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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
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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.modifiers;
  
  import java.util.List;
  
  import org.hipparchus.CalculusFieldElement;
  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.frames.TopocentricFrame;
  import org.orekit.models.earth.ionosphere.IonosphericModel;
  import org.orekit.propagation.FieldSpacecraftState;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.utils.Constants;
  import org.orekit.utils.ParameterDriver;
  
  /** Class modifying theoretical TDOA measurements with ionospheric delay.
   * <p>
   * The effect of ionospheric correction on the TDOA is a time delay computed
   * directly from the difference in ionospheric delays for each downlink.
   * </p><p>
   * The ionospheric delay depends on the frequency of the signal.
   * </p>
   * @author Pascal Parraud
   * @since 11.2
   */
  public class TDOAIonosphericDelayModifier implements EstimationModifier<TDOA> {
  
      /** Ionospheric delay model. */
      private final IonosphericModel ionoModel;
  
      /** Frequency [Hz]. */
      private final double frequency;
  
      /** Constructor.
       *
       * @param model ionospheric model appropriate for the current TDOA measurement method
       * @param freq  frequency of the signal in Hz
       */
      public TDOAIonosphericDelayModifier(final IonosphericModel model,
                                          final double freq) {
          ionoModel = model;
          frequency = freq;
      }
  
      /** {@inheritDoc} */
      @Override
      public String getEffectName() {
          return "ionosphere";
      }
  
      /** Compute the measurement error due to ionosphere on a single downlink.
       * @param station station
       * @param state spacecraft state
       * @return the measurement error due to ionosphere (s)
       */
      private double timeErrorIonosphericModel(final GroundStation station,
                                               final SpacecraftState state) {
          // base frame associated with the station
          final TopocentricFrame baseFrame = station.getBaseFrame();
          // delay in meters
          final double delay = ionoModel.pathDelay(state, baseFrame, frequency, ionoModel.getParameters(state.getDate()));
          // return delay in seconds
          return delay / Constants.SPEED_OF_LIGHT;
      }
  
      /** Compute the measurement error due to ionosphere on a single downlink.
       * @param <T> type of the elements
       * @param station station
       * @param state spacecraft state
       * @param parameters ionospheric model parameters
       * @return the measurement error due to ionosphere (s)
       */
      private <T extends CalculusFieldElement<T>> T timeErrorIonosphericModel(final GroundStation station,
                                                                              final FieldSpacecraftState<T> state,
                                                                              final T[] parameters) {
          // Base frame associated with the station
          final TopocentricFrame baseFrame = station.getBaseFrame();
          // Delay in meters
          final T delay = ionoModel.pathDelay(state, baseFrame, frequency, parameters);
          // return delay in seconds
         return delay.divide(Constants.SPEED_OF_LIGHT);
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return ionoModel.getParametersDrivers();
     }
 
     @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::timeErrorIonosphericModel,
                                                   this);
 
     }
 
     @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, ionoModel,
                                 new ModifierGradientConverter(state, 6, new FrameAlignedProvider(state.getFrame())),
                                 primeStation, secondStation,
                                 this::timeErrorIonosphericModel,
                                 this::timeErrorIonosphericModel,
                                 this);
 
     }
 
 }