Effects of local ionospheric anomalies on navigation performance and integrity using WAAS

Ionospheric delay is probably the most serious error source for the Wide Area Augmentation System (WAAS). Recent research demonstrates that under normal conditions, user ionosphere ranging errors are acceptable. However, local anomalies within the coarse surface fit provided by the WAAS ionospheric grid may pose a significant threat to user integrity. To study this problem, covariance simulation is used along with a search for the parameters of bubbles that give the “worst-case” ionospheric correction error for particular WAAS networks. These results indicate which classes of local ionospheric anomalies would provide the greatest threat to WAAS user integrity. Ionosphere observations collected by the Stanford WAAS network are being used to search for the presence of local anomalies behavior along these lines. As more data is collected, a better model of anomaly characteristics and probabilities can be constructed. These results will help determine the magnitude of this threat to user navigation and also evaluate risk-mitigation effects