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# c2teqx

erfa.c2teqx(rbpn, gst, rpom)

  Assemble the celestial to terrestrial matrix from equinox-based components (the celestial-to-true matrix, the Greenwich Apparent Sidereal Time and the polar motion matrix).

  Parameters:
    rbpn double array
    gst double array
    rpom double array
  Returns:
    rc2t double array

  Notes

  Wraps ERFA function `eraC2teqx`. The ERFA documentation is:

      - - - - - - - - - -
       e r a C 2 t e q x
      - - - - - - - - - -

      Assemble the celestial to terrestrial matrix from equinox-based
      components (the celestial-to-true matrix, the Greenwich Apparent
      Sidereal Time and the polar motion matrix).

      Given:
         rbpn   double[3][3]  celestial-to-true matrix
         gst    double        Greenwich (apparent) Sidereal Time (radians)
         rpom   double[3][3]  polar-motion matrix

      Returned:
         rc2t   double[3][3]  celestial-to-terrestrial matrix (Note 2)

      Notes:

      1) This function constructs the rotation matrix that transforms
         vectors in the celestial system into vectors in the terrestrial
         system.  It does so starting from precomputed components, namely
         the matrix which rotates from celestial coordinates to the
         true equator and equinox of date, the Greenwich Apparent Sidereal
         Time and the polar motion matrix.  One use of the present function
         is when generating a series of celestial-to-terrestrial matrices
         where only the Sidereal Time changes, avoiding the considerable
         overhead of recomputing the precession-nutation more often than
         necessary to achieve given accuracy objectives.

      2) The relationship between the arguments is as follows:

            [TRS] = rpom * R_3(gst) * rbpn * [CRS]

                  = rc2t * [CRS]

         where [CRS] is a vector in the Geocentric Celestial Reference
         System and [TRS] is a vector in the International Terrestrial
         Reference System (see IERS Conventions 2003).

      Called:
         eraCr        copy r-matrix
         eraRz        rotate around Z-axis
         eraRxr       product of two r-matrices

      Reference:

         McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
         IERS Technical Note No. 32, BKG (2004)

      This revision:  2021 May 11

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.