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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
    * the License.  You may obtain a copy of the License at
    *
    *   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;
  
  import org.hipparchus.analysis.UnivariateFunction;
  import org.hipparchus.analysis.solvers.BracketingNthOrderBrentSolver;
  import org.hipparchus.analysis.solvers.UnivariateSolver;
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.hipparchus.util.FastMath;
  import org.orekit.estimation.Context;
  import org.orekit.frames.Frame;
  import org.orekit.frames.Transform;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.utils.Constants;
  import org.orekit.utils.PVCoordinates;
  import org.orekit.utils.ParameterDriver;
  
  import java.util.Arrays;
  
  /**
   * Creates a list of {@link BistaticRangeRate bistatic range-rate measurements}.
   * @author Pascal Parraud
   */
  public class BistaticRangeRateMeasurementCreator extends MeasurementCreator {
  
      private final Context context;
      private final GroundStation emitter;
      private final GroundStation receiver;
      private final ObservableSatellite satellite;
  
      public BistaticRangeRateMeasurementCreator(final Context context) {
          this.context   = context;
          this.emitter   = context.BRRstations.getKey();
          this.receiver  = context.BRRstations.getValue();
          this.satellite = new ObservableSatellite(0);
      }
  
      public ObservableSatellite getSatellite() {
          return satellite;
      }
  
      public void init(SpacecraftState s0, AbsoluteDate t, double step) {
          for (final GroundStation station : Arrays.asList(context.BRRstations.getKey(),
                                                           context.BRRstations.getValue())) {
              for (ParameterDriver driver : Arrays.asList(station.getClockOffsetDriver(),
                                                          station.getEastOffsetDriver(),
                                                          station.getNorthOffsetDriver(),
                                                          station.getZenithOffsetDriver(),
                                                          station.getPrimeMeridianOffsetDriver(),
                                                          station.getPrimeMeridianDriftDriver(),
                                                          station.getPolarOffsetXDriver(),
                                                          station.getPolarDriftXDriver(),
                                                          station.getPolarOffsetYDriver(),
                                                          station.getPolarDriftYDriver())) {
                  if (driver.getReferenceDate() == null) {
                      driver.setReferenceDate(s0.getDate());
                  }
              }
          }
      }
  
      public void handleStep(final SpacecraftState currentState) {
  
          final AbsoluteDate  date     = currentState.getDate();
          final Frame         inertial = currentState.getFrame();
          final Vector3D      position = currentState.getPosition();
          final Vector3D      velocity = currentState.getPVCoordinates().getVelocity();
  
          // Create a BRR measurement only if elevation for both stations is higher than 30°
          if ((emitter.getBaseFrame().getTrackingCoordinates(position, inertial, date).getElevation()  > FastMath.toRadians(30.0)) &&
              (receiver.getBaseFrame().getTrackingCoordinates(position, inertial, date).getElevation() > FastMath.toRadians(30.0))) {
  
              // The solver used
              final UnivariateSolver solver = new BracketingNthOrderBrentSolver(1.0e-12, 5);
  
              final double downLinkDelay  = solver.solve(1000, new UnivariateFunction() {
                  public double value(final double x) {
                      final Transform t = receiver.getOffsetToInertial(inertial, date.shiftedBy(x), false);
                      final double d = Vector3D.distance(position, t.transformPosition(Vector3D.ZERO));
                      return d - x * Constants.SPEED_OF_LIGHT;
                  }
              }, -1.0, 1.0);
  
              final AbsoluteDate receptionDate = currentState.getDate().shiftedBy(downLinkDelay);
             final PVCoordinates receiverPV   = receiver.getOffsetToInertial(inertial, receptionDate, false)
                                                        .transformPVCoordinates(PVCoordinates.ZERO);
 
             // line of sight at reception
             final Vector3D receptionLOS = (position.subtract(receiverPV.getPosition())).normalize();
 
             // relative velocity, spacecraft-station, at the date of reception
             final Vector3D deltaVr = velocity.subtract(receiverPV.getVelocity());
 
             final double upLinkDelay = solver.solve(1000, new UnivariateFunction() {
                 public double value(final double x) {
                     final Transform t = emitter.getOffsetToInertial(inertial, date.shiftedBy(-x), false);
                     final double d = Vector3D.distance(position, t.transformPosition(Vector3D.ZERO));
                     return d - x * Constants.SPEED_OF_LIGHT;
                 }
             }, -1.0, 1.0);
 
             final AbsoluteDate emissionDate = currentState.getDate().shiftedBy(-upLinkDelay);
             final PVCoordinates emitterPV   = emitter.getOffsetToInertial(inertial, emissionDate, false)
                                                      .transformPVCoordinates(PVCoordinates.ZERO);
 
             // line of sight at emission
             final Vector3D emissionLOS = (position.subtract(emitterPV.getPosition())).normalize();
 
             // relative velocity, spacecraft-station, at the date of emission
             final Vector3D deltaVe = velocity.subtract(emitterPV.getVelocity());
 
             // range rate at the date of reception
             final double rr = deltaVr.dotProduct(receptionLOS) + deltaVe.dotProduct(emissionLOS);
 
             addMeasurement(new BistaticRangeRate(emitter, receiver, receptionDate, rr, 1.0, 10, satellite));
 
         }
 
     }
 }