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    * Licensed to CS Communication & Systèmes (CS) under one or more
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  package org.orekit.models.earth.troposphere;
  
  import java.util.Collections;
  import java.util.List;
  
  import org.hipparchus.CalculusFieldElement;
  import org.orekit.bodies.FieldGeodeticPoint;
  import org.orekit.bodies.GeodeticPoint;
  import org.orekit.models.earth.weather.FieldPressureTemperatureHumidity;
  import org.orekit.models.earth.weather.PressureTemperatureHumidity;
  import org.orekit.models.earth.weather.PressureTemperatureHumidityProvider;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.utils.Constants;
  import org.orekit.utils.FieldTrackingCoordinates;
  import org.orekit.utils.ParameterDriver;
  import org.orekit.utils.TrackingCoordinates;
  
  /** The Askne Nordius model.
   * <p>
   * The hydrostatic part is equivalent to Saastamoinen, whereas the wet part takes
   * into account {@link PressureTemperatureHumidity#getTm() mean temperature weighted
   * with water vapor pressure} and {@link PressureTemperatureHumidity#getLambda() water
   * vapor decrease factor}.
   * </p>
   * @author Luc Maisonobe
   * @see "J. Askne and H. Nordius, Estimation of tropospheric delay for microwaves
   *      from surface weather data, Radio Science, volume 22, number 3, pages 379-386,
   *      May-June 1987"
   * @see "Landskron D (2017) Modeling tropospheric delays for space geodetic
   *      techniques. Dissertation, Department of Geodesy and Geoinformation, TU Wien, Supervisor: J. Böhm.
   *      http://repositum.tuwien.ac.at/urn:nbn:at:at-ubtuw:1-100249"
   * @since 12.1
   */
  public class AskneNordiusModel implements TroposphericModel {
  
      /** Lowest acceptable elevation angle [rad]. */
      public static final double LOW_ELEVATION_THRESHOLD = 0.05;
  
      /** Base delay coefficient (from Saastamoninen model). */
      private static final double L0 = 2.2768e-5;
  
      /** Askne-Nordius coefficient k'₂. */
      private static final double K_PRIME_2 = 16.5203;
  
      /** Askne-Nordius coefficient k₃. */
      private static final double K_3 = 377600;
  
      /** Gas constant for dry components. */
      private static final double RD = 287.0464;
  
      /** Unit consversion factor. */
      private static final double FACTOR = 1.0e-6;
  
      /** Mapping function. */
      private final TroposphereMappingFunction mappingFunction;
  
      /** Provider for pressure, temperature and humidity.
       * @since 13.0
       */
      private final PressureTemperatureHumidityProvider pthProvider;
  
      /** Create a new Askne Nordius model.
       * @param mappingFunction mapping function
       * @param pthProvider provider for pressure, temperature and humidity
       */
      public AskneNordiusModel(final TroposphereMappingFunction mappingFunction,
                               final PressureTemperatureHumidityProvider pthProvider) {
          this.mappingFunction = mappingFunction;
          this.pthProvider     = pthProvider;
      }
  
      /** {@inheritDoc} */
      @Override
      public TroposphericDelay pathDelay(final TrackingCoordinates trackingCoordinates, final GeodeticPoint point,
                                         final double[] parameters, final AbsoluteDate date) {
  
          final double[] mf = mappingFunction.mappingFactors(trackingCoordinates, point, date);
  
          // compute weather parameters
          final PressureTemperatureHumidity weather = pthProvider.getWeatherParameters(point, date);
  
          // compute the path delay
         final double zh     = L0 * weather.getPressure();
         final double zw     = FACTOR * (K_PRIME_2 + K_3 / weather.getTm()) *
                               RD * weather.getWaterVaporPressure() /
                               (Constants.G0_STANDARD_GRAVITY * (weather.getLambda() + 1.0));
         final double sh     = zh * mf[0];
         final double sw     = zw * mf[1];
         return new TroposphericDelay(zh, zw, sh, sw);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> FieldTroposphericDelay<T> pathDelay(final FieldTrackingCoordinates<T> trackingCoordinates,
                                                                                    final FieldGeodeticPoint<T> point,
                                                                                    final T[] parameters, final FieldAbsoluteDate<T> date) {
 
         final T[] mf = mappingFunction.mappingFactors(trackingCoordinates, point, date);
 
         // compute weather parameters
         final FieldPressureTemperatureHumidity<T> weather = pthProvider.getWeatherParameters(point, date);
 
         // compute the path delay
         final T zh     = weather.getPressure().multiply(L0);
         final T zw     = weather.getTm().reciprocal().multiply(K_3).add(K_PRIME_2).
                          multiply(weather.getWaterVaporPressure().multiply(RD)).
                          divide(weather.getLambda().add(1.0).multiply(Constants.G0_STANDARD_GRAVITY)).
                          multiply(FACTOR);
         final T sh     = zh.multiply(mf[0]);
         final T sw     = zw.multiply(mf[1]);
         return new FieldTroposphericDelay<>(zh, zw, sh, sw);
 
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return Collections.emptyList();
     }
 
 }