An Evaluation of Various GPS Satellite Ephemerides

Adam Roulston, Nicholas Talbot and Kefei Zhang

Abstract:	High precision orbital information is extremely important for the determination of GPS-derived position. The Broadcast ephemeris generated by the GPS Operational Control Segment (OCS), is freely available to users who decode the GPS Navigation Message. Although the Broadcast ephemeris is sufficiently accurate for most navigation users, high-precision applications benefit from improved orbital information. There are a number of ephemeris sources now routinely available via the Internet for post-processing, near-real-time, and real-time applications. Ephemerides are generated by: The International GPS Service (IGS); Jet Propulsion Laboratory (JPL); Centre for Orbit Determination in Europe (CODE); U.S. National Geodetic Survey (NGS); the U.S. National Imagery and Mapping Agency (NIMA); and other organisations. The performance of the ephemeris services can be gauged by the distribution of tracking stations used in the orbit generation, underlying computation procedures, availability and latency. An evaluation of six ephemeris products was undertaken over two separate weeks during 1999 and 2000. Satellite locations at each 15 minute epoch in the orbits were compared to the IGS Final ephemeris. It is believed that the IGS Final ephemeris is the most precise and therefore was considered as truth in the comparison. Based on the evaluation, the IGS Predicted and JPL Ultra-Rapid ephemerides were within an accuracy of 3m and 1.5m respectively, of truth. However, both ephemeris sources showed problematic modelling for some satellites. The Broadcast ephemeris, on the other hand, gave consistent results across all satellites. All precise orbits gave comparable accuracy, with the official military ephemeris generated by NIMA, the worst. Results suggest that the IGS Predicted, or JPL Ultra-Rapid ephemeris have the potential to replace the Broadcast ephemeris for real-time applications. However the Broadcast ephemeris is sufficient for all but the most demanding applications. The Precise ephemerides have standard deviations at the 10cm level and are suitable for high-precision post- processing applications.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	45 - 54
Cite this article:	Roulston, Adam, Talbot, Nicholas, Zhang, Kefei, "An Evaluation of Various GPS Satellite Ephemerides," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 45-54.
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