MISSION ANALYSIS OF CLUSTERS OF SATELLITES

An innovative satellite system that provides high precision localisati on of beacon positions consists of a cluster of satellites, i.e. a gro up of satellites that maintain assigned positions at relatively short distances from each other. Compared to a single satellite, the interes t of such a cluster lies in its ability to synthesise antenna bases mu ch longer than those who can be physically mounted on one satellite. E ach satellite of the cluster measures the time-of-arrival of the signa l transmitted by the beacon. The derived time-differences-of-arrival ( TDOA) are processed to estimate the beacon position. At first, this pa per summarises the investigations performed on the localisation accura cy that have yielded the optimal cluster geometry. In a previous paper [E. Frayssinhes and E. Lansard, AAS paper 95-334 (1995)], Alcatel Esp ace has proposed a mathematical formulation relying on a strong analog y with GPS geometrical characterisation of navigation performances. Th e effects of geometry are expressed by geometric dilution of precision (GDOP) parameters. Such parameters are obtained by solving the TDOA m easurement equations for the beacon position using an iterated-least-s quares procedure. Then, the paper focuses at the system level on the p eculiar problems that arise when such a satellite cluster system is de alt with, and more particularly the launch and early operations phases , the station-keeping strategies of manoeuvres, and the relative local isation and clock synchronisation of the satellites. In particular, it is shown that even with the ''civil'' C/A GPS measurements, different ial techniques can yield respective accuracies better than 5 m r.m.s. and 15 ns r.m.s. (C) 1997 Published by Elsevier Science Ltd.