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  package org.orekit.estimation.measurements.modifiers;
  
  import org.hipparchus.geometry.euclidean.threed.Vector3D;
  import org.hipparchus.util.FastMath;
  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.bodies.GeodeticPoint;
  import org.orekit.bodies.OneAxisEllipsoid;
  import org.orekit.estimation.measurements.EstimatedMeasurement;
  import org.orekit.estimation.measurements.GroundStation;
  import org.orekit.estimation.measurements.ObservableSatellite;
  import org.orekit.estimation.measurements.gnss.AmbiguityCache;
  import org.orekit.estimation.measurements.gnss.Phase;
  import org.orekit.frames.FramesFactory;
  import org.orekit.frames.TopocentricFrame;
  import org.orekit.gnss.PredefinedGnssSignal;
  import org.orekit.orbits.CartesianOrbit;
  import org.orekit.propagation.SpacecraftState;
  import org.orekit.propagation.analytical.tle.TLE;
  import org.orekit.propagation.analytical.tle.TLEPropagator;
  import org.orekit.utils.Constants;
  import org.orekit.utils.IERSConventions;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TimeStampedPVCoordinates;
  
  import java.util.Arrays;
  
  /**
   * 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 RelativisticJ2ClockPhaseModifierTest {
  
  
      @Test
      public void testRelativisticClockCorrection() {
  
          // Station
          final OneAxisEllipsoid earth = new OneAxisEllipsoid(Constants.WGS84_EARTH_EQUATORIAL_RADIUS,
                                                              Constants.WGS84_EARTH_FLATTENING,
                                                              FramesFactory.getITRF(IERSConventions.IERS_2010, true));
          final GeodeticPoint point    = new GeodeticPoint(FastMath.toRadians(42.0), FastMath.toRadians(1.0), 100.0);
          final TopocentricFrame topo  = new TopocentricFrame(earth, point, "");
          final GroundStation station  = new GroundStation(topo);
  
          // Satellite (GPS orbit from TLE)
          final TLE tle = new TLE("1 28474U 04045A   20252.59334296 -.00000043  00000-0  00000-0 0  9998",
                                  "2 28474  55.0265  49.5108 0200271 267.9106 149.0797  2.00552216116165");
          final TimeStampedPVCoordinates satPV = TLEPropagator.selectExtrapolator(tle).getPVCoordinates(tle.getDate(), FramesFactory.getEME2000());
          final SpacecraftState state = new SpacecraftState(new CartesianOrbit(satPV, FramesFactory.getEME2000(), Constants.WGS84_EARTH_MU));
  
          // Set reference date to station drivers
          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(state.getDate());
              }
          }
  
          // Station PV
          final Vector3D zero = Vector3D.ZERO;
          final TimeStampedPVCoordinates stationPV = station.getOffsetToInertial(state.getFrame(), state.getDate(), false).transformPVCoordinates(new TimeStampedPVCoordinates(state.getDate(), zero, zero, zero));
  
          // phase measurement
          final Phase phase = new Phase(station, state.getDate(), 26584264.45, PredefinedGnssSignal.G01.getWavelength(),
                                  1.0, 1.0, new ObservableSatellite(0),
                                        new AmbiguityCache());
         final EstimatedMeasurement<Phase> estimated = new EstimatedMeasurement<>(phase, 0, 0,
                         new SpacecraftState[] {state},
                         new TimeStampedPVCoordinates[] {state.getPVCoordinates(), stationPV});
         estimated.setEstimatedValue(phase.getObservedValue()[0]);
         Assertions.assertEquals(0.0, estimated.getObservedValue()[0] - estimated.getEstimatedValue()[0], 1.0e-3);
 
         // Measurement modifier
         final RelativisticJ2ClockPhaseModifier modifier = new RelativisticJ2ClockPhaseModifier(Constants.WGS84_EARTH_MU,
                 Constants.WGS84_EARTH_C20, Constants.WGS84_EARTH_EQUATORIAL_RADIUS );
         modifier.modify(estimated);
         Assertions.assertEquals(0, modifier.getParametersDrivers().size());
 
         // Verify : According to Teunissen and Montenbruck, the delay is supposed to be around 60ps for Galileo.
         //          The computed value is equal to 64.745 ps, therefore lying in the supposed range.
         Assertions.assertEquals(-0.10202, estimated.getObservedValue()[0] - estimated.getEstimatedValue()[0], 1.0e-2);
         Assertions.assertEquals(1,
                                 estimated.getAppliedEffects().entrySet().stream().
                                 filter(e -> e.getKey().getEffectName().equals("J₂ clock relativity")).count());
 
     }
 
     @BeforeEach
     public void setUp() {
         Utils.setDataRoot("regular-data");
     }
 
 }