Ionospheric Estimation and Integrity Threat Detection

Andrew J. Hansen, Y.C. Chao, Todd Walter, Per Enge

Abstract:	A full realization of the Wide Area Augmentation Sys- tem (WAAS) is intended to provide aircraft guidance throughout the en route, terminal, non-precision and precision approach phases of flight. The most demand- ing phase is precision approach where vertical position- ing accuracy of ones of meters is necessary. Integrity requirements ensuring safety of life specify that any vertical position errors greater than the Vertical Pro- tection Limit be enunciated to the flight crew within six seconds. The ionosphere is the foremost impedi- ment to such a guarantee. Stanford, as a member of the National Satellite Test Bed (NSTB), is developing techniques for estimating the ionosphere in real-time to provide high accuracy position corrections. Paramount in the estimation pro- cess is the detection of ionospheric integrity threats. Modal decomposition of the ionosphere is the founda- tion of that process. Ionospheric modes which cannot be observed by the sparse WAAS network are integrity threats if they produce large vertical position errors at the aircraft. Likewise, observable modes may threaten integrity if they either cannot be conveyed to the aircraft or de- correlate too quickly in distance or time. We present a thought experiment for measuring the integrity of wide-area ionospheric corrections. This experiment suggests a theoretic lower bound on the ionospheric component of the vertical protection level the WAAS system can provide to the aircraft. The results of this analysis provide a point of departure for developing an integrity metric for both the WAAS system provider and aircraft on precision approach.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	883 - 889
Cite this article:	Hansen, Andrew J., Chao, Y.C., Walter, Todd, Enge, Per, "Ionospheric Estimation and Integrity Threat Detection," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 883-889.
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