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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
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  package org.orekit.models.earth.troposphere;
  
  import java.util.Collections;
  import java.util.List;
  
  import org.hipparchus.Field;
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.MathArrays;
  import org.orekit.annotation.DefaultDataContext;
  import org.orekit.bodies.FieldGeodeticPoint;
  import org.orekit.bodies.GeodeticPoint;
  import org.orekit.data.DataContext;
  import org.orekit.time.AbsoluteDate;
  import org.orekit.time.DateTimeComponents;
  import org.orekit.time.FieldAbsoluteDate;
  import org.orekit.time.TimeScale;
  import org.orekit.utils.ParameterDriver;
  
  /** The Vienna1 tropospheric delay model for radio techniques.
   * The Vienna model data are given with a time interval of 6 hours
   * as well as on a global 2.5° * 2.0° grid.
   *
   * This version considered the height correction for the hydrostatic part
   * developed by Niell, 1996.
   *
   * @see "Boehm, J., Werl, B., and Schuh, H., (2006),
   *       Troposhere mapping functions for GPS and very long baseline
   *       interferometry from European Centre for Medium-Range Weather
   *       Forecasts operational analysis data, J. Geophy. Res., Vol. 111,
   *       B02406, doi:10.1029/2005JB003629"
   *
   * @author Bryan Cazabonne
   */
  public class ViennaOneModel implements DiscreteTroposphericModel, MappingFunction {
  
      /** The a coefficient for the computation of the wet and hydrostatic mapping functions.*/
      private final double[] coefficientsA;
  
      /** Values of hydrostatic and wet delays as provided by the Vienna model. */
      private final double[] zenithDelay;
  
      /** UTC time scale. */
      private final TimeScale utc;
  
      /** Build a new instance.
       *
       * <p>This constructor uses the {@link DataContext#getDefault() default data context}.
       *
       * @param coefficientA The a coefficients for the computation of the wet and hydrostatic mapping functions.
       * @param zenithDelay Values of hydrostatic and wet delays
       * @see #ViennaOneModel(double[], double[], TimeScale)
       */
      @DefaultDataContext
      public ViennaOneModel(final double[] coefficientA, final double[] zenithDelay) {
          this(coefficientA, zenithDelay,
               DataContext.getDefault().getTimeScales().getUTC());
      }
  
      /**
       * Build a new instance.
       *
       * @param coefficientA The a coefficients for the computation of the wet and
       *                     hydrostatic mapping functions.
       * @param zenithDelay  Values of hydrostatic and wet delays
       * @param utc          UTC time scale.
       * @since 10.1
       */
      public ViennaOneModel(final double[] coefficientA,
                            final double[] zenithDelay,
                            final TimeScale utc) {
          this.coefficientsA = coefficientA.clone();
          this.zenithDelay   = zenithDelay.clone();
          this.utc           = utc;
      }
  
      /** {@inheritDoc} */
      @Override
      public double pathDelay(final double elevation, final GeodeticPoint point,
                              final double[] parameters, final AbsoluteDate date) {
          // zenith delay
          final double[] delays = computeZenithDelay(point, parameters, date);
          // mapping function
         final double[] mappingFunction = mappingFactors(elevation, point, date);
         // Tropospheric path delay
         return delays[0] * mappingFunction[0] + delays[1] * mappingFunction[1];
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> T pathDelay(final T elevation, final FieldGeodeticPoint<T> point,
                                                        final T[] parameters, final FieldAbsoluteDate<T> date) {
         // zenith delay
         final T[] delays = computeZenithDelay(point, parameters, date);
         // mapping function
         final T[] mappingFunction = mappingFactors(elevation, point, date);
         // Tropospheric path delay
         return delays[0].multiply(mappingFunction[0]).add(delays[1].multiply(mappingFunction[1]));
     }
 
     /** This method allows the  computation of the zenith hydrostatic and
      * zenith wet delay. The resulting element is an array having the following form:
      * <ul>
      * <li>T[0] = D<sub>hz</sub> → zenith hydrostatic delay
      * <li>T[1] = D<sub>wz</sub> → zenith wet delay
      * </ul>
      * @param point station location
      * @param parameters tropospheric model parameters
      * @param date current date
      * @return a two components array containing the zenith hydrostatic and wet delays.
      */
     public double[] computeZenithDelay(final GeodeticPoint point, final double[] parameters, final AbsoluteDate date) {
         return zenithDelay.clone();
     }
 
