Estimation of low degree geopotential coefficients using SLR data

Abstract Geodetic satellites have been providing the low frequency part of the geopotential models used for precise orbit determination purposes (e.g. JGM3, EGM96, &). Nevertheless they can be used to estimate the temporal variation of selected coefficients, helping to clarify the complex interrelations in the earth-ocean-atmosphere system. In this paper we present the two years long analysis of SLR data from the seven available geodetic satellites (Lageos III, Stella, Starlette, Ajisai, Etalon III) to recover monthly estimates of low degree geopotential coefficients; the results are obtained analysing the satellites separately and in proper combination. An accurate modelling of the satellite orbits is required in order to separate the geopotential coefficients: we assume as a priori geopotential the JGM3 model together with its associated tides and we take care of non-gravitational effects on the satellites by means of proper empirical estimated accelerations. The time series of the estimated coefficients ( J 2 , J 3 , J 4 , J 5 ) are inspected to detect the sub-annual perturbations related to seasonal variation of mass distribution. Huge residual seasonal signals in the orbit of Stella indicate a strong model deficiency related to the Suns influence on the environment. The remaining six satellites are homogeneously modelled and build up a three cycles per year oscillation on J 2 and a seasonal oscillation (1 year and six month periods) revealed on the J 4 . The origin and possible causes of these signals are further discussed in the text. We also present a preliminary J 2 estimate, using twelve years of Lageos-I and Lageos-II observations, that is compared with previous obtained values.