/* 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
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    *
    *   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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  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.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 phase measurement.
   * <p>
   * This class can be used in precise orbit determination applications
   * for modeling a phase measurement between a GNSS satellite (emitter)
   * and a LEO satellite (receiver).
   * <p>
   * The one-way GNSS phase 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 InterSatellitesPhase} measurement
   * class. However, using the one-way GNSS phase 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 OneWayGNSSPhase extends AbstractMeasurement<OneWayGNSSPhase> {
  
      /** Name for ambiguity driver. */
      public static final String AMBIGUITY_NAME = "ambiguity";
  
      /** Driver for ambiguity. */
      private final ParameterDriver ambiguityDriver;
  
      /** Emitting satellite. */
      private final PVCoordinatesProvider remote;
  
      /** Clock offset of the emitting satellite. */
      private final double dtRemote;
  
      /** Wavelength of the phase observed value [m]. */
      private final double wavelength;
  
      /** 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 phase observed value, in cycles
       * @param wavelength phase observed value wavelength, in meters
       * @param sigma theoretical standard deviation
       * @param baseWeight base weight
       * @param local satellite which receives the signal and perform the measurement
       */
      public OneWayGNSSPhase(final PVCoordinatesProvider remote,
                             final double dtRemote,
                             final AbsoluteDate date,
                             final double phase, final double wavelength, final double sigma,
                             final double baseWeight, final ObservableSatellite local) {
          // Call super constructor
          super(date, phase, sigma, baseWeight, Collections.singletonList(local));
  
          // Initialize phase ambiguity driver
          ambiguityDriver = new ParameterDriver(AMBIGUITY_NAME, 0.0, 1.0,
                                                Double.NEGATIVE_INFINITY, Double.POSITIVE_INFINITY);
  
          // The local satellite clock offset affects the measurement
          addParameterDriver(ambiguityDriver);
          addParameterDriver(local.getClockOffsetDriver());
  
          // Initialise fields
         this.dtRemote   = dtRemote;
         this.remote     = remote;
         this.wavelength = wavelength;
     }
 
     /** Get the wavelength.
      * @return wavelength (m)
      */
     public double getWavelength() {
         return wavelength;
     }
 
     /** Get the driver for phase ambiguity.
      * @return the driver for phase ambiguity
      */
     public ParameterDriver getAmbiguityDriver() {
         return ambiguityDriver;
     }
 
     /** {@inheritDoc} */
     @Override
     protected EstimatedMeasurement<OneWayGNSSPhase> theoreticalEvaluation(final int iteration,
                                                                           final int evaluation,
                                                                           final SpacecraftState[] states) {
 
         // Phase derivatives are computed with respect to spacecrafts states in inertial frame
         // Parameters:
         //  - 0..2  - Position of the receiver satellite in inertial frame
         //  - 3..5  - Velocity of the receiver satellite in inertial frame
         //  - 6..n  - Measurement parameters: ambiguity and clock offset
         int nbEstimatedParamsPhase = 6;
         final Map<String, Integer> parameterIndicesPhase = new HashMap<>();
         for (ParameterDriver phaseMeasurementDriver : getParametersDrivers()) {
             if (phaseMeasurementDriver.isSelected()) {
                 parameterIndicesPhase.put(phaseMeasurementDriver.getName(), nbEstimatedParamsPhase++);
             }
         }
 
         // Coordinates of both satellites
         final SpacecraftState localState  = states[0];
         final TimeStampedFieldPVCoordinates<Gradient> pvaLocal  = getCoordinates(localState, 0, nbEstimatedParamsPhase);
         final TimeStampedPVCoordinates                pvaRemote = remote.getPVCoordinates(getDate(), localState.getFrame());
 
         // Downlink delay
         final Gradient dtLocal = getSatellites().get(0).getClockOffsetDriver().getValue(nbEstimatedParamsPhase, parameterIndicesPhase);
         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);
 
         // prepare the evaluation
         final EstimatedMeasurement<OneWayGNSSPhase> estimatedPhase =
                         new EstimatedMeasurement<>(this, iteration, evaluation,
                                                    new SpacecraftState[] {
                                                        localState.shiftedBy(deltaMTauD.getValue())
                                                    }, new TimeStampedPVCoordinates[] {
                                                        pvaRemote.shiftedBy(delta - tauD.getValue()),
                                                        localState.shiftedBy(delta).getPVCoordinates()
                                                    });
 
         // Phase value
         final double   cOverLambda      = Constants.SPEED_OF_LIGHT / wavelength;
         final Gradient ambiguity        = ambiguityDriver.getValue(nbEstimatedParamsPhase, parameterIndicesPhase);
         final Gradient phase            = tauD.add(dtLocal).subtract(dtRemote).multiply(cOverLambda).add(ambiguity);
         final double[] phaseDerivatives = phase.getGradient();
 
         // Set value and state derivatives of the estimated measurement
         estimatedPhase.setEstimatedValue(phase.getValue());
         estimatedPhase.setStateDerivatives(0, Arrays.copyOfRange(phaseDerivatives, 0,  6));
 
         // Set partial derivatives with respect to parameters
         for (final ParameterDriver phaseMeasurementDriver : getParametersDrivers()) {
             final Integer index = parameterIndicesPhase.get(phaseMeasurementDriver.getName());
             if (index != null) {
                 estimatedPhase.setParameterDerivatives(phaseMeasurementDriver, phaseDerivatives[index]);
             }
         }
 
         // Return the estimated measurement
         return estimatedPhase;
 
     }
 
 }