GPS SPS Performance in 2012 – How Good Was It?

B. Renfro, J. Kammerman, M. Bratton, D. Munton

Abstract:	The 2008 Standard Positioning Service (SPS) Performance Standard (SPS PS) [1] provides the U.S. government’s assertions regarding the expected performance of GPS. The SPS PS defines SPS as that service delivered through the L1 C/A code signal; the only GNSS civil signal for which such a specification exists. As such, the SPS PS is relevant to developing GNSS signals because it may serve as a model for any future performance specifications for other GNSS signals. Therefore, it is important to test the SPS PS assertions and verify both the usefulness of the metrics and the quality of service being delivered. We will characterize the performance of GPS for 2012 in terms of the SPS PS and compare this performance with that of recent years. Our analysis addresses metrics that are stated in terms of performance to be expected by a single-frequency user, and our emphasis will be on those assertions which can be verified by anyone with knowledge of standard GPS data analysis practices, familiarity with the relevant signal specification, and access to a data archive. The observation data used in the analysis consists of observation data collected from a subset of the IGS network [2,3] and data collected from the National Geospatial-Intelligence Agency (NGA) GPS monitor station network [4]. To determine satellite positions we use both an archive of broadcast ephemeris data, and the NGA precise ephemeris product. The analysis tools used are either part of, or built from, the open-source GPS Toolkit (GPSTk) project [5]. The SPSPS assertions evaluated will include those associated with the accuracy, continuity, availability of the GPS signal-in-space (SIS), and the position performance standards (through the position dilution of precision). We will describe the various metrics and the conditions that apply to them, our method of analysis, and the overall results of the analysis. We will provide examples of some of the intricacies involved in evaluating the SPS PS metrics. Our analysis will confirm that the GPS met the SPSPS performance metrics during 2012. In past years we have seen brief outages, not covered by Notice Advisory to Navstar Users, across multiple stations. We will compare this past behavior with observations in 2012, and discuss why this behavior is not a violation of the SPS PS. Finally, we will briefly describe how the SPSPS might be extended to the newer GPS civil signals and other GNSS signals. [1] U.S. Department of Defense Standard Positioning Service Performance Standard, 4th Edition [Report]. - 2008. [2] Beutler G. [et al.] The International GPS Service (IGS): An Interdisciplinary Service in Support of Earth Science [Journal], Advances in Space Research. - 1999. - 4 : Vol. Vol 23. - pp. 631-635. [3] “The International GNSS Service (IGS) in a Changing Landscape of Global Navigation Satellite Systems", Journal of Geodesy, Vol 83 (3-4), March 2009 [4] Renfro B., Munton D. and Mach R. Around the World for 26 Years - A Brief History of the NGA Monitor Station Network [Conference], ION International Technical Meeting. - Newport Beach, CA : Institute of Navigation, 2012. [5] Harris, R. B., and R. G. Mach, The GPSTk: an open source GPS toolkit, GPS Solutions 2007 11.2 145-150.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	959 - 971
Cite this article:	Renfro, B., Kammerman, J., Bratton, M., Munton, D., "GPS SPS Performance in 2012 – How Good Was It?," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 959-971.
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