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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
    * the License.  You may obtain a copy of the License at
    *
    *   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.Collections;
  import java.util.List;
  
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.orekit.estimation.measurements.EstimatedMeasurementBase;
  import org.orekit.estimation.measurements.EstimationModifier;
  import org.orekit.estimation.measurements.TurnAroundRange;
  import org.orekit.frames.StaticTransform;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  /** On-board antenna offset effect on turn around range measurements.
   * @author Luc Maisonobe
   * @since 9.0
   */
  public class OnBoardAntennaTurnAroundRangeModifier implements EstimationModifier<TurnAroundRange> {
  
      /** Position of the Antenna Phase Center in satellite frame. */
      private final Vector3D antennaPhaseCenter;
  
      /** Simple constructor.
       * @param antennaPhaseCenter position of the Antenna Phase Center in satellite frame
       */
      public OnBoardAntennaTurnAroundRangeModifier(final Vector3D antennaPhaseCenter) {
          this.antennaPhaseCenter = antennaPhaseCenter;
      }
  
      /** {@inheritDoc} */
      @Override
      public List<ParameterDriver> getParametersDrivers() {
          return Collections.emptyList();
      }
  
      /** {@inheritDoc} */
      @Override
      public void modifyWithoutDerivatives(final EstimatedMeasurementBase<TurnAroundRange> estimated) {
  
          // the participants are primary station at emission, spacecraft during leg 1,
          // secondary station at rebound, spacecraft during leg 2, primary station at reception
          final TimeStampedPVCoordinates[] participants      = estimated.getParticipants();
          final Vector3D                   pPrimaryEmission  = participants[0].getPosition();
          final AbsoluteDate               transitDateLeg1   = participants[1].getDate();
          final Vector3D                   pSecondaryRebound = participants[2].getPosition();
          final AbsoluteDate               transitDateLeg2   = participants[3].getDate();
          final Vector3D                   pPrimaryReception = participants[4].getPosition();
  
          // transforms from spacecraft to inertial frame at transit dates
          final SpacecraftState refState              = estimated.getStates()[0];
          final SpacecraftState transitStateLeg1      = refState.shiftedBy(transitDateLeg1.durationFrom(refState.getDate()));
          final StaticTransform spacecraftToInertLeg1 = transitStateLeg1.toStaticTransform().getInverse();
          final SpacecraftState transitStateLeg2      = refState.shiftedBy(transitDateLeg2.durationFrom(refState.getDate()));
          final StaticTransform spacecraftToInertLeg2 = transitStateLeg2.toStaticTransform().getInverse();
  
          // compute the geometrical value of the turn-around range directly from participants positions.
          // Note that this may be different from the value returned by estimated.getEstimatedValue(),
          // because other modifiers may already have been taken into account
          final Vector3D pSpacecraftLeg1 = spacecraftToInertLeg1.transformPosition(Vector3D.ZERO);
          final Vector3D pSpacecraftLeg2 = spacecraftToInertLeg2.transformPosition(Vector3D.ZERO);
          final double turnAroundRangeUsingSpacecraftCenter =
                          0.5 * (Vector3D.distance(pPrimaryEmission,  pSpacecraftLeg1) +
                                 Vector3D.distance(pSpacecraftLeg1,   pSecondaryRebound)   +
                                 Vector3D.distance(pSecondaryRebound, pSpacecraftLeg2) +
                                 Vector3D.distance(pSpacecraftLeg2,   pPrimaryReception));
  
          // compute the geometrical value of the range replacing
          // the spacecraft positions with antenna phase center positions
          final Vector3D pAPCLeg1 = spacecraftToInertLeg1.transformPosition(antennaPhaseCenter);
          final Vector3D pAPCLeg2 = spacecraftToInertLeg2.transformPosition(antennaPhaseCenter);
          final double turnAroundRangeUsingAntennaPhaseCenter =
                          0.5 * (Vector3D.distance(pPrimaryEmission,  pAPCLeg1)          +
                                 Vector3D.distance(pAPCLeg1,          pSecondaryRebound) +
                                 Vector3D.distance(pSecondaryRebound, pAPCLeg2)          +
                                 Vector3D.distance(pAPCLeg2,          pPrimaryReception));
  
          // get the estimated value before this modifier is applied
          final double[] value = estimated.getEstimatedValue();
  
          // modify the value
          value[0] += turnAroundRangeUsingAntennaPhaseCenter - turnAroundRangeUsingSpacecraftCenter;
         estimated.modifyEstimatedValue(this, value);
 
     }
 
 }