Sensitivity of LAGEOS to changes in Earth's (2, 2) gravity coefficients

Abstract

In recent years it has become of geophysical interest to detect a possible time variation of the low-degree coefficients of the Earth's gravity field, in particular the (2, 2) tesseral harmonic. We investigate the possibility of detecting such a phenomenon via analysis of the tracking data from LAGEOS, the passive geodynamics satellite tracked by laser stations on the ground. For this purpose the main problems are caused: (i) for short orbital arcs, by the irregular distribution in time of the tracking data, and by the dynamical effects of oceanic tides, which cannot be easily separated from the effects of geopotential changes; (ii) for long orbital arcs, by the difficulty of reliably predicting the variable drag-like force that is causing a slow semimajor axis decay. We estimate that a relative accuracy of the order of 10⁻⁴ in the (2, 2) coefficients can be reached provided that a larger number of higher technology laser stations is available, and that better modelling is possible of the drag-like force. Both these conditions seem quite unrealistic at present. If a relative accuracy better than 10⁻⁴ has to be reached, an effective separation of the tidal perturbations is also needed, since they give rise to perturbations with a similar signature.