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# c2t06a

erfa.c2t06a(tta, ttb, uta, utb, xp, yp)

  Form the celestial to terrestrial matrix given the date, the UT1 and the polar motion, using the IAU 2006/2000A precession-nutation model.

  Parameters:
    tta double array
    ttb double array
    uta double array
    utb double array
    xp double array
    yp double array
  Returns:
    rc2t double array

  Notes

  Wraps ERFA function `eraC2t06a`. The ERFA documentation is:

      - - - - - - - - - -
       e r a C 2 t 0 6 a
      - - - - - - - - - -

      Form the celestial to terrestrial matrix given the date, the UT1 and
      the polar motion, using the IAU 2006/2000A precession-nutation
      model.

      Given:
         tta,ttb  double         TT as a 2-part Julian Date (Note 1)
         uta,utb  double         UT1 as a 2-part Julian Date (Note 1)
         xp,yp    double         coordinates of the pole (radians, Note 2)

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

      Notes:

      1) The TT and UT1 dates tta+ttb and uta+utb are Julian Dates,
         apportioned in any convenient way between the two arguments.  For
         example, JD(UT1)=2450123.7 could be expressed in any of
         these ways, among others:

                 uta            utb

             2450123.7           0.0       (JD method)
             2451545.0       -1421.3       (J2000 method)
             2400000.5       50123.2       (MJD method)
             2450123.5           0.2       (date & time method)

         The JD method is the most natural and convenient to use in
         cases where the loss of several decimal digits of resolution is
         acceptable.  The J2000 and MJD methods are good compromises
         between resolution and convenience.  In the case of uta,utb, the
         date & time method is best matched to the Earth rotation angle
         algorithm used:  maximum precision is delivered when the uta
         argument is for 0hrs UT1 on the day in question and the utb
         argument lies in the range 0 to 1, or vice versa.

      2) The arguments xp and yp are the coordinates (in radians) of the
         Celestial Intermediate Pole with respect to the International
         Terrestrial Reference System (see IERS Conventions 2003),
         measured along the meridians 0 and 90 deg west respectively.

      3) The matrix rc2t transforms from celestial to terrestrial
         coordinates:

            [TRS] = RPOM * R_3(ERA) * RC2I * [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), RC2I is the
         celestial-to-intermediate matrix, ERA is the Earth rotation
         angle and RPOM is the polar motion matrix.

      Called:
         eraC2i06a    celestial-to-intermediate matrix, IAU 2006/2000A
         eraEra00     Earth rotation angle, IAU 2000
         eraSp00      the TIO locator s', IERS 2000
         eraPom00     polar motion matrix
         eraC2tcio    form CIO-based celestial-to-terrestrial matrix

      Reference:

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

      This revision:  2023 January 18

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