This report summarizes recent progress in orbit determination and the role of Satellite Laser Ranging (SLR) in advancing the state of its art. In the past four years, major improvements in our ability to position a near-Earth SLR satellite within a well understood and stable geocentric frame have occurred. Key enabling factors are upgraded gravitational and non-conservative force models and extensive advancement in tracking technologies. In combination, these capabilities allowed orbit analysts to reach their most ambitious goal, that of providing better than 4 cm RMS radial orbit knowledge in support of the joint USA/French TOPEX/POSEIDON Mission (T/P) [ Fu et al., 1994].
SLR has provided an essential resource for improving the force models needed to achieve this level of orbit modeling. Most important, these observations have permitted major advances in gravity modeling [Nerem, 1995 this issue]. SLR acquired on the Lageos-1, Lageos-2, Starlette, Ajisai and Stella satellites provide the foundation for determining the long wavelength field which is critical for orbit modeling applications. T/P, by acquiring a continuous mapping of sea surface topography, provides a strong test bed for evaluating radial orbit accuracy. Through the comparisons with tide gauge data and the employment of various orbital testing procedures, T/P orbit modeling accuracy has been confirmed.
The modeling and expected accuracy sought for T/P is discussed in Nerem et al., [1993a] and Tapley et al., [1990]. Since T/P reached orbit in August 1992, its radial orbit has consistently been determined at the 2 to 4 cm RMS levels providing an unprecedented geocentric reference for the two radar altimeters flown on this satellite. Tapley et al., [1994a], Nouel et al., [1994], and Putney et al., [1993] reviewed T/P orbit accuracies achieved using data from the SLR and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) networks; Schutz et al., [1994], Yunck et al., [1994], Bertiger et al., [1994] and Melbourne et al., [1994] discussed those determined using the Global Positioning System (GPS). This report focuses on the contribution of U.S. groups to satellite dynamical force modeling and related orbit determination advances, and serves to underscore the unique contribution of the laser satellite constellation in the areas of static and tidal geopotential modeling, and for studying non-tidal mass redistribution, apparent motion of the geocenter and global scale determination.