Combined VLBI/GPS series of precession-nutation and comparison with IAU2000 model

The IAU adopted in 2000 a new model of precession-nutation that is to be used from January 1, 2003. It is based on the theory of nutation for the rigid Earth by Souchay et al. ([CITE]) and the transfer function of a complicated geophysical Earth model computed by Mathews et al. ([CITE]). Some of the constants of this model are derived from the celestial pole offsets observed by Very Long-Baseline Interferometry (VLBI). Since 1994, series of celestial pole offset rates, observed by Global Positioning System (GPS), have been made available. The observations of Earth Orientation Parameters (EOP) made by different techniques have specific behavior at different frequencies. VLBI is very stable in long-term sense but does not provide very dense observations; satellite methods (such as GPS) have a good short-term precision and high resolution but their long-term stability is not good. The recently proposed method of combined smoothing (Vondrák & Čepek [CITE]) is capable of combining non-evenly distributed observations of a parameter with its observed rates, obtained in epochs that do not need to be identical with the epochs of the observed parameter. Using this method, the combination of VLBI-based celestial pole offsets with the GPS-based celestial pole offset rates is presented, and compared with the new IAU2000 model of precession-nutation. The new model is shown to agree with the combined solution at the level of present observational accuracy, and the Free Core Nutation (FCN) is shown to be dominant in the residuals.