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    * contributor license agreements.  See the NOTICE file distributed with
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    * 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
   * distributed under the License is distributed on an "AS IS" BASIS,
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  package org.orekit.estimation.measurements.modifiers;
  
  import java.util.List;
  
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.Field;
  import org.hipparchus.geometry.euclidean.threed.FieldVector3D;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.orekit.attitudes.FrameAlignedProvider;
  import org.orekit.estimation.measurements.EstimatedMeasurement;
  import org.orekit.estimation.measurements.EstimatedMeasurementBase;
  import org.orekit.estimation.measurements.EstimationModifier;
  import org.orekit.estimation.measurements.GroundStation;
  import org.orekit.estimation.measurements.TDOA;
  import org.orekit.models.earth.troposphere.TroposphericModel;
  import org.orekit.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.utils.Constants;
  import org.orekit.utils.FieldTrackingCoordinates;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TrackingCoordinates;
  
  /** Class modifying theoretical TDOA measurements with tropospheric delay.
   * <p>
   * The effect of tropospheric correction on the TDOA is a time delay computed
   * directly from the difference in tropospheric delays for each downlink.
   * </p><p>
   * Tropospheric delay is not frequency dependent for signals up to 15 GHz.
   * </p>
   * @author Pascal Parraud
   * @since 11.2
   */
  public class TDOATroposphericDelayModifier implements EstimationModifier<TDOA> {
  
      /** Tropospheric delay model. */
      private final TroposphericModel tropoModel;
  
      /** Constructor.
       *
       * @param model tropospheric model appropriate for the current TDOA measurement method.
       * @since 12.1
       */
      public TDOATroposphericDelayModifier(final TroposphericModel model) {
          tropoModel = model;
      }
  
      /** {@inheritDoc} */
      @Override
      public String getEffectName() {
          return "troposphere";
      }
  
      /** Compute the measurement error due to Troposphere on a single downlink.
       * @param station station
       * @param state spacecraft state
       * @return the measurement error due to Troposphere (s)
       */
      private double timeErrorTroposphericModel(final GroundStation station, final SpacecraftState state) {
          final Vector3D position = state.getPosition();
  
          // tracking
          final TrackingCoordinates trackingCoordinates =
                          station.getBaseFrame().getTrackingCoordinates(position, state.getFrame(), state.getDate());
  
          // only consider measurements above the horizon
          if (trackingCoordinates.getElevation() > 0) {
              // Delay in meters
              final double delay = tropoModel.pathDelay(trackingCoordinates,
                                                        station.getOffsetGeodeticPoint(state.getDate()),
                                                        tropoModel.getParameters(state.getDate()), state.getDate()).
                                   getDelay();
              // return delay in seconds
              return delay / Constants.SPEED_OF_LIGHT;
          }
  
          return 0;
      }
  
      /** Compute the measurement error due to Troposphere on a single downlink.
       * @param <T> type of the element
       * @param station station
       * @param state spacecraft state
       * @param parameters tropospheric model parameters
      * @return the measurement error due to Troposphere (s)
      */
     private <T extends CalculusFieldElement<T>> T timeErrorTroposphericModel(final GroundStation station,
                                                                              final FieldSpacecraftState<T> state,
                                                                              final T[] parameters) {
         // Field
         final Field<T> field = state.getDate().getField();
         final T zero         = field.getZero();
 
         // tracking
         final FieldVector3D<T> pos = state.getPosition();
         final FieldTrackingCoordinates<T> trackingCoordinates =
                         station.getBaseFrame().getTrackingCoordinates(pos, state.getFrame(), state.getDate());
 
         // only consider measurements above the horizon
         if (trackingCoordinates.getElevation().getReal() > 0) {
             // delay in meters
             final T delay = tropoModel.pathDelay(trackingCoordinates,
                                                  station.getOffsetGeodeticPoint(state.getDate()),
                                                  parameters, state.getDate()).
                             getDelay();
             // return delay in seconds
             return delay.divide(Constants.SPEED_OF_LIGHT);
         }
 
         return zero;
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return tropoModel.getParametersDrivers();
     }
 
     /** {@inheritDoc} */
     @Override
     public void modifyWithoutDerivatives(final EstimatedMeasurementBase<TDOA> estimated) {
 
         final TDOA            measurement   = estimated.getObservedMeasurement();
         final GroundStation   primeStation  = measurement.getPrimeStation();
         final GroundStation   secondStation = measurement.getSecondStation();
 
         TDOAModifierUtil.modifyWithoutDerivatives(estimated,  primeStation, secondStation,
                                                   this::timeErrorTroposphericModel, this);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public void modify(final EstimatedMeasurement<TDOA> estimated) {
 
         final TDOA            measurement   = estimated.getObservedMeasurement();
         final GroundStation   primeStation  = measurement.getPrimeStation();
         final GroundStation   secondStation = measurement.getSecondStation();
         final SpacecraftState state         = estimated.getStates()[0];
 
         TDOAModifierUtil.modify(estimated, tropoModel,
                                 new ModifierGradientConverter(state, 6, new FrameAlignedProvider(state.getFrame())),
                                 primeStation, secondStation,
                                 this::timeErrorTroposphericModel,
                                 this::timeErrorTroposphericModel,
                                 this);
 
     }
 
 }