The short - arc approach to laser ranging

We present the scientiic motivations and the rst results obtained with an alternative method for the analysis of laser ranging data for the determination of the coordinates of ground stations and their temporal variations. The method makes use of very short orbital arcs (lasting less than two hours for LAGEOS) observed almost simultaneously by at least three stations and is particularly addressed to the solution of \local" networks (of continental or sub-continental scale). The method is very robust with respect to errors in the force model and can attain cen-timetric accuracy in the determination of station positions even with a simpliied dynamics, as it is shown by a test campaign involving six European stations for a duration of seven years. We propose a modiication of the method which should reduce its sensitivity to large systematic errors in the observations and improve the stochastic model. 1. The need for a short-arc approach During the past years (from 1986 up to present) our group has been working on the deenition and implementation of an alternative method for the analysis of satellite laser ranging data with the purpose of determining the positions and the relative displacements of the ground stations with the best possible accuracy. The main features of the method are the use of very short orbital arcs (lasting less than one orbital revolution of the satellite) and the application to a local network: the ground stations are distributed over a region having a limited extension, so that they can track the satellite almost simultaneously. After a period of initial tests, these concepts led to the development of a software package for orbital analysis (ORBIT10) which is now fully operational.