OBSERVATIONS OF TIDALLY COHERENT DIURNAL AND SEMIDIURNAL VARIATIONS IN THE GEOCENTER

The center of mass of the Earth, about which a satellite orbits, is de termined by the mass distribution of the solid Earth (including the Ea rth's interior), the oceans, and the atmosphere. The tracking sites, h owever, are all located on the lithosphere only, and so the network or igin will not generally coincide with the center of mass. Changes in t he vector offset between these two points, referred to as apparent geo center motion, are due to global scale movement of mass. In this study we have extended the approach used by space geodetic techniques for d etermination of tidally coherent nearly diurnal and semidiurnal variat ions in the Earth's orientation to include geocenter variations due to ocean tides. We demonstrate the weaker observability of nearly diurna l retrograde motions in x and y, and hence the utility of their separa tion from the other equatorial terms. We find statistically significan t terms at the 99.5% level due to O-1, P-1, K-1, and M-2. The solution s, which indicate geocenter motions at the few millimeter level per ti de, compare well with predicted values from theoretical ocean tide mod els and from TOPEX/Poseidon radar altimetry.