Estimation of low degree geopotential coefficients using SLR data

Geodetic satellites have been providing the low frequency part of the geopo tential models used for precise orbit determination purposes (e.g. JGM3, EG M96,...). 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 l ong analysis of SLR data from the seven available geodetic satellites (Lage os I-II, Stella, Starlette, Ajisai, Etalon I-II) to recover monthly estimat es of low degree geopotential coefficients; the results are obtained analys ing the satellites separately and in proper combination. An accurate modell ing of the satellite orbits is required in order to separate the geopotenti al 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 t ime series of the estimated coefficients (J(2), J(3), J(4), J(5)) are inspe cted to detect the sub-annual perturbations related to seasonal variation o f mass distribution. Huge residual seasonal signals in the orbit of Stella indicate a strong model deficiency related to the Sun's influence on the en vironment. The remaining six satellites are homogeneously modeled 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 possibl e causes of these signals are further discussed in the text. We also presen t a preliminary j(2) estimate, using twelve years of Lageos-I and Lageos-II observations, that is compared with previous obtained values. (C) 1998 Els evier Science Ltd. All rights reserved.