/* Copyright 2002-2021 CS GROUP
    * Licensed to CS GROUP (CS) under one or more
    * 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.gnss;
  
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Map;
  
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.orekit.estimation.measurements.AbstractMeasurement;
  import org.orekit.estimation.measurements.EstimatedMeasurement;
  import org.orekit.estimation.measurements.InterSatellitesRange;
  import org.orekit.estimation.measurements.ObservableSatellite;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.utils.Constants;
  import org.orekit.utils.PVCoordinatesProvider;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TimeStampedFieldPVCoordinates;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /** One-way GNSS range measurement.
   * <p>
   * This class can be used in precise orbit determination applications
   * for modeling a range measurement between a GNSS satellite (emitter)
   * and a LEO satellite (receiver).
   * <p>
   * The one-way GNSS range measurement assumes knowledge of the orbit and
   * the clock offset of the emitting GNSS satellite. For instance, it is
   * possible to use a SP3 file or a GNSS navigation message to recover
   * the satellite's orbit and clock.
   * <p>
   * This class is very similar to {@link InterSatellitesRange} measurement
   * class. However, using the one-way GNSS range measurement, the orbit and clock
   * of the emitting GNSS satellite are <b>NOT</b> estimated simultaneously with
   * LEO satellite coordinates.
   *
   * @author Bryan Cazabonne
   * @since 10.3
   */
  public class OneWayGNSSRange extends AbstractMeasurement<OneWayGNSSRange> {
  
      /** Emitting satellite. */
      private final PVCoordinatesProvider remote;
  
      /** Clock offset of the emitting satellite. */
      private final double dtRemote;
  
      /** Simple constructor.
       * @param remote provider for GNSS satellite which simply emits the signal
       * @param dtRemote clock offset of the GNSS satellite, in seconds
       * @param date date of the measurement
       * @param range observed value
       * @param sigma theoretical standard deviation
       * @param baseWeight base weight
       * @param local satellite which receives the signal and perform the measurement
       */
      public OneWayGNSSRange(final PVCoordinatesProvider remote,
                             final double dtRemote,
                             final AbsoluteDate date,
                             final double range, final double sigma,
                             final double baseWeight, final ObservableSatellite local) {
          // Call super constructor
          super(date, range, sigma, baseWeight, Collections.singletonList(local));
          // The local satellite clock offset affects the measurement
          addParameterDriver(local.getClockOffsetDriver());
          // Initialise fields
          this.dtRemote = dtRemote;
          this.remote   = remote;
      }
  
      /** {@inheritDoc} */
      @Override
      protected EstimatedMeasurement<OneWayGNSSRange> theoreticalEvaluation(final int iteration,
                                                                            final int evaluation,
                                                                            final SpacecraftState[] states) {
  
          // Range derivatives are computed with respect to spacecraft state in inertial frame
          // Parameters:
          //  - 0..2 - Position of the spacecraft in inertial frame
          //  - 3..5 - Velocity of the spacecraft in inertial frame
          //  - 6..n - measurements parameters (clock offset, etc)
          int nbEstimatedParams = 6;
         final Map<String, Integer> parameterIndices = new HashMap<>();
         for (ParameterDriver measurementDriver : getParametersDrivers()) {
             if (measurementDriver.isSelected()) {
                 parameterIndices.put(measurementDriver.getName(), nbEstimatedParams++);
             }
         }
 
         // Coordinates of both satellites in local satellite frame
         final SpacecraftState localState  = states[0];
         final TimeStampedFieldPVCoordinates<Gradient> pvaLocal  = getCoordinates(localState, 0, nbEstimatedParams);
         final TimeStampedPVCoordinates                pvaRemote = remote.getPVCoordinates(getDate(), localState.getFrame());
 
         // Downlink delay
         final Gradient dtLocal = getSatellites().get(0).getClockOffsetDriver().getValue(nbEstimatedParams, parameterIndices);
         final FieldAbsoluteDate<Gradient> arrivalDate = new FieldAbsoluteDate<>(getDate(), dtLocal.negate());
 
         final TimeStampedFieldPVCoordinates<Gradient> s1Downlink = pvaLocal.shiftedBy(arrivalDate.durationFrom(pvaLocal.getDate()));
         final Gradient tauD = signalTimeOfFlight(new TimeStampedFieldPVCoordinates<>(pvaRemote.getDate(), dtLocal.getField().getOne(), pvaRemote),
                                                  s1Downlink.getPosition(), arrivalDate);
 
         // Transit state
         final double   delta      = getDate().durationFrom(pvaRemote.getDate());
         final Gradient deltaMTauD = tauD.negate().add(delta);
 
         // Estimated measurement
         final EstimatedMeasurement<OneWayGNSSRange> estimatedRange =
                         new EstimatedMeasurement<>(this, iteration, evaluation,
                                                    new SpacecraftState[] {
                                                        localState.shiftedBy(deltaMTauD.getValue())
                                                    }, new TimeStampedPVCoordinates[] {
                                                        pvaRemote.shiftedBy(delta - tauD.getValue()),
                                                        localState.shiftedBy(delta).getPVCoordinates()
                                                    });
 
         // Range value
         final Gradient range            = tauD.add(dtLocal).subtract(dtRemote).multiply(Constants.SPEED_OF_LIGHT);
         final double[] rangeDerivatives = range.getGradient();
 
         // Set value and state derivatives of the estimated measurement
         estimatedRange.setEstimatedValue(range.getValue());
         estimatedRange.setStateDerivatives(0, Arrays.copyOfRange(rangeDerivatives, 0,  6));
 
         // Set partial derivatives with respect to parameters
         for (final ParameterDriver measurementDriver : getParametersDrivers()) {
             final Integer index = parameterIndices.get(measurementDriver.getName());
             if (index != null) {
                 estimatedRange.setParameterDerivatives(measurementDriver, rangeDerivatives[index]);
             }
         }
 
         // Return the estimated measurement
         return estimatedRange;
 
     }
 
 }