How are topocentric angles (hour angle/declination or azimuth/elevation) be derived from X, Y and theta?
The ICRS coordinates must first be transformed into the GCRS frame by taking into account space motion, parallax, light deflection and aberration. Then given the TT date, the quantity s can be computed. X,Y,s can then be used to form the rotation matrix that transforms from GCRS to the intermediate frame (which is the CEO-based counterpart of the classical true equator and equinox of date frame). The Earth Rotation Angle, theta, is then applied in the same way as Greenwich Sidereal Time formerly. The polar motion is then applied, involving forming a further rotation matrix that uses the pole coordinates xp,yp and a small quantity s’, yielding the ITRS coordinates. Expressed as spherical coordinates, the ITRS vector is (-GHA,Dec), and conventional transformations can then be used to predict the local azimuth and altitude for a site at known geodetic coordinates.
