The Precise Positioning Service (PPS) Performance Standard (PS)

Karl Kovach, Jason Taylor, Andrew Elliott, and David Steare

Abstract:	The Navstar Global Positioning System (GPS) Precise Positioning Service (PPS) Performance Standard (PS) was published on 23 February 2007. The PPS PS represents a major milestone in the evolution of GPS. It is the first public disclosure of the PPS performance characteristics. It allows the PPS to take its place alongside the Standard Positioning Service (SPS) as the only two fully operational Global Navigation Satellite System (GNSS) services whose performance is backed by a documented commitment from the service provider. The PPS PS differs significantly from the SPS PS published in 2001. Besides the obvious PPS-versus-SPS differences (e.g., PPS is dual-frequency) and the accumulated GPS improvements which have occurred in the six years since the SPS PS was published (e.g., the expandable 24-slot constellation), the focus of the PPS PS has shifted towards directly specifying the performance of the broadcast signal-in-space (SIS) which all PPS user equipment (UE) “plug into” rather than indirectly specifying performance as the output of an idealized UE. The focus on the UE-independent SIS performance allows the PPS PS to address the fundamental characteristics and limitations of the system which are within the purview of the GPS Space and Control Segments (i.e., those aspects of GPS performance the U.S. Government, as the service provider, has responsibility for). To explicitly provide a basis for certification of PPS avionics, the PPS SIS performance is specified in aviation-centric parameters of coverage, accuracy, integrity, continuity, and availability. This paper provides important background information about the PPS PS. It explains the rationale behind the PPS PS and places it into the context of a necessary basis for safety-of-flight certification. This paper also describes some of the technical analyses which went into the development and validation of the numeric values in the PPS PS -- many of the performance standards in the PPS PS were actually derived from other GPS specifications that exist in different forms elsewhere. For instance, the first and second time derivatives of the SIS pseudorange accuracy (URRE or pseudorange velocity accuracy, and URRRE or pseudorange acceleration accuracy) come from the Allan variance for the on-orbit satellite clocks. The framework for translating PPS SIS performance standards into UE-dependent position, velocity, and time (PVT) terms is discussed along with some interesting examples which demonstrate that the old adage “beauty is in the eye of the beholder” can certainly apply to GPS performance as viewed by different classes of UE. The subject of “operating margin” is addressed at length in this paper. The SPS PS has been widely criticized as having excessive operating margin. Many SPS users report that the SPS performance observed in day-to-day operation is significantly better than that committed to in the SPS PS -- some reports suggest that actual performance may be better by a factor of ten or more. The PPS PS is sure to be similarly criticized, though perhaps not quite as severely. For example, there is a roughly 50% reduction of the operating margin in the performance standard for SIS pseudorange accuracy in the PPS PS as compared to the SPS PS (e.g., 5.9 m 95% in the PPS PS versus 11.8 m 95% in the SPS PS). The PPS PS is serving as a template for a near-future update of the SPS PS. This paper should thus be of keen interest to all GPS UE manufacturers and users, both PPS and SPS alike. The PPS PS defines the performance levels available from today’s PPS, but it is really a living document. As each new GPS SIS or significant system upgrade approaches its initial operational capability (IOC), the PPS PS will be updated to reflect the improved performance standards that manufacturers and users will be able to rely on. The standards established in the PPS PS will undoubtedly influence the future PSs for GPS augmentation services and the PSs for other GNSS services. Just like GPS UE manufacturers need to know the performance they can count on receiving from the input GPS SIS in order to compute a realistic estimate of the output performance their GPS UE will provide to users, GPS augmentation service designers and operators need to know the performance they can count on from the GPS SIS so they can augment it in some reasonably optimal fashion (e.g., keeping up with the SIS’s pseudorange velocity accuracy can use up a lot, or a little, of the bandwidth in a differential GPS system). The explicit and testable performance standards in the PPS PS set a high standard that other GNSS service providers will be compared against.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2529 - 2540
Cite this article:	Kovach, Karl, Taylor, Jason, Elliott, Andrew, Steare, David, "The Precise Positioning Service (PPS) Performance Standard (PS)," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2529-2540.
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