Improvements to the WAAS ionospheric algorithms

Eric Altshuler, Daniel Cormier, and Helena Go

Abstract:	The Wide Area Augmentation System (WAAS) is a safety-critical, software-intensive system, augmenting the satellite-based Global Positioning System (GPS) to provide users with airborne positions of adequate accuracy, availability, and integrity during different phases of flight. Under the "Free Flight" concept of the National Airspace System (NAS) adopted by the Federal Aviation Administration (FAA) at the turn of the century, the GPS/WAAS infrastructure is assuming a critical role in ensuring the safe and efficient flight operating capability of the NAS. A primary feature of WAAS is that it provides corrections to the user which account for the delays of the GPS satellite signals as they pass through the ionosphere. These corrections are determined through dual-frequency differential delays processed at the WAAS Reference Stations under the assumption of a thin shell model of the ionosphere. WAAS uses these delays to provide the user with delay values and error bounds interpolated via a least-squares planar fit to a fixed grid of locations throughout the WAAS service volume. The ionosphere delays are modeled locally as a flat plane in the East and North directions. Grid Iono Vertical Errors (GIVEs) and ionospheric delays are calculated at Iono Grid Points (IGPs) which make up a virtual grid 350 km above the surface of the earth. These GIVEs are used to estimate the overbound of a user error due to the ionosphere. The GIVEs must account for the deviations from this planar model that the user might experience when the IGP containing that GIVE is to used in the calculation of the user’s Vertical Protection Limit (VPL). During nominal conditions, the ionosphere is described very well by the planar model, and a constant decorrelation term is sufficient to describe deviations from planarity. In this study we examine ways to improve this process by allowing for more accurate error estimation. The benefits are twofold. First, under nominal conditions producing more accurate error estimates will allow GIVEs to be reduced. This has a corresponding effect on user VPLs to allow service levels below the present 50 meter Vertical Alarm Limit (VAL). Second, more accurate error estimates enhance integrity, adding extra assurance that error estimates sufficiently bound actual user errors under all conditions. We focus on three terms in the GIVE equation which combined represent spatial ionospheric threats to the user. We show that for each of these terms, minor modifications to the current WAAS algorithms can significantly reduce GIVEs for a typical user and allow the user to achieve a 20 meter VPL. Finally, we show that the current system is relatively insensitive to the number of IPPs seen by the system. For example, currently in regions which have an IPP density of about 20 IPPs per 1000 km radius, the GIVEs are approximately 3.7 meters, whereas in regions which have about 50 IPPs per 1000 km radius, the GIVEs are approximately 3.2 meters. This is an improvement of only a single index level as the IPP density varies by a factor of two and a half. This implies that the addition of new stations without further algorithm enhancements may not be sufficient to achieve service levels beyond the current 50 meter VAL.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	2256 - 2261
Cite this article:	Altshuler, Eric, Cormier, Daniel, Go, Helena, "Improvements to the WAAS ionospheric algorithms," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 2256-2261.
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