LAGEOS spin axis and non-gravitational excitations of its orbit

The analysis of the orbit residuals of LAGEOS-type satellites provides an opportunity to measure tiny perturbing forces of non-gravitational origin. Some of these effects require a knowledge of the satellite spin axis orientation. Until 1996, a standard dynamical model of the LAGEOS spin axis evolution was able to explain most of the residual excitations. This is no longer the case after 1996. However, the orbit residuals can still be successfully explained if the axis dynamical model is supplemented by empirical corrections. A good agreement with direct observations of the LAGEOS axis orientation in 1995 and 1996 enhances credibility of these empirical modifications. Their theoretical understanding remains an interesting challenge for the future. (C) 1999 COSPAR. Published by Elsevier Science Ltd.