Analytical and Experimental Observations of Ionospheric and Tropospheric Decorrelation Effects for Differential Satellite Navigation during Precision Approach

Jock R. I. Christie, Ping-Ya Ko§, Boris S. Pervan, Per K. Enge, J. David Powell, Bradford W. Parkinson, Chung Shan

Abstract:	The introduction of the Global Positioning System (GPS) ushered in a new era of affordable precise navigation. Differential GPS (DGPS) corrections have reduced positioning errors from about 100 meters to roughly 1 meter, which has led to their proposed use in aircraft precision approach. However, the accuracy of differential corrections is somewhat limited by spatial and temporal decorrelation of the atmosphere. Current requirements for precision landing, which limit integrity risk to one undetected navigation hazard in a billion trials, suggest that we must fully characterize ionospheric and tropospheric decorrelation. This paper will present both theoretical and experimental results in this regard. Currently assumed values of spatial decorrelation for the ionosphere and troposphere are about 2 and 1 mm/km respectively. This suggests an error of only 2 centimeters error due to the ionosphere, and 1 centimeter error due to the tropo-sphere at 10 kilometers from the DGPS reference station. However, these are simply average values based on relatively smooth models of the ionosphere and troposphere. These val-ues seem overly optimistic when compared to a documented case of ionospheric decorrelation that resulted in a 50 centime-ter difference between ionospheric estimates over a 9 kilometer baseline. Models of the troposphere were also analyzed to exam-ine the variability due to changing surface conditions. Archived inflight data from a prototype Local Area Augmenta-tion System (LAAS) architecture was post processed using several different tropospheric models. Carrier phase residuals were calculated at each epoch as a measure of model consis-tency. The actual displacement due to a change in model was examined to determine the sensitivity to ground measurements of temperature, pressure and humidity. The archived LAAS flight data was input to a Carrier Smoothed Code algorithm, along with an assumed ionospheric gradient. The impact of filter time constants on pseudorange error was calculated. This paper seeks to obtain statistical bounds for the tro-pospheric and ionospheric decorrelation effects on a LAAS architecture.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	739 - 747
Cite this article:	Christie, Jock R. I., Ko§, Ping-Ya, Pervan, Boris S., Enge, Per K., Powell, J. David, Parkinson, Bradford W., Shan, Chung, "Analytical and Experimental Observations of Ionospheric and Tropospheric Decorrelation Effects for Differential Satellite Navigation during Precision Approach," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 739-747.
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