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    * 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
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   * Unless required by applicable law or agreed to in writing, software
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  package org.orekit.estimation.measurements.gnss;
  
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.Map;
  
  import org.hipparchus.analysis.differentiation.Gradient;
  import org.hipparchus.analysis.differentiation.GradientField;
  import org.orekit.estimation.measurements.AbstractMeasurement;
  import org.orekit.estimation.measurements.AbstractParticipant;
  import org.orekit.estimation.measurements.EstimatedMeasurement;
  import org.orekit.estimation.measurements.EstimatedMeasurementBase;
  import org.orekit.estimation.measurements.MeasurementQuality;
  import org.orekit.estimation.measurements.ObservableSatellite;
  import org.orekit.estimation.measurements.Observer;
  import org.orekit.estimation.measurements.SignalBasedMeasurement;
  import org.orekit.frames.FieldTransform;
  import org.orekit.frames.Frame;
  import org.orekit.frames.Transform;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.signal.AdjustableEmitterSignalTimer;
  import org.orekit.signal.FieldAdjustableEmitterSignalTimer;
  import org.orekit.signal.FieldSignalReceptionCondition;
  import org.orekit.signal.SignalReceptionCondition;
  import org.orekit.signal.SignalTravelTimeModel;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.utils.Constants;
  import org.orekit.utils.FieldPVCoordinates;
  import org.orekit.utils.FieldPVCoordinatesProvider;
  import org.orekit.utils.PVCoordinates;
  import org.orekit.utils.PVCoordinatesProvider;
  import org.orekit.utils.drivers.ParameterDriver;
  import org.orekit.utils.TimeStampedFieldPVCoordinates;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /** Class modeling a phase measurement from a ground station.
   * <p>
   * The measurement is considered to be a signal emitted from
   * a spacecraft and received on a ground station.
   * Its value is the number of cycles between emission and
   * reception. The motion of both the station and the
   * spacecraft during the signal flight time are taken into
   * account. The date of the measurement corresponds to the
   * reception on ground of the emitted signal.
   * </p>
   * @author Thierry Ceolin
   * @author Luc Maisonobe
   * @author Maxime Journot
   * @since 9.2
   */
  public class Phase extends SignalBasedMeasurement<Phase> {
  
      /** Type of the measurement. */
      public static final String MEASUREMENT_TYPE = "Phase";
  
      /** Driver for ambiguity. */
      private final AmbiguityDriver ambiguityDriver;
  
      /** Wavelength of the phase observed value [m]. */
      private final double wavelength;
  
      /** Observer that receives signal from satellite. */
      private final Observer observer;
  
      /** Simple constructor.
       * @param observer observer that performs the measurement
       * @param date date of the measurement
       * @param phase observed value (cycles)
       * @param wavelength phase observed value wavelength (m)
       * @param sigma theoretical standard deviation
       * @param baseWeight base weight
       * @param satellite satellite related to this measurement
       * @param cache from which ambiguity drive should come
       * @since 12.1
       */
      public Phase(final Observer observer, final AbsoluteDate date,
                   final double phase, final double wavelength, final double sigma,
                   final double baseWeight, final ObservableSatellite satellite,
                   final AmbiguityCache cache) {
          this(observer, date, phase, wavelength, new MeasurementQuality(sigma, baseWeight), new SignalTravelTimeModel(),
                  satellite, cache);
      }
 
     /** Simple constructor.
      * @param observer observer that performs the measurement
      * @param date date of the measurement
      * @param phase observed value (cycles)
      * @param wavelength phase observed value wavelength (m)
      * @param measurementQuality measurement quality data as used in orbit determination
      * @param signalTravelTimeModel signal model
      * @param satellite satellite related to this measurement
      * @param cache from which ambiguity drive should come
      * @since 14.0
      */
     public Phase(final Observer observer, final AbsoluteDate date,
                  final double phase, final double wavelength, final MeasurementQuality measurementQuality,
                  final SignalTravelTimeModel signalTravelTimeModel, final ObservableSatellite satellite,
                  final AmbiguityCache cache) {
         super(date, false, phase, measurementQuality, signalTravelTimeModel,
                 Collections.singletonList(satellite));
         ambiguityDriver = cache.getAmbiguity(satellite.getName(), observer.getName(), wavelength);
         addParametersDrivers(observer.getParametersDrivers());
         addParameterDriver(ambiguityDriver);
         this.observer = observer;
         this.wavelength = wavelength;
     }
 
