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# c2tpe

erfa.c2tpe(tta, ttb, uta, utb, dpsi, deps, xp, yp)

  Form the celestial to terrestrial matrix given the date, the UT1, the nutation and the polar motion.

  Parameters
    tta double array
    ttb double array
    uta double array
    utb double array
    dpsi double array
    deps double array
    xp double array
    yp double array
  Returns
    rc2t double array

  Notes

  Wraps ERFA function `eraC2tpe`. The ERFA documentation is:

      - - - - - - - - -
       e r a C 2 t p e
      - - - - - - - - -

      Form the celestial to terrestrial matrix given the date, the UT1,
      the nutation and the polar motion.  IAU 2000.

      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)
         dpsi,deps  double        nutation (Note 2)
         xp,yp      double        coordinates of the pole (radians, Note 3)

      Returned:
         rc2t       double[3][3]  celestial-to-terrestrial matrix (Note 4)

      Notes:

      1) The TT and UT1 dates tta+ttb and uta+utb are Julian Dates,
         apportioned in any convenient way between the arguments uta and
         utb.  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 caller is responsible for providing the nutation components;
         they are in longitude and obliquity, in radians and are with
         respect to the equinox and ecliptic of date.  For high-accuracy
         applications, free core nutation should be included as well as
         any other relevant corrections to the position of the CIP.

      3) 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.

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

            [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), RBPN is the
         bias-precession-nutation matrix, GST is the Greenwich (apparent)
         Sidereal Time and RPOM is the polar motion matrix.

      5) Although its name does not include "00", This function is in fact
         specific to the IAU 2000 models.

      Called:
         eraPn00      bias/precession/nutation results, IAU 2000
         eraGmst00    Greenwich mean sidereal time, IAU 2000
         eraSp00      the TIO locator s', IERS 2000
         eraEe00      equation of the equinoxes, IAU 2000
         eraPom00     polar motion matrix
         eraC2teqx    form equinox-based celestial-to-terrestrial matrix

      Reference:

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

      This revision:  2021 May 11

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.