era.c2teqx.md

eraC2teqx

rc2t = 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).

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) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.