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    * The ASF 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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   * See the License for the specific language governing permissions and
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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.EstimatedMeasurement;
  import org.orekit.estimation.measurements.EstimationModifier;
  import org.orekit.estimation.measurements.gnss.Phase;
  import org.orekit.frames.Transform;
  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 phase measurements.
   * @author David Soulard
   * @since 10.2
   */
  public class OnBoardAntennaPhaseModifier implements EstimationModifier<Phase> {
  
      /** 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 OnBoardAntennaPhaseModifier(final Vector3D antennaPhaseCenter) {
          this.antennaPhaseCenter = antennaPhaseCenter;
      }
  
      /** {@inheritDoc} */
      @Override
      public List<ParameterDriver> getParametersDrivers() {
          return Collections.emptyList();
      }
  
      /** {@inheritDoc} */
      @Override
      public void modify(final EstimatedMeasurement<Phase> estimated) {
          // the participants are spacecraft at emission, ground station at reception
          final TimeStampedPVCoordinates[] participants = estimated.getParticipants();
          final AbsoluteDate               emissionDate = participants[0].getDate();
          final Vector3D                   pReception   = participants[1].getPosition();
  
          // transform from spacecraft to inertial frame at emission date
          final SpacecraftState refState          = estimated.getStates()[0];
          final SpacecraftState emissionState     = refState.shiftedBy(emissionDate.durationFrom(refState.getDate()));
          final Transform       spacecraftToInert = emissionState.toTransform().getInverse();
  
          // compute the geometrical value of the 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 pSpacecraft = spacecraftToInert.transformPosition(Vector3D.ZERO);
          final double rangeUsingSpacecraftCenter = Vector3D.distance(pSpacecraft, pReception);
  
          // compute the geometrical value of the range replacing
          // the spacecraft position with antenna phase center position
          final Vector3D pAPC = spacecraftToInert.transformPosition(antennaPhaseCenter);
          final double rangeUsingAntennaPhaseCenter = Vector3D.distance(pAPC, pReception);
  
          // get the estimated value before this modifier is applied
          final double[] value = estimated.getEstimatedValue();
  
          // modify the value
          value[0] += (rangeUsingAntennaPhaseCenter - rangeUsingSpacecraftCenter) / estimated.getObservedMeasurement().getWavelength();
          estimated.setEstimatedValue(value);
      }
  
  }