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  package org.orekit.estimation.measurements.modifiers;
  
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.junit.jupiter.api.Assertions;
  import org.junit.jupiter.api.BeforeEach;
  import org.junit.jupiter.api.Test;
  import org.orekit.Utils;
  import org.orekit.estimation.measurements.EstimatedMeasurement;
  import org.orekit.estimation.measurements.EstimatedMeasurementBase;
  import org.orekit.estimation.measurements.EstimationModifier;
  import org.orekit.estimation.measurements.InterSatellitesRange;
  import org.orekit.estimation.measurements.ObservableSatellite;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.analytical.tle.TLE;
  import org.orekit.propagation.analytical.tle.TLEPropagator;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.TimeScalesFactory;
  import org.orekit.utils.Constants;
  
  /**
   * Check against prediction in
   *
   * "Springer Handbook oƒ Global Navigation Satellite Systems, Teunissen, Montenbruck"
   *
   * An approximate value is given in terms of delay for Galileo satellites.
   * As these satellites are close to GPS satellites, we consider the delays to be
   * of the same order, namely around 62ps.
   *
   * The values produced by the modifiers are translated in terms of delay and checked against
   * the approximate value.
   */
  
  public class RelativisticJ2ClockInterSatellitesRangeModifierTest {
  
      /** Date. */
      private static AbsoluteDate date;
  
      /** Spacecraft states. */
      private static SpacecraftState[] states;
  
      @Test
      public void testRelativisticClockCorrection() {
  
          // Measurement
          final InterSatellitesRange range = new InterSatellitesRange(new ObservableSatellite(0), new ObservableSatellite(1),
                                                                      false, date,
                                                                      Vector3D.distance(states[0].getPosition(),
                                                                                        states[1].getPosition()),
                                                                      1.0, 1.0);
  
          // Inter-satellites range before applying the modifier
          final EstimatedMeasurementBase<InterSatellitesRange> estimatedBefore = range.estimateWithoutDerivatives(states);
  
          // Inter-satellites range before applying the modifier
          final EstimationModifier<InterSatellitesRange> modifier = new RelativisticJ2ClockInterSatellitesRangeModifier(Constants.WGS84_EARTH_MU,
                  Constants.WGS84_EARTH_C20, Constants.WGS84_EARTH_EQUATORIAL_RADIUS );
          range.addModifier(modifier);
          final EstimatedMeasurement<InterSatellitesRange> estimatedAfter = range.estimate(0, 0, states);
  
          // Verify : According to Teunissen and Montenbruck, the delay is supposed to be around 62 ps for Galileo.
          //          The computed value is equal to 67.375 ps, therefore lying in the supposed range.
          Assertions.assertEquals(-0.0202125, estimatedBefore.getEstimatedValue()[0] - estimatedAfter.getEstimatedValue()[0], 1.0e-2);
          Assertions.assertEquals(0, modifier.getParametersDrivers().size());
          Assertions.assertEquals(1,
                                  estimatedAfter.getAppliedEffects().entrySet().stream().
                                  filter(e -> e.getKey().getEffectName().equals("J₂ clock relativity")).count());
  
      }
  
      @BeforeEach
      public void setUp() {
          // Data root
          Utils.setDataRoot("regular-data");
  
          // Date
          date = new AbsoluteDate("2004-01-13T00:00:00.000", TimeScalesFactory.getUTC());
  
          // Spacecraft states
          states = new SpacecraftState[2];
          final TLE local = new TLE("1 27642U 03002A   04013.91734903  .00000108  00000-0  12227-4 0  3621",
                                    "2 27642  93.9970   6.8623 0003169  80.1383 280.0205 14.90871424 54508");
          final TLE remote = new TLE("1 20061U 89044A   04013.44391333  .00000095  00000-0  10000-3 0  3242",
                                     "2 20061  53.4233 172.2072 0234017 261.4179  95.8975  2.00577231106949");
         states[0] = TLEPropagator.selectExtrapolator(local).propagate(date);
         states[1] = TLEPropagator.selectExtrapolator(remote).propagate(date);
     }
 
 }