     /** Get receiving observer.
      * @return observer
      */
     public final Observer getObserver() {
         return observer;
     }
 
     /** Get the wavelength.
      * @return wavelength (m)
      */
     public double getWavelength() {
         return wavelength;
     }
 
     /** Get the driver for phase ambiguity.
      * @return the driver for phase ambiguity
      * @since 10.3
      */
     public AmbiguityDriver getAmbiguityDriver() {
         return ambiguityDriver;
     }
 
     /** {@inheritDoc} */
     @Override
     protected EstimatedMeasurementBase<Phase> theoreticalEvaluationWithoutDerivatives(final int iteration,
                                                                                       final int evaluation,
                                                                                       final SpacecraftState[] states,
                                                                                       final boolean fillParticipants) {
         // Coordinates of the measured spacecraft
         final SpacecraftState state = states[0];
         final Frame frame = state.getFrame();
         final TimeStampedPVCoordinates pva   = state.getPVCoordinates();
 
         // transform between remote observer frame and inertial frame
         final AbsoluteDate measurementDate = getDate();
         final Transform offsetToInertialDownlink = getObserver().getOffsetToInertial(frame, measurementDate, false);
         final AbsoluteDate downlinkDate             = offsetToInertialDownlink.getDate();
 
         // Observer position in inertial frame at end of the downlink leg
         final TimeStampedPVCoordinates origin = new TimeStampedPVCoordinates(downlinkDate, PVCoordinates.ZERO);
         final TimeStampedPVCoordinates satelliteDownlink = offsetToInertialDownlink.transformPVCoordinates(origin);
 
         // Coordinates provider for emitting object (observed spacecraft)
         final PVCoordinatesProvider pvCoordinatesProvider = AbstractParticipant.extractPVCoordinatesProvider(states[0], pva);
 
         // Downlink delay / determine time of emission of signal by ObservableSatellite
         final AdjustableEmitterSignalTimer signalTimeOfFlight = getSignalTravelTimeModel()
                 .getAdjustableEmitterComputer(pvCoordinatesProvider);
         final SignalReceptionCondition receptionCondition = new SignalReceptionCondition(downlinkDate,
                 satelliteDownlink.getPosition(), frame);
         final double tauD = signalTimeOfFlight.computeDelay(receptionCondition, pva.getDate());
 
         // Transit state & Transit state (re)computed with gradients
         final double          delta             = downlinkDate.durationFrom(state.getDate());
         final double          deltaMTauD        = delta - tauD;
         final SpacecraftState transitState      = states[0].shiftedBy(deltaMTauD);
 
         // prepare the evaluation
         final EstimatedMeasurementBase<Phase> estimated = new EstimatedMeasurementBase<>(this, iteration, evaluation,
                                                        new SpacecraftState[] { transitState }, fillParticipants ? new TimeStampedPVCoordinates[] {
                                                            transitState.getPVCoordinates(), satelliteDownlink } : new TimeStampedPVCoordinates[0]);
 
         // Clock offsets
         final ObservableSatellite satellite = getSatellites().getFirst();
 
         final double dts = satellite.getOffsetValue(state.getDate());
         final double dtg = getObserver().getOffsetValue(getDate());
 
         // Phase value
         final double cOverLambda = Constants.SPEED_OF_LIGHT / wavelength;
         final double ambiguity   = ambiguityDriver.getValue();
         final double phase       = (tauD + dtg - dts) * cOverLambda + ambiguity;
 
         estimated.setEstimatedValue(phase);
 
         return estimated;
 
     }
 
     /** {@inheritDoc} */
     @Override
     protected EstimatedMeasurement<Phase> theoreticalEvaluation(final int iteration,
                                                                 final int evaluation,
                                                                 final SpacecraftState[] states) {
         // Create the parameter indices map
         final SpacecraftState state = states[0];
         final Frame                frame        = state.getFrame();
         final Map<String, Integer> paramIndices = getParameterIndices(states);
         final int                  nbParams     = 6 * states.length + paramIndices.size();
 
