Time series of low degree zonals obtained analyzing different geodetic satellites

The geodetic satellite orbit is used as a gravity probe to sense the time varying
field. We analysed ten years of Satellite Laser Ranging (SLR) observations of the
LAGEOS-I, LAGEOS-II, Starlette and Stella satellites and estimated the time series of the
low degree zonal coefficients in the Earth gravity field up to degree six. The corresponding
secular drifts have been derived and compared with other published results.