The geodetic precision of VLBI (very long baseline interferometry) has improved to the level that baseline lengths can be measured with a wrms precision of 1 ppb and site positions with precision of 8-14 mm in the vertical and 2-3 mm in the horizontal components. Observed VLBI site position variations contain significant contributions at seasonal (annual and semiannual) frequencies. Here, we will concentrate on the site vertical annual amplitudes that mostly range from 2-6 mm. We determine the extent to which annual site variations can be explained by mass loading signals. Specifically, we examine the contributions from atmosphere pressure loading, hydrologic loading, and nontidal ocean loading. We have also examined the effect of two modeling errors specific to the VLBI measurements: tropospheric delay mismodeling (hydrostatic mapping function) and antenna thermal deformation.
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