     /** This method allows the  computation of the zenith hydrostatic and
      * zenith wet delay. The resulting element is an array having the following form:
      * <ul>
      * <li>T[0] = D<sub>hz</sub> → zenith hydrostatic delay
      * <li>T[1] = D<sub>wz</sub> → zenith wet delay
      * </ul>
      * @param <T> type of the elements
      * @param point station location
      * @param parameters tropospheric model parameters
      * @param date current date
      * @return a two components array containing the zenith hydrostatic and wet delays.
      */
     public <T extends CalculusFieldElement<T>> T[] computeZenithDelay(final FieldGeodeticPoint<T> point, final T[] parameters,
                                                                   final FieldAbsoluteDate<T> date) {
         final Field<T> field = date.getField();
         final T zero = field.getZero();
         final T[] delays = MathArrays.buildArray(field, 2);
         delays[0] = zero.add(zenithDelay[0]);
         delays[1] = zero.add(zenithDelay[1]);
         return delays;
     }
 
     /** {@inheritDoc} */
     @Override
     public double[] mappingFactors(final double elevation, final GeodeticPoint point,
                                    final AbsoluteDate date) {
         // Day of year computation
         final DateTimeComponents dtc = date.getComponents(utc);
         final int dofyear = dtc.getDate().getDayOfYear();
 
         // General constants | Hydrostatic part
         final double bh  = 0.0029;
         final double c0h = 0.062;
         final double c10h;
         final double c11h;
         final double psi;
 
         // Latitude of the station
         final double latitude = point.getLatitude();
 
         // sin(latitude) > 0 -> northern hemisphere
         if (FastMath.sin(latitude) > 0) {
             c10h = 0.001;
             c11h = 0.005;
             psi  = 0;
         } else {
             c10h = 0.002;
             c11h = 0.007;
             psi  = FastMath.PI;
         }
 
         // Temporal factor
         double t0 = 28;
         if (latitude < 0) {
             // southern hemisphere: t0 = 28 + an integer half of year
             t0 += 183;
         }
         // Compute hydrostatique coefficient c
         final double coef = ((dofyear - t0) / 365) * 2 * FastMath.PI + psi;
         final double ch = c0h + ((FastMath.cos(coef) + 1) * (c11h / 2) + c10h) * (1 - FastMath.cos(latitude));
 
         // General constants | Wet part
         final double bw = 0.00146;
         final double cw = 0.04391;
 
         final double[] function = new double[2];
         function[0] = TroposphericModelUtils.mappingFunction(coefficientsA[0], bh, ch, elevation);
         function[1] = TroposphericModelUtils.mappingFunction(coefficientsA[1], bw, cw, elevation);
 
         // Apply height correction
         final double correction = TroposphericModelUtils.computeHeightCorrection(elevation, point.getAltitude());
         function[0] = function[0] + correction;
 
         return function;
     }
 
     /** {@inheritDoc} */
     @Override
     public <T extends CalculusFieldElement<T>> T[] mappingFactors(final T elevation, final FieldGeodeticPoint<T> point,
                                                               final FieldAbsoluteDate<T> date) {
         final Field<T> field = date.getField();
         final T zero = field.getZero();
 
         // Day of year computation
         final DateTimeComponents dtc = date.getComponents(utc);
         final int dofyear = dtc.getDate().getDayOfYear();
 
         // General constants | Hydrostatic part
         final T bh  = zero.add(0.0029);
         final T c0h = zero.add(0.062);
         final T c10h;
         final T c11h;
         final T psi;
 
         // Latitude and longitude of the station
         final T latitude = point.getLatitude();
 
         // sin(latitude) > 0 -> northern hemisphere
         if (FastMath.sin(latitude.getReal()) > 0) {
             c10h = zero.add(0.001);
             c11h = zero.add(0.005);
             psi  = zero;
         } else {
             c10h = zero.add(0.002);
             c11h = zero.add(0.007);
             psi  = zero.getPi();
         }
 
         // Compute hydrostatique coefficient c
         // Temporal factor
         double t0 = 28;
         if (latitude.getReal() < 0) {
             // southern hemisphere: t0 = 28 + an integer half of year
             t0 += 183;
         }
         final T coef = psi.add(zero.getPi().multiply(2.0).multiply((dofyear - t0) / 365));
         final T ch = c11h.divide(2.0).multiply(FastMath.cos(coef).add(1.0)).add(c10h).multiply(FastMath.cos(latitude).negate().add(1.)).add(c0h);
 
         // General constants | Wet part
         final T bw = zero.add(0.00146);
         final T cw = zero.add(0.04391);
 
         final T[] function = MathArrays.buildArray(field, 2);
         function[0] = TroposphericModelUtils.mappingFunction(zero.add(coefficientsA[0]), bh, ch, elevation);
         function[1] = TroposphericModelUtils.mappingFunction(zero.add(coefficientsA[1]), bw, cw, elevation);
 
         // Apply height correction
         final T correction = TroposphericModelUtils.computeHeightCorrection(elevation, point.getAltitude(), field);
         function[0] = function[0].add(correction);
 
         return function;
     }
 
     /** {@inheritDoc} */
     @Override
     public List<ParameterDriver> getParametersDrivers() {
         return Collections.emptyList();
     }
 
 }