121.         return secondaryStation;
  122.     }

  124.     /** {@inheritDoc} */
  125.     @Override
  126.     protected EstimatedMeasurement<TurnAroundRange> theoreticalEvaluation(final int iteration, final int evaluation,
  127.                                                                           final SpacecraftState[] states) {

  129.         final SpacecraftState state = states[0];

  131.         // Turn around range derivatives are computed with respect to:
  132.         // - Spacecraft state in inertial frame
  133.         // - Primary station parameters
  134.         // - Secondary station parameters
  135.         // --------------------------
  136.         //
  137.         //  - 0..2 - Position of the spacecraft in inertial frame
  138.         //  - 3..5 - Velocity of the spacecraft in inertial frame
  139.         //  - 6..n - stations' parameters (clock offset, station offsets, pole, prime meridian...)
  140.         int nbParams = 6;
  141.         final Map<String, Integer> indices = new HashMap<>();
  142.         for (ParameterDriver driver : getParametersDrivers()) {
  143.             // we have to check for duplicate keys because primary and secondary station share
  144.             // pole and prime meridian parameters names that must be considered
  145.             // as one set only (they are combined together by the estimation engine)
  146.             if (driver.isSelected() && !indices.containsKey(driver.getName())) {
  147.                 indices.put(driver.getName(), nbParams++);
  148.             }
  149.         }
  150.         final Field<Gradient>         field   = GradientField.getField(nbParams);
  151.         final FieldVector3D<Gradient> zero    = FieldVector3D.getZero(field);

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

  156.         // The path of the signal is divided in two legs.
  157.         // Leg1: Emission from primary station to satellite in primaryTauU seconds
  158.         //     + Reflection from satellite to secondary station in secondaryTauD seconds
  159.         // Leg2: Reflection from secondary station to satellite in secondaryTauU seconds
  160.         //     + Reflection from satellite to primary station in primaryTaudD seconds
  161.         // The measurement is considered to be time stamped at reception on ground
  162.         // by the primary station. All times are therefore computed as backward offsets
  163.         // with respect to this reception time.
  164.         //
  165.         // Two intermediate spacecraft states are defined:
  166.         //  - transitStateLeg2: State of the satellite when it bounced back the signal
  167.         //                      from secondary station to primary station during the 2nd leg
  168.         //  - transitStateLeg1: State of the satellite when it bounced back the signal
  169.         //                      from primary station to secondary station during the 1st leg

  171.         // Compute propagation time for the 2nd leg of the signal path
  172.         // --

  174.         // Time difference between t (date of the measurement) and t' (date tagged in spacecraft state)
  175.         // (if state has already been set up to pre-compensate propagation delay,
  176.         // we will have delta = primaryTauD + secondaryTauU)
  177.         final double delta = getDate().durationFrom(state.getDate());

  179.         // transform between primary station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
  180.         final FieldTransform<Gradient> primaryToInert =
  181.                         primaryStation.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
  182.         final FieldAbsoluteDate<Gradient> measurementDateDS = primaryToInert.getFieldDate();

  184.         // Primary station PV in inertial frame at measurement date
  185.         final TimeStampedFieldPVCoordinates<Gradient> primaryArrival =
  186.                         primaryToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(measurementDateDS,
  187.                                                                                                   zero, zero, zero));

  189.         // Compute propagation times
  190.         final Gradient primaryTauD = signalTimeOfFlight(pvaDS, primaryArrival.getPosition(), measurementDateDS);

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

  195.         // Transit state where the satellite reflected the signal from secondary to primary station
  196.         final SpacecraftState transitStateLeg2 = state.shiftedBy(dtLeg2.getValue());

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

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

  207.         // Secondary station PV in inertial frame at approximate rebound date on secondary station
  208.         final TimeStampedFieldPVCoordinates<Gradient> QSecondaryApprox =
  209.                         secondaryToInertApprox.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(approxReboundDate,
  210.                                                                                                           zero, zero, zero));

  212.         // Uplink time of flight from secondary station to transit state of leg2
  213.         final Gradient secondaryTauU = signalTimeOfFlight(QSecondaryApprox,
  214.                                                           transitStateLeg2PV.getPosition(),
  215.                                                           transitStateLeg2PV.getDate());

  217.         // Total time of flight for leg 2
  218.         final Gradient tauLeg2 = primaryTauD.add(secondaryTauU);

  220.         // Compute propagation time for the 1st leg of the signal path
  221.         // --

  223.         // Absolute date of rebound of the signal to secondary station
  224.         final FieldAbsoluteDate<Gradient> reboundDateDS = measurementDateDS.shiftedBy(tauLeg2.negate());
  225.         final FieldTransform<Gradient> secondaryToInert =
  226.                         secondaryStation.getOffsetToInertial(state.getFrame(), reboundDateDS, nbParams, indices);

