/* Copyright 2002-2021 CS GROUP
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    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * CS licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
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   * See the License for the specific language governing permissions and
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  package org.orekit.estimation.measurements;
  
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Map;
  
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.analysis.differentiation.GradientField;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.orekit.frames.FieldTransform;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.utils.Constants;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TimeStampedFieldPVCoordinates;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /** Class modeling one-way or two-way range rate measurement between two vehicles.
   * One-way range rate (or Doppler) measurements generally apply to specific satellites
   * (e.g. GNSS, DORIS), where a signal is transmitted from a satellite to a
   * measuring station.
   * Two-way range rate measurements are applicable to any system. The signal is
   * transmitted to the (non-spinning) satellite and returned by a transponder
   * (or reflected back)to the same measuring station.
   * The Doppler measurement can be obtained by multiplying the velocity by (fe/c), where
   * fe is the emission frequency.
   *
   * @author Thierry Ceolin
   * @author Joris Olympio
   * @since 8.0
   */
  public class RangeRate extends AbstractMeasurement<RangeRate> {
  
      /** Ground station from which measurement is performed. */
      private final GroundStation station;
  
      /** Flag indicating whether it is a two-way measurement. */
      private final boolean twoway;
  
      /** Simple constructor.
       * @param station ground station from which measurement is performed
       * @param date date of the measurement
       * @param rangeRate observed value, m/s
       * @param sigma theoretical standard deviation
       * @param baseWeight base weight
       * @param twoway if true, this is a two-way measurement
       * @param satellite satellite related to this measurement
       * @since 9.3
       */
      public RangeRate(final GroundStation station, final AbsoluteDate date,
                       final double rangeRate, final double sigma, final double baseWeight,
                       final boolean twoway, final ObservableSatellite satellite) {
          super(date, rangeRate, sigma, baseWeight, Collections.singletonList(satellite));
          addParameterDriver(station.getClockOffsetDriver());
          addParameterDriver(station.getClockDriftDriver());
          addParameterDriver(satellite.getClockDriftDriver());
          addParameterDriver(station.getEastOffsetDriver());
          addParameterDriver(station.getNorthOffsetDriver());
          addParameterDriver(station.getZenithOffsetDriver());
          addParameterDriver(station.getPrimeMeridianOffsetDriver());
          addParameterDriver(station.getPrimeMeridianDriftDriver());
          addParameterDriver(station.getPolarOffsetXDriver());
          addParameterDriver(station.getPolarDriftXDriver());
          addParameterDriver(station.getPolarOffsetYDriver());
          addParameterDriver(station.getPolarDriftYDriver());
          this.station = station;
          this.twoway  = twoway;
      }
  
      /** Check if the instance represents a two-way measurement.
       * @return true if the instance represents a two-way measurement
       */
      public boolean isTwoWay() {
          return twoway;
      }
  
      /** Get the ground station from which measurement is performed.
       * @return ground station from which measurement is performed
       */
      public GroundStation getStation() {
          return station;
     }
 
     /** {@inheritDoc} */
     @Override
     protected EstimatedMeasurement<RangeRate> theoreticalEvaluation(final int iteration, final int evaluation,
                                                                     final SpacecraftState[] states) {
 
         final SpacecraftState state = states[0];
 
         // Range-rate derivatives are computed with respect to spacecraft state in inertial frame
         // and station position in station's offset frame
         // -------
         //
         // Parameters:
         //  - 0..2 - Position of the spacecraft in inertial frame
         //  - 3..5 - Velocity of the spacecraft in inertial frame
         //  - 6..n - station parameters (clock offset, clock drift, station offsets, pole, prime meridian...)
         int nbParams = 6;
         final Map<String, Integer> indices = new HashMap<>();
         for (ParameterDriver driver : getParametersDrivers()) {
             if (driver.isSelected()) {
                 indices.put(driver.getName(), nbParams++);
             }
         }
         final FieldVector3D<Gradient> zero = FieldVector3D.getZero(GradientField.getField(nbParams));
 
