First and Second Degree Gravitational Variations From Satellite Laser Ranging

Satellite Laser Ranging (SLR) is a key ingredient for the definition of the Terrestrial Reference System (TRF). SLR defines uniquely the origin and in part the scale of the TRF. Driven by numerous geophysical processes, continuous mass redistribution within the Earth system causes concomitant changes in the Stokes coefficients describing the terrestrial gravity field. Seasonal changes in these coefficients have been closely correlated with mass transfer in the atmosphere, hydrosphere and oceans and they are now routinely monitored from dedicated space missions (e.g. GRACE). The stability, integrity and applicability of the TRF are directly related to the accuracy and fidelity with which such motions can be observed or modeled during the position determination of the defining sites. Variations in the very low degree and order harmonics, produce geometric effects that are manifested as changes in the origin and orientation relationship between the instantaneous and the mean reference frame, as well as the orientation of the axes of figure. SLR has contributed the most accurate weekly observations of these effects yet, demonstrating millimeter level accuracy. Our multi-year TRF definition is done incrementally, adding every week, the new weekly data set, thereby extending the validity of the TRF and thus allowing the accurate observation of several significant geophysical events, recent and in the past, involving mass motion in the hydrosphere and the solid Earth. We now have a new series of weekly results, consistent with the IERS Conventions 2003, using the latest improvements in modeling SLR observations. We will present our results from several years of LAGEOS and ETALON SLR data, assess their accuracy, compare them the previous series, to geophysical signals, and to the monthly series from the GRACE project.