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    * contributor license agreements.  See the NOTICE file distributed with
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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
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    *
    *   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.propagation.events;
  
  import org.hipparchus.ode.events.Action;
  import org.hipparchus.util.FastMath;
  import org.junit.After;
  import org.junit.Assert;
  import org.junit.Before;
  import org.junit.Test;
  import org.orekit.Utils;
  import org.orekit.bodies.BodyShape;
  import org.orekit.bodies.GeodeticPoint;
  import org.orekit.bodies.OneAxisEllipsoid;
  import org.orekit.frames.FramesFactory;
  import org.orekit.frames.TopocentricFrame;
  import org.orekit.orbits.KeplerianOrbit;
  import org.orekit.orbits.Orbit;
  import org.orekit.orbits.PositionAngle;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.analytical.KeplerianPropagator;
  import org.orekit.propagation.events.handlers.EventHandler;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.TimeScale;
  import org.orekit.time.TimeScalesFactory;
  import org.orekit.utils.Constants;
  import org.orekit.utils.IERSConventions;
  
  public class BackAndForthDetectorTest {
  
      @Test
      public void testBackAndForth() {
  
          final TimeScale utc = TimeScalesFactory.getUTC();
  
          final AbsoluteDate date0 = new AbsoluteDate(2006, 12, 27, 12,  0, 0.0, utc);
          final AbsoluteDate date1 = new AbsoluteDate(2006, 12, 27, 22, 50, 0.0, utc);
          final AbsoluteDate date2 = new AbsoluteDate(2006, 12, 27, 22, 58, 0.0, utc);
  
          // Orbit
          final double a = 7274000.;
          final double e = 0.00127;
          final double i = FastMath.toRadians(90.);
          final double w = FastMath.toRadians(0.);
          final double raan = FastMath.toRadians(12.5);
          final double lM = FastMath.toRadians(60.);
          Orbit iniOrb = new KeplerianOrbit(a, e, i, w, raan, lM,
                                            PositionAngle.MEAN, FramesFactory.getEME2000(), date0,
                                            Constants.WGS84_EARTH_MU);
  
          // Propagator
          KeplerianPropagator propagator = new KeplerianPropagator(iniOrb);
  
          // Station
          final GeodeticPoint stationPosition = new GeodeticPoint(FastMath.toRadians(0.), FastMath.toRadians(100.), 110.);
          final BodyShape earth = new OneAxisEllipsoid(Constants.WGS84_EARTH_EQUATORIAL_RADIUS,
                                                       Constants.WGS84_EARTH_FLATTENING,
                                                       FramesFactory.getITRF(IERSConventions.IERS_2010, true));
          final TopocentricFrame stationFrame = new TopocentricFrame(earth, stationPosition, "");
  
          // Detector
          final Visibility visi = new Visibility();
          propagator.addEventDetector(new ElevationDetector(stationFrame).
                                      withConstantElevation(FastMath.toRadians(10.)).
                                      withHandler(visi));
  
          // Forward propagation (AOS + LOS)
          propagator.propagate(date1);
          propagator.propagate(date2);
          // Backward propagation (AOS + LOS)
          propagator.propagate(date1);
          propagator.propagate(date0);
  
          Assert.assertEquals(4, visi.getVisiNb());
  
      }
  
      private static class Visibility implements EventHandler<ElevationDetector> {
          private int _visiNb;
  
          public Visibility() {
              _visiNb = 0;
          }
  
          public int getVisiNb() {
             return _visiNb;
         }
 
         public Action eventOccurred(SpacecraftState s, ElevationDetector ed, boolean increasing) {
             _visiNb++;
             return Action.CONTINUE;
         }
 
     }
 
     @Before
     public void setUp() {
         Utils.setDataRoot("regular-data");
     }
 
     @After
     public void tearDown() {
     }
 
 }