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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.ParameterDriver;
  
  import java.util.Arrays;
  
  /**
   * Creates a list of {@link TDOA} measurements.
   * @author Pascal Parraud
   */
  public class TDOAMeasurementCreator extends MeasurementCreator {
  
      private final Context context;
      private final GroundStation primary;
      private final GroundStation secondary;
      private final ObservableSatellite satellite;
  
      public TDOAMeasurementCreator(final Context context) {
          this.context   = context;
          this.primary   = context.TDOAstations.getKey();
          this.secondary = context.TDOAstations.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.TDOAstations.getKey(),
                                                           context.TDOAstations.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();
  
          // Create a BRR measurement only if elevation for both stations is higher than 30°
          if ((primary.getBaseFrame().getTrackingCoordinates(position, inertial, date).getElevation()  > FastMath.toRadians(30.0)) &&
              (secondary.getBaseFrame().getTrackingCoordinates(position, inertial, date).getElevation() > FastMath.toRadians(30.0))) {
  
              // The solver used
              final UnivariateSolver solver = new BracketingNthOrderBrentSolver(1.0e-12, 5);
  
              // Signal time of flight to primary station
              final double referenceDelay = solver.solve(1000, new UnivariateFunction() {
                  public double value(final double x) {
                      final Transform t = primary.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  = date.shiftedBy(referenceDelay);
  
              // Signal time of flight to secondary station
             final double relativeDelay = solver.solve(1000, new UnivariateFunction() {
                 public double value(final double x) {
                     final Transform t = secondary.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);
 
             // time difference on arrival
             final double tdoa = referenceDelay - relativeDelay;
 
             addMeasurement(new TDOA(primary, secondary, receptionDate, tdoa, 1.0, 10, satellite));
 
         }
 
     }
 }