On-Orbit Validation of GPS IIR Autonomous Navigation

J.A. Rajan, M. Orr, P. Wang

Abstract:	Global Positioning System (GPS) is a national utility providing navigation and timing service worldwide. The Block IIR GPS system is the set of 21 satellites that are slated to replenish Block II/IIA GPS satellites as they go out of service due to failure or degradation in performance. Currently there are eight GPS IIR satellites on-orbit. These satellites have the new capability to autonomously self navigate using crosslink ranging measurements. This capability is referred to as Autonomous Navigation (AutoNav). AutoNav design includes non-interfering background operation capability. This on-orbit operational data is post processed on the ground for performance validation. This paper presents the performance analysis of the on-orbit data. The analysis of the on-orbit data indicates that the measured crosslink ranges have coarse errors and fine errors. The coarse errors are due to unintentional interference over various areas of the world. Fortunately, most coarse errors can be identified and edited out of further processing. The fine errors are potentially due to the inability to correctly estimate plasmaspheric propagation delays. The original operational concept to compensate for plasmaspheric delays is to use a dual-frequency technique. With this technique, crosslink ranges between AutoNav satellites are measured at two different frequencies (fa and fb). By comparing crosslink ranges at fa and fb, ranging errors caused by plasma delays can be estimated and removed. However, analysis of the on-orbit test data has indicated the ranging signals are experiencing other effects such as multipath due to structures on the satellite. These other effects result in fine periodic errors in the ranging residuals. This paper discusses the development and performance of three different techniques to remove the periodic errors in the range residual. The correction algorithms were implemented and the corrected measurements were of sufficient quality to be used for AutoNav processing. The corrected measurements were further processed by the AutoNav algorithms and were found to yield excellent results. The performance and relative merits of the three algorithms are also discussed.
Published in:	Proceedings of the 59th Annual Meeting of The Institute of Navigation and CIGTF 22nd Guidance Test Symposium (2003) June 23 - 25, 2003 Hyatt Regency Hotel Albuquerque, NM
Pages:	411 - 419
Cite this article:	Rajan, J.A., Orr, M., Wang, P., "On-Orbit Validation of GPS IIR Autonomous Navigation," Proceedings of the 59th Annual Meeting of The Institute of Navigation and CIGTF 22nd Guidance Test Symposium (2003), Albuquerque, NM, June 2003, pp. 411-419.
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