A Preliminary Study of the Effect of Ionospheric Scintillation on WAAS User Availability in Equatorial Regions

Sam Pullen, Guttorm Opshaug, Andrew Hansen, Todd Walter, Per Enge, and Bradford Parkinson

Abstract:	While Wide Area Augmentation System (WAAS) user performance is expected meet aviation requirements under most conditions, infrequent ionospheric anomalies may result in significant degradation at certain times and locations. Scintillation, caused by local ionospheric plasma anomalies that intercept the path of GPS signals, can cause severe signal fades and phase vacillations that may lead to loss of signal lock or increased measurement errors. The degree to which scintillation will affect user availability is not well known, however, and detailed models of scintillation phenomena are limited by the sparse amount of data available. To evaluate the threat that scintillation may pose to WAAS users, existing scintillation models have been adapted to provide probability distributions for signal amplitude and phase variations. Samples from these distributions are fed into a simplified receiver simulation that outputs the probability of losing signal lock given a specified nominal signal-to-noise ratio. These results are integrated with an existing WAAS Service Volume Model (SVM) to assess the resulting user availability for an example WAAS network covering South America. The results indicate that in susceptible locations, moderate scintillation results in a high probability of loss-of-lock for low-elevation satellites, and strong scintillation does the same for higher-elevation satellites. The problem is particularly bad for codeless L2 tracking used by existing WAAS networks to generate ionosphere measurements. However, when compared to the case of large ionosphere delays without scintillation, precision approach availability is not much worse because these periods have higher ionosphere errors and thus signifi-cantly diminished availability relative to nominal ionosphere conditions. Non-precision applications of WAAS retain high availability even under scintillation. Better assessments of the effect of scintillation on WAAS will be possible once data from the upcoming solar peak in 2000-01 is delivered by the National Satellite Testbed (NSTB) and by Phase 1 WAAS reference stations.
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:	687 - 699
Cite this article:	Pullen, Sam, Opshaug, Guttorm, Hansen, Andrew, Walter, Todd, Enge, Per, Parkinson, Bradford, "A Preliminary Study of the Effect of Ionospheric Scintillation on WAAS User Availability in Equatorial Regions," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 687-699.
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