Optical Tracking of Synchronous Satellites for Geophysical Purposes

The tracking of synchronous satellites, combined with an orbit propagation model of comparable accuracy, allows the determination of the resonant geopotential coefficients (l-m even) of low degree l (mainly C22 and S22,which correspond to the Earth’s equator ellipticity). The radio tracking techniques used in the sixties with the first telecommunication satellites (Wagner 1965;1966) were not very accurate, so that the resulting relative accuracy in was not better than few percents.In the present situation, the coefficients C22, S22 relevant for synchronous orbits derived within the existing global models show rather large discrepancies going from 10−3 to 10−1 of their values. Much more accurate tracking techniques are now available (LASER, Doppler twin band), but only the synchronous satellite SIRIO2, scheduled for launch by ESA in 1982,will have a laser retroflector array on board; no synchronous satellite has — nor is planned to have — the required payload on board to allow Doppler twin-band tracking. The ground laser stations tracking SIRIO2 will provide ranging data with an uncertainty in the radial direction less than 1 m. Due to the bad geometry of the range measurements, the corresponding uncertainty in longitude will be about 10 times larger, providing, in principle, a relative accuracy in C22,S22 of the order of 10−5. Unfortunately the results which could be obtained by using LASER data are seriously limited by radiation pressure perturbations (Anselmo et al.,1981),which are very difficult to model (unless ad hoc spherical or very expensive drag-free satellites are used). The bad modelling of the radiation pressure perturbation causes the longitude uncertainties to grow up to 100÷1000m, so that the resulting relative accuracy in C22,S22 will not be anyway better than 10−3÷10−4.