The New Generation GEOSAT Follow-On Precise Orbit Ephemeris Using ITRF2005 and Other Improvements

Launched in 1998, the U.S. Navy GEOSAT Follow-On spacecraft (GFO) began continuous radar altimeter coverage of the oceans in 2000 and is still operating, although currently with reduced capability. By providing high quality altimeter data GFO can supplement Jason, TOPEX/POSEIDON (T/P), and Envisat, providing a different synoptic sampling of the oceans with its 17-day ground track repeat cycle. GFO altimeter data is also currently being used to aid in the monitoring of lake and reservoir levels worldwide under a USDA funded project. Altimeter crossover analysis suggests GFO is capable of POSEIDON class altimetry, showing crossover residuals averaging below 7.5 cm, with 5-cm orbit error the largest contributor to the altimeter error budget. Non- conservative forces are believed to be the largest single contributor to orbit error. Satellite laser ranging (SLR), especially in combination with altimeter crossover data, offer the only means of determining high-quality precise orbits. SLR tracking is also augmented by Doppler (Tranet style) beacons. This study evaluates possible improvements to the current GFO GDR orbits. POD model improvements include using model standards consistent with the latest generation of GSFC reprocessed TOPEX and Jason orbits (for example GGM02C GRACE gravity, and time-varying gravity models) as well as a new high-resolution radiation pressure model developed by University College London. We also document the impact of the new ITRF2005 reference frame on the GFO GDR orbit products. Orbit accuracy is evaluated using GFO tracking data and altimeter crossover and collinear sea surface height residuals, and analysis of coefficients used to adjust GFO data into the TOPEX frame.