THE TEMPORAL AND SPATIAL CHARACTERISTICS OF TOPEX POSEIDON RADIAL ORBIT ERROR/

Satellite orbit error has long been the bane of oceanographers who ana lyze altimetry data. However, radial orbit error on TOPEX/POSEIDON (T/ P) has been reduced to the 3 to 4-cm root-mean-square (rms) level over a 10-day repeat cycle, which represents an order of magnitude improve ment over earlier altimetry missions such as Geosat. Consequently, oce anographers are now able to directly evaluate the absolute ocean topog raphy to unprecedented accuracy levels. While significantly reduced, t he T/P orbit error still requires quantification. This study examines the spatial and temporal characteristics of the T/P radial orbit error , as assessed through the analysis of laser tracking residuals and orb it comparisons with independently generated trajectories. Spectral ana lyses of the orbit differences between the orbits determined from sate llite laser ranging and Doppler Orbitography and Radiopositioning Inte grated by Satellite data and the independently determined reduced dyna mic Global Positioning System (GPS) ephemerides indicate that the pred ominant power is at the once-per-orbital revolution frequency with 2- to 3-cm peaks. When the orbit differences are colinearly aligned to a fixed geographic grid and spectral analysis is performed at each geogr aphic grid point, a nearly 60-day period is found with maximum amplitu des in the 2- to 4-cm range. The contribution of both conservative and nonconservative force and measurement mismodeling to this error signa l are assessed. We demonstrate that the similar to 60-day error period seen at fixed geographic locations arises from weaknesses in the dyna mic ocean tidal models used in the orbit calculations. New tidal model s have been developed which significantly reduce this error. Second-ge neration orbits incorporating many model improvements have been comput ed and demonstrate a significant reduction in the radial orbit error s ignals. Some orbit error still exists, and methods for further model i mprovements and the possibility of achieving 1-cm radial rms orbit acc uracy in T/P are discussed.