         // Coordinates of the spacecraft expressed as a gradient
         final TimeStampedFieldPVCoordinates<Gradient> pva = AbstractMeasurement.getCoordinates(states[0], 0, nbParams);
 
         // transform between Observer object and inertial frame, expressed as a gradient
         // The components of the Observer's position in offset frame are the 3 last derivative parameters
         final FieldTransform<Gradient> offsetToInertialDownlink = getObserver().
                 getOffsetToInertial(frame, getDate(), nbParams, paramIndices);
         final FieldAbsoluteDate<Gradient> downlinkDate = offsetToInertialDownlink.getFieldDate();
 
         // Observer position in inertial frame at end of the downlink leg
         final GradientField field = GradientField.getField(nbParams);
         final TimeStampedFieldPVCoordinates<Gradient> satelliteDownlink =
                 offsetToInertialDownlink.transformPVCoordinates(new TimeStampedFieldPVCoordinates<>(downlinkDate,
                         FieldPVCoordinates.getZero(field)));
 
         // Form coordinates provider
         final FieldPVCoordinatesProvider<Gradient> fieldPVCoordinatesProvider = AbstractParticipant.extractFieldPVCoordinatesProvider(states[0], pva);
 
         // Downlink delay
         final FieldAdjustableEmitterSignalTimer<Gradient> fieldComputer = getSignalTravelTimeModel()
                 .getFieldAdjustableEmitterComputer(field, fieldPVCoordinatesProvider);
         final FieldSignalReceptionCondition<Gradient> receptionCondition = new FieldSignalReceptionCondition<>(downlinkDate,
                 satelliteDownlink.getPosition(), frame);
         final Gradient tauD = fieldComputer.computeDelay(receptionCondition, pva.getDate());
 
         // Transit state & Transit state (re)computed with gradients
         final Gradient        delta        = downlinkDate.durationFrom(states[0].getDate());
         final Gradient        deltaMTauD   = tauD.negate().add(delta);
         final SpacecraftState transitState = states[0].shiftedBy(deltaMTauD.getValue());
         final FieldAbsoluteDate<Gradient> fieldDate = new FieldAbsoluteDate<>(field, states[0].getDate()).shiftedBy(deltaMTauD);
         final TimeStampedFieldPVCoordinates<Gradient> transitPV = fieldPVCoordinatesProvider.getPVCoordinates(fieldDate, frame);
 
         // prepare the evaluation
         final EstimatedMeasurement<Phase> estimated =
                         new EstimatedMeasurement<>(this, iteration, evaluation,
                                 new SpacecraftState[] { transitState},
                                 new TimeStampedPVCoordinates[] {
                                         transitPV.toTimeStampedPVCoordinates(),
                                         satelliteDownlink.toTimeStampedPVCoordinates()});
 
         // Clock offsets
         final ObservableSatellite satellite = getSatellites().getFirst();
 
         final Gradient dts = satellite.getFieldOffsetValue(nbParams, paramIndices, state.getDate());
         final Gradient dtg = getObserver().getFieldOffsetValue(nbParams, paramIndices, getDate());
 
         // Phase value
         final double   cOverLambda = Constants.SPEED_OF_LIGHT / wavelength;
         final Gradient ambiguity   = ambiguityDriver.getValue(nbParams, paramIndices);
         final Gradient phase       = tauD.add(dtg).subtract(dts).multiply(cOverLambda).add(ambiguity);
 
         estimated.setEstimatedValue(phase.getValue());
 
         // Phase first order derivatives with respect to state
         final double[] derivatives = phase.getGradient();
         estimated.setStateDerivatives(0, Arrays.copyOfRange(derivatives, 0, 6));
 
         // Set first order derivatives with respect to parameters
         for (final ParameterDriver driver : getParametersDrivers()) {
             final Integer index = paramIndices.get(driver.getName());
             if (index != null) {
                 estimated.setParameterDerivatives(driver, derivatives[index]);
             }
         }
 
         return estimated;
 
     }
 
 }