Improvement of the NIMA Precise Orbit and Clock Estimates

James P. Cunningham, Everett R. Swift, Frank Mueller

Abstract:	The National Imagery and Mapping Agency (NIMA) computes precise GPS orbit and clock estimates using data collected by a globally distributed set of twelve tracking stations consisting of their own seven tracking stations and the five operational Air Force monitor stations. The Naval Surface Warfare Center, Dahlgren Division developed and continues to enhance the OMNIS software system used for these computations and provides analysis support. Several modifications to the estimation procedures over the last two years have resulted in improved accuracy in the orbit and clock estimates. In October 1996 NIMA began using new GPS-realized World Geodetic System 1984 coordinates for the twelve tracking stations with an estimated accuracy of better than 5 cm, one sigma, per component. Simultaneously, a double-differencing technique using both smoothed pseudorange and carrier phase observations was implemented for generating the precise orbit estimates. These estimates are then held fixed while the satellite and station clocks are estimated using stochastic models. Previously, the orbits and clocks were estimated simultaneously using smoothed pseudorange data only processed in a non-differenced mode. In December 1997 NIMA started using an improved tropospheric refraction model, the new Earth Gravitational Model 1996, and refined statistics for all stochastic parameters. The NIMA precise orbit estimates are evaluated by comparing them with the International GPS Service (IGS) final orbits. These IGS orbits are based on combining orbits generated by seven organizations using data from an average of over 50 tracking stations. The orbit differences are expressed in terms of both orbit-frame differences and rms User Range Errors (UREs) using the IGS orbits as truth. The rms UREs for the NIMA precise orbit estimates based on 12 stations decreased from about 30 cm before the 1996 improvements to better than 10 cm after the 1997 improvements. This paper will discuss the two sets of processing improvements, the comparisons made with the IGS orbits, and comparisons of the NIMA Earth orientation estimates with the International Earth Rotation Service final values.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	1587 - 1596
Cite this article:	Cunningham, James P., Swift, Everett R., Mueller, Frank, "Improvement of the NIMA Precise Orbit and Clock Estimates," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 1587-1596.
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