Sub-Daily to Interannual Geocenter Motion - Direct and Inverse GPS Results

The motion of center-of-mass (CM) of the entire Earth system with respect to the surface of the solid Earth is a consequence of mass redistribution on the surface and in the interior. Understanding of such motion is important for the definition and maintenance of the origin of global geodetic reference frames. Accurate measurements of such motion can also provide us with a window of opportunity to infer a global mode of mass variation with the largest spatial scale. In certain cases, determination of geocenter motion is essential when the goal is to know the complete mass variation spectrum. Driven by surface mass variation, the motion of CM with respect to the center-of-figure (CF) of the solid Earth surface has a one-to-one correspondence with degree 1 surface mass load. However, geodetic tracking networks have only finite number of stations, and can be very sparse in many cases. The motions of CM with respect to various networks therefore will be different from that with respect to CF and different among themselves. The network-based measurements will be further complicated by higher degree surface load induced deformation. These issues will be examined as we report our direct and inverse measurements of sub-daily to interannual geocenter motions through GPS orbit dynamics and relative surface deformation. We will also discuss implications of geocenter motion on the study of surface mass variations using time-variable gravity data.