Improving WAAS Availability Along the Coast of California

Lawrence Sparks, Eric Altshuler

Abstract: The Wide Area Augmentation System (WAAS) broadcasts information that allows users to bound the positioning error that arises from various sources, including, in particular, the ionospheric delay of signals emitted by Global Positioning System satellites. Loss of WAAS availability occurs locally when a WAAS user computes a horizontal or vertical protection limit that exceeds a specified maximum associated with a given navigation mode and level of service. Along the coast of California, availability is routinely found to be less than 100%, even under nominal ionospheric conditions. The largest contribution to the user’s computed protection limit generally comes from the ionospheric threat model that protects the user from the adverse influence of delay estimation error due to undersampled ionospheric irregularities. This paper describes a methodology for improving WAAS availability over coastal regions, in particular California and Alaska, by making the ionospheric threat model dependent upon the overall level of ionospheric disturbance. To determine the local state of the ionosphere, we propose to introduce into WAAS a moderate storm detector. Threats that cause the moderate storm detector to trip are eliminated from the quiet-time branch of the ionospheric threat model. Using this quiet-time branch under nominal ionospheric conditions significantly reduces the magnitude of the users’ computed protection limits, thereby dramatically improving WAAS availability. An analysis of WAAS data over the past three years indicates that the fraction of time in which the quiet-time threat model branch could have been used throughout the WAAS coverage region exceeds 99.94%. Adopting this branch substantially increases availability at the edges of coverage – the difference in availability achieved by implementing the moderate storm detector is found, at some ionospheric grid points, to be as much as 75%. The WAAS Integrity and Performance Panel has recommended to the Federal Aviation Administration (FAA) that this proposal be implemented in WAAS.
Published in: Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014)
September 8 - 12, 2014
Tampa Convention Center
Tampa, Florida
Pages: 3299 - 3311
Cite this article: Sparks, Lawrence, Altshuler, Eric, "Improving WAAS Availability Along the Coast of California," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 3299-3311.
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