WAAS Performance Evaluation under Increased Ionospheric Activity

R. Yousuf and S. Skone

Abstract:	One of the greatest challenges in developing accurate and reliable satellite-based augmentation systems (SBAS) is modeling of ionospheric effects. Wide area GPS networks are generally sparse (station spacings of 500-1000 km), and ionosphere models can suffer degraded performance in regions where large spatial gradients in total electron content (TEC) exist. Of particular concern for WAAS users is the ionospheric feature called storm enhanced density (SED). In order to assess the WAAS performance – including both the WAAS positioning accuracies and errors in the WAAS ionosphere models - the University of Calgary operates the Canadian GPS Network for Ionosphere Monitoring (CANGIM). This network consists of a number of sites in Canada, augmented by several sites in the northern United States, which collect WAAS geostationary downlink messages. These messages are collected continuously and decoded (post-mission) at University of Calgary. Localization schemes have been developed to compute WAAS corrections for any location in North America. In this paper, issues associated with obtaining optimal WAAS accuracies under increased ionospheric conditions are investigated. An extensive analysis of WAAS positioning accuracies is conducted for North America during the intense ionospheric storm events, using data from both IGS and CORS reference stations with localized WAAS corrections. By estimating WAAS position errors at hundreds of independent reference stations across the United States and Canada, the spatial variations in positioning accuracies are determined and correlated with development of local ionospheric effects. It is observed that the sparse WAAS reference network may not adequately model local ionospheric effects during increased ionospheric activity. Positioning accuracies are degraded by a factor of ten in regions extending several thousand kilometers. Such position errors can persist for several hours. Analysis of future system enhancements, such as the CWAAS, are investigated to improve positioning accuracies and service availability.