         // Coordinates of the spacecraft expressed as a gradient
         final TimeStampedFieldPVCoordinates<Gradient> pvaDS = getCoordinates(state, 0, nbParams);
 
         // transform between station and inertial frame, expressed as a gradient
         // The components of station's position in offset frame are the 3 last derivative parameters
         final FieldTransform<Gradient> offsetToInertialDownlink =
                         station.getOffsetToInertial(state.getFrame(), getDate(), nbParams, indices);
         final FieldAbsoluteDate<Gradient> downlinkDateDS =
                         offsetToInertialDownlink.getFieldDate();
 
         // Station position in inertial frame at end of the downlink leg
         final TimeStampedFieldPVCoordinates<Gradient> stationDownlink =
                         offsetToInertialDownlink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDateDS,
                                                                                                             zero, zero, zero));
 
         // Compute propagation times
         // (if state has already been set up to pre-compensate propagation delay,
         //  we will have delta == tauD and transitState will be the same as state)
 
         // Downlink delay
         final Gradient tauD = signalTimeOfFlight(pvaDS, stationDownlink.getPosition(), downlinkDateDS);
 
         // Transit state
         final Gradient        delta        = downlinkDateDS.durationFrom(state.getDate());
         final Gradient        deltaMTauD   = tauD.negate().add(delta);
         final SpacecraftState transitState = state.shiftedBy(deltaMTauD.getValue());
 
         // Transit state (re)computed with gradients
         final TimeStampedFieldPVCoordinates<Gradient> transitPV = pvaDS.shiftedBy(deltaMTauD);
 
         // one-way (downlink) range-rate
         final EstimatedMeasurement<RangeRate> evalOneWay1 =
                         oneWayTheoreticalEvaluation(iteration, evaluation, true,
                                                     stationDownlink, transitPV, transitState, indices, nbParams);
         final EstimatedMeasurement<RangeRate> estimated;
         if (twoway) {
             // one-way (uplink) light time correction
             final FieldTransform<Gradient> offsetToInertialApproxUplink =
                             station.getOffsetToInertial(state.getFrame(),
                                                         downlinkDateDS.shiftedBy(tauD.multiply(-2)), nbParams, indices);
             final FieldAbsoluteDate<Gradient> approxUplinkDateDS =
                             offsetToInertialApproxUplink.getFieldDate();
 
             final TimeStampedFieldPVCoordinates<Gradient> stationApproxUplink =
                             offsetToInertialApproxUplink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(approxUplinkDateDS,
                                                                                                                     zero, zero, zero));
 
             final Gradient tauU = signalTimeOfFlight(stationApproxUplink, transitPV.getPosition(), transitPV.getDate());
 
             final TimeStampedFieldPVCoordinates<Gradient> stationUplink =
                             stationApproxUplink.shiftedBy(transitPV.getDate().durationFrom(approxUplinkDateDS).subtract(tauU));
 
             final EstimatedMeasurement<RangeRate> evalOneWay2 =
                             oneWayTheoreticalEvaluation(iteration, evaluation, false,
                                                         stationUplink, transitPV, transitState, indices, nbParams);
 
             // combine uplink and downlink values
             estimated = new EstimatedMeasurement<>(this, iteration, evaluation,
                                                    evalOneWay1.getStates(),
                                                    new TimeStampedPVCoordinates[] {
                                                        evalOneWay2.getParticipants()[0],
                                                        evalOneWay1.getParticipants()[0],
                                                        evalOneWay1.getParticipants()[1]
                                                    });
             estimated.setEstimatedValue(0.5 * (evalOneWay1.getEstimatedValue()[0] + evalOneWay2.getEstimatedValue()[0]));
 
             // combine uplink and downlink partial derivatives with respect to state
             final double[][] sd1 = evalOneWay1.getStateDerivatives(0);
             final double[][] sd2 = evalOneWay2.getStateDerivatives(0);
             final double[][] sd = new double[sd1.length][sd1[0].length];
             for (int i = 0; i < sd.length; ++i) {
                 for (int j = 0; j < sd[0].length; ++j) {
                     sd[i][j] = 0.5 * (sd1[i][j] + sd2[i][j]);
                 }
             }
             estimated.setStateDerivatives(0, sd);
 
