Burst Mode Message Loss Effects On WAAS Availability

Richard Fuller, Todd Walter and Per Enge

Abstract:	The Wide Area Augmentation System (WAAS) is a GPS- based navigation aid currently under development by the Federal Aviation Administration (FAA). WAAS will provide corrections to aviation users for the GPS clock, its ephemeris, and for the delay in its signal as it passes through the ionosphere. These corrections will be broadcast to users throughout the United States via geostationary satellites. A master station that combines data from a continental network of reference GPS receivers will create these messages. The geostationary satellites serve both as wide-area differential GPS data links as well as additional ranging sources. The data message stream of WAAS enhances the accuracy and integrity of the GPS signal for aviation. Simultaneously, the satellite ranging-source increases the percentage of time that the precise signal is available. In this way, WAAS provides needed improvements in four metrics over the standard GPS signal: accuracy, integrity, availability, and continuity. The WAAS Signal-In-Space (SIS) has a limited data message bandwidth of 250 bits-per-second. This data bandwidth was chosen to balance two concerns. First, the power of the signal must not be so strong as to jam GPS. Second, the signal must provide the minimum amount of information necessary to ensure adequate accuracy and integrity for aviation users over the entire geostationary satellite footprint. The required message loss is specified not to exceed a rate of 0.001 (one loss per one-thousand messages) to ensure adequate system continuity and availability. The WAAS message structure is not particularly sensitive to independent message losses below the specified rate. Groups of missed messages (burst-mode) can prove to be a challenge in maintaining a continuous WAAS solution. The effects of burst-mode losses on the quality of the WAAS solution is presented and a Markov model for the burst message loss is developed. This research shows that WAAS availability can be achieved even with a message loss rate up to of 0.005 (i.e., five times the specification) even in the presence of burst-mode outages. This proves that the specification will ensure availability despite having been derived for independent message loss as opposed to burst losses. Flight tests were conducted in California and Alaska to establish actual message loss profiles for aircraft. These flight test results were modeled and used in conjunction with NSTB reference station data to establish availability of WAAS solutions for various locations in the US. This research demonstrates that the GEO satellite fulfills its goals of providing accuracy, integrity, availability and continuity.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	230 - 241
Cite this article:	Fuller, Richard, Walter, Todd, Enge, Per, "Burst Mode Message Loss Effects On WAAS Availability," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 230-241.
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