  228.         // Secondary station PV in inertial frame at rebound date on secondary station
  229.         final TimeStampedFieldPVCoordinates<Gradient> secondaryRebound =
  230.                         secondaryToInert.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(reboundDateDS,
  231.                                                                                                     FieldPVCoordinates.getZero(field)));

  233.         // Downlink time of flight from transitStateLeg1 to secondary station at rebound date
  234.         final Gradient secondaryTauD = signalTimeOfFlight(transitStateLeg2PV,
  235.                                                           secondaryRebound.getPosition(),
  236.                                                           reboundDateDS);


  239.         // Elapsed time between state date t' and signal arrival to the transit state of the 1st leg
  240.         final Gradient dtLeg1 = dtLeg2.subtract(secondaryTauU).subtract(secondaryTauD);

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

  245.         // transform between primary station topocentric frame (east-north-zenith) and inertial frame expressed as gradients
  246.         // The components of primary station's position in offset frame are the 3 third derivative parameters
  247.         final FieldAbsoluteDate<Gradient> approxEmissionDate =
  248.                         measurementDateDS.shiftedBy(-2 * (secondaryTauU.getValue() + primaryTauD.getValue()));
  249.         final FieldTransform<Gradient> primaryToInertApprox =
  250.                         primaryStation.getOffsetToInertial(state.getFrame(), approxEmissionDate, nbParams, indices);

  252.         // Primary station PV in inertial frame at approximate emission date
  253.         final TimeStampedFieldPVCoordinates<Gradient> QPrimaryApprox =
  254.                         primaryToInertApprox.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(approxEmissionDate,
  255.                                                                                                         zero, zero, zero));

  257.         // Uplink time of flight from primary station to transit state of leg1
  258.         final Gradient primaryTauU = signalTimeOfFlight(QPrimaryApprox,
  259.                                                         transitStateLeg1PV.getPosition(),
  260.                                                         transitStateLeg1PV.getDate());

  262.         // Primary station PV in inertial frame at exact emission date
  263.         final AbsoluteDate emissionDate = transitStateLeg1PV.getDate().toAbsoluteDate().shiftedBy(-primaryTauU.getValue());
  264.         final TimeStampedPVCoordinates primaryDeparture =
  265.                         primaryToInertApprox.shiftedBy(emissionDate.durationFrom(primaryToInertApprox.getDate())).
  266.                         transformPVCoordinates(new TimeStampedPVCoordinates(emissionDate, PVCoordinates.ZERO)).
  267.                         toTimeStampedPVCoordinates();

  269.         // Total time of flight for leg 1
  270.         final Gradient tauLeg1 = secondaryTauD.add(primaryTauU);


  273.         // --
  274.         // Evaluate the turn-around range value and its derivatives
  275.         // --------------------------------------------------------

  277.         // The state we use to define the estimated measurement is a middle ground between the two transit states
  278.         // This is done to avoid calling "SpacecraftState.shiftedBy" function on long duration
  279.         // Thus we define the state at the date t" = date of rebound of the signal at the secondary station
  280.         // Or t" = t -primaryTauD -secondaryTauU
  281.         // The iterative process in the estimation ensures that, after several iterations, the date stamped in the
  282.         // state S in input of this function will be close to t"
  283.         // Therefore we will shift state S by:
  284.         //  - +secondaryTauU to get transitStateLeg2
  285.         //  - -secondaryTauD to get transitStateLeg1
  286.         final EstimatedMeasurement<TurnAroundRange> estimated =
  287.                         new EstimatedMeasurement<>(this, iteration, evaluation,
  288.                                                    new SpacecraftState[] {
  289.                                                        transitStateLeg2.shiftedBy(-secondaryTauU.getValue())
  290.                                                    },
  291.                                                    new TimeStampedPVCoordinates[] {
  292.                                                        primaryDeparture,
  293.                                                        transitStateLeg1PV.toTimeStampedPVCoordinates(),
  294.                                                        secondaryRebound.toTimeStampedPVCoordinates(),
  295.                                                        transitStateLeg2.getPVCoordinates(),
  296.                                                        primaryArrival.toTimeStampedPVCoordinates()
  297.                                                    });

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

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

  308.         // set partial derivatives with respect to parameters
  309.         // (beware element at index 0 is the value, not a derivative)
  310.         for (final ParameterDriver driver : getParametersDrivers()) {
  311.             final Integer index = indices.get(driver.getName());
  312.             if (index != null) {
  313.                 estimated.setParameterDerivatives(driver, derivatives[index]);
  314.             }
  315.         }

  317.         return estimated;

  319.     }

  321. }
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