Position variations of 40 VLBI stations at 32 tidal frequencies were
obtained from analysis of 3 million measurements of group delays from 1980 to
2002. Residual displacements after the removal of the effect of solid Earth 
tides were studied. The purpose of this study is to investigate a harmonic 
signal at any frequency where it is expected and to assess quantitatively 
whether the models of vertical and horizontal site position variations agree 
with the observations. It was found that the estimates of station displacements 
are generally in a good agreement with the ocean loading computed on the basis 
of ocean tide models for the main diurnal and semi-diurnal tides. Moreover, 
VLBI results allow us to discriminate between different ocean tide models. 
However, discrepancies between VLBI results and all models of ocean loading for 
K1, K2 and S2 tides exceed both the errors of the VLBI estimates and the errors 
of ocean loading displacements based on the reported formal uncertainties of 
ocean tide models. It was found that there is a significant non-tidal signal 
at diurnal and annual frequencies. Applying a model of hydrological loading 
reduces the variance of the residual vertical displacements at the annual 
frequency at 40. Using an empirical model of harmonic site position 
variations in VLBI processing provides a better fit and improves the baseline 
length repeatability.