Observations of tidally coherent diurnal and semidiurnal variations in the geocenter

The center of mass of the Earth, about which a satellite orbits, is determined by the mass distribution of the solid Earth (including the Earth's interior), the oceans, and the atmosphere. The tracking sites, however, are all located on the lithosphere only, and so the network origin will not generally coincide with the center of mass. Changes in the vector offset between these two points, referred to as apparent geocenter motion, are due to global scale movement of mass. In this study we have extended the approach used by space geodetic techniques for determination of tidally coherent nearly diurnal and semidiurnal variations in the Earth's orientation to include geocenter variations due to ocean tides. We demonstrate the weaker observability of nearly diurnal retrograde motions in x and y, and hence the utility of their separation from the other equatorial terms. We find statistically significant terms at the 99.5% level due to O-1, P-1, K-1, and M-2. The solutions, which indicate geocenter motions at the few millimeter level per tide, compare well with predicted values from theoretical ocean tide models and from TOPEX/Poseidon radar altimetry.