The Effects of Local Ionospheric Decorrelation on LAAS: Theory and Experimental Results

Jock R. I. Christie, Ping-Ya Ko, Andrew Hansen, Donghai Dai, Samuel Pullen, Boris S. Pervan, Bradford W. Parkinson

Abstract:	Solar cycle 23 is expected to reach a maximum in the year 2000. This has produced a recent flurry of research about the possible adverse effects of the ionosphere on the millions of GPS users. The introduction of the Global Positioning Sys-tem (GPS) ushered in a new era of affordable precise navigation. Differential GPS (DGPS) corrections have reduced positioning errors from about 100 meters to roughly meter, which has led to their proposed use in aircraft precision approach. However, the accuracy of differential corrections somewhat limited by spatial and temporal decorrelation of the atmosphere. Much of the recent increase in GPS usage, as well as the development of precision landing requirements were developed near solar minimum. Although consideration was made for increased levels of sunspot and ionospheric activity, it is prudent to better characterize ionospheric decorrelation. This paper will present both theoretical and experimental results in this regard. Currently assumed value of spatial decorrelation for the ionosphere is about 2 mm/km, which is equivalent to an error of only 3 centimeters error due to the ionosphere at 15 kilome-ters from the DGPS reference station. However, this value based on relatively smooth models of the ionosphere. This value seems overly optimistic when compared to a documented ionospheric gradient of 55 mm/km, which is certainly too conservative. Much of the development of the prototype landing sys-tems has occurred in the last three or four years, close to the solar minimum. Current requirements for precision landing, which limit integrity risk to one undetected navigation hazard in a billion trials, suggest that we must fully characterize iono-spheric decorrelation. This paper seeks to obtain statistical bounds for the ionospheric decorrelation effects on a LAAS architecture. Archived data from the National Satellite Test Bed (NSTB = WAAS Prototype) was analyzed to characterize the spatial gradients. The impact of filter time constants on pseu-dorange error was calculated. This delay artifact of the CSC architecture was examined to ensure integrity.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	769 - 777
Cite this article:	Christie, Jock R. I., Ko, Ping-Ya, Hansen, Andrew, Dai, Donghai, Pullen, Samuel, Pervan, Boris S., Parkinson, Bradford W., "The Effects of Local Ionospheric Decorrelation on LAAS: Theory and Experimental Results," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 769-777.
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