             // combine uplink and downlink partial derivatives with respect to parameters
             evalOneWay1.getDerivativesDrivers().forEach(driver -> {
                 final double[] pd1 = evalOneWay1.getParameterDerivatives(driver);
                 final double[] pd2 = evalOneWay2.getParameterDerivatives(driver);
                 final double[] pd = new double[pd1.length];
                 for (int i = 0; i < pd.length; ++i) {
                     pd[i] = 0.5 * (pd1[i] + pd2[i]);
                 }
                 estimated.setParameterDerivatives(driver, pd);
             });
 
         } else {
             estimated = evalOneWay1;
         }
 
         return estimated;
 
     }
 
     /** Evaluate measurement in one-way.
      * @param iteration iteration number
      * @param evaluation evaluations counter
      * @param downlink indicator for downlink leg
      * @param stationPV station coordinates when signal is at station
      * @param transitPV spacecraft coordinates at onboard signal transit
      * @param transitState orbital state at onboard signal transit
      * @param indices indices of the estimated parameters in derivatives computations
      * @param nbParams the number of estimated parameters in derivative computations
      * @return theoretical value
      * @see #evaluate(SpacecraftStatet)
      */
     private EstimatedMeasurement<RangeRate> oneWayTheoreticalEvaluation(final int iteration, final int evaluation, final boolean downlink,
                                                                         final TimeStampedFieldPVCoordinates<Gradient> stationPV,
                                                                         final TimeStampedFieldPVCoordinates<Gradient> transitPV,
                                                                         final SpacecraftState transitState,
                                                                         final Map<String, Integer> indices,
                                                                         final int nbParams) {
 
         // prepare the evaluation
         final EstimatedMeasurement<RangeRate> estimated =
                         new EstimatedMeasurement<RangeRate>(this, iteration, evaluation,
                                                             new SpacecraftState[] {
                                                                 transitState
                                                             }, new TimeStampedPVCoordinates[] {
                                                                 (downlink ? transitPV : stationPV).toTimeStampedPVCoordinates(),
                                                                 (downlink ? stationPV : transitPV).toTimeStampedPVCoordinates()
                                                             });
 
         // range rate value
         final FieldVector3D<Gradient> stationPosition  = stationPV.getPosition();
         final FieldVector3D<Gradient> relativePosition = stationPosition.subtract(transitPV.getPosition());
 
         final FieldVector3D<Gradient> stationVelocity  = stationPV.getVelocity();
         final FieldVector3D<Gradient> relativeVelocity = stationVelocity.subtract(transitPV.getVelocity());
 
         // radial direction
         final FieldVector3D<Gradient> lineOfSight      = relativePosition.normalize();
 
         // line of sight velocity
         final Gradient lineOfSightVelocity = FieldVector3D.dotProduct(relativeVelocity, lineOfSight);
 
         // range rate
         Gradient rangeRate = lineOfSightVelocity;
 
         if (!twoway) {
             // clock drifts, taken in account only in case of one way
             final ObservableSatellite satellite    = getSatellites().get(0);
             final Gradient            dtsDot       = satellite.getClockDriftDriver().getValue(nbParams, indices);
             final Gradient            dtgDot       = station.getClockDriftDriver().getValue(nbParams, indices);
 
             final Gradient clockDriftBiais = dtgDot.subtract(dtsDot).multiply(Constants.SPEED_OF_LIGHT);
 
             rangeRate = rangeRate.add(clockDriftBiais);
         }
 
         estimated.setEstimatedValue(rangeRate.getValue());
 
         // compute partial derivatives of (rr) with respect to spacecraft state Cartesian coordinates
         final double[] derivatives = rangeRate.getGradient();
         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
 
         // set partial derivatives with respect to parameters
         // (beware element at index 0 is the value, not a derivative)
         for (final ParameterDriver driver : getParametersDrivers()) {
             final Integer index = indices.get(driver.getName());
             if (index != null) {
                 estimated.setParameterDerivatives(driver, derivatives[index]);
             }
         }
 
         return estimated;
 
     }
 
 }