PoleTides.cpp

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//==============================================================================
//
//  This file is part of GNSSTk, the ARL:UT GNSS Toolkit.
//
//  The GNSSTk is free software; you can redistribute it and/or modify
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//  This software was developed by Applied Research Laboratories at the
//  University of Texas at Austin.
//  Copyright 2004-2022, The Board of Regents of The University of Texas System
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//==============================================================================
 
//==============================================================================
//
//  This software was developed by Applied Research Laboratories at the
//  University of Texas at Austin, under contract to an agency or agencies
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//  DISTRIBUTION STATEMENT A: This software has been approved for public
//                            release, distribution is unlimited.
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//==============================================================================
 
#include "PoleTides.hpp"
#include "CivilTime.hpp"
#include "MJD.hpp"
 
 
namespace gnsstk
{
 
      /* Returns the effect of pole tides (meters) at the given
       * position and epoch, in the Up-East-North (UEN) reference frame.
       *
       * @param[in]  t Epoch to look up
       * @param[in]  p Position of interest
       *
       * @return a Triple with the pole tide effect, in meters and in
       *    the UEN reference frame.
       *
       * @throw InvalidRequest If the request can not be completed for
       *    any reason, this is thrown. The text may have additional
       *    information about the reason the request failed.
       *
       * @warning In order to use this method, you must have previously
       *    set the current pole displacement parameters
       *
       */
   Triple PoleTides::getPoleTide( const CommonTime& t,
                                  const Position& p )
   {
 
         // We will store here the results
      Triple res(0.0, 0.0, 0.0);
 
         // Declare J2000 reference time: January 1st, 2000, at noon
      const CivilTime j2000(2000, 1, 1, 12, 0, 0.0);
 
 
      try
      {
 
            // Get current position's latitude and longitude, in radians
         double latitude(p.geodeticLatitude()*DEG_TO_RAD);
         double longitude(p.longitude()*DEG_TO_RAD);
 
            // Compute appropriate running averages
            // Get time difference between current epoch and
            // J2000.0, in years
         double timedif(static_cast<double>((MJD(t).mjd - MJD(j2000).mjd)/365.25));
 
         double xpbar(0.054 + timedif*0.00083);
         double ypbar(0.357 + timedif*0.00395);
 
            // Now, compute m1 and m2 parameters
         double m1(xdisp-xpbar);
         double m2(ypbar-ydisp);
 
            // Now, compute some useful values
         double sin2lat(std::sin(2.0*latitude));
         double cos2lat(std::cos(2.0*latitude));
         double sinlat(std::sin(latitude));
         double sinlon(std::sin(longitude));
         double coslon(std::cos(longitude));
 
            // Finally, get the pole tide values, in UEN reference
            // frame and meters
         res[0] = -0.033 * sin2lat * ( m1*coslon + m2*sinlon );
         res[1] = +0.009 * sinlat  * ( m1*sinlon - m2*coslon );
         res[2] = -0.009 * cos2lat * ( m1*coslon + m2*sinlon );
 
            // Please be aware that the former equations take into account
            // that the IERS pole tide equations use CO-LATITUDE instead
            // of LATITUDE. See Wahr, 1985.
 
      } // End of try block
      catch(...)
      {
 
         InvalidRequest ir("Unknown error when computing pole tides.");
         GNSSTK_THROW(ir);
 
      }
 
      return res;
 
   }  // End of method 'PoleTides::getPoleTide()'
 
 
 
      /* Method to set the pole displacement parameters
       *
       * @param x     Pole displacement x, in arcseconds
       * @param y     Pole displacement y, in arcseconds
       *
       * @return This same object
       */
   PoleTides& PoleTides::setXY( const double& x,
                                const double& y )
   {
 
      xdisp = x;
      ydisp = y;
 
      return (*this);
 
   }  // End of method 'PoleTides::setXY()'
 
 
 
}  // End of namespace gnsstk