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From USNO http://aa.usno.navy.mil/publications/docs/Circular_180C_Fortran.pdf :

1.6 Terrestrial-Celestial Relationships

NOVAS uses the International Terrestrial Reference System (ITRS) for specifying locations and directions on or near the surface of the Earth. As mentioned at the end of section 1.1, the ITRS is consistent, to within a few centimeters, with WGS-84 coordinates provided by GPS, and it is sometimes referred to as the Earth-centered Earth-fixed system (ECEF). The ITRS is a geocentric system with the directions of its axes defined by the coordinates of a large number of observing stations, in a way completely analogous to the definition of the ICRS by the coordinates of extragalactic radio sources. The ITRS z-axis is toward the north geodetic pole and its x-axis is toward a point at longitude and latitude zero; the y-axis forms a right-handed system with the other two axes.

Practical applications of astronomical data often require relating terrestrial coordinates to celestial coordinates and vice versa. For example, we may want the position of a celestial object expressed with respect to the local horizon system. [ZDAZ] Or, we may have a vector, expressed in an Earth-fixed system, that represents some instrumental axis, and we would like to know where that vector is pointed on the celestial sphere. NOVAS can perform the terrestrial-to-celestial transformation or its inverse; specifically, the transformation from the ITRS to the GCRS, or the GCRS to the ITRS. [TERCEL] These transformations are a series of rotations that, taken together, represent the instantaneous orientation of the Earth in space. [PRECES, NUTATE, CIOBAS, SIDTIM, EROT, WOBBLE]

Not all aspects of the Earth’s orientation are predictable. Polar motion represents the small shift of the geodetic north pole (the ITRS z-axis) with respect to the rotational axis (the CIP), the largest part of which must be determined from observations. Typically, the total shift amounts to a few tenths of an arcsecond (1-2 μrad, 10 meters) and is specified by the parameters xp and yp. The observational determinations are designated simply as x and y, and current values are available from IERS Bulletin A.18EO web site Past values can be obtained at the .19USNO Circular 179 For most purposes we can set xp=x and yp=y (see 20
section 6.5.2 or the IERS Conventions 2009 section 5.5.1). Several NOVAS subroutines require as input the xp and yp values for the date of interest, although, if the final accuracy requirements are no better than 1 arcsecond, these values can be set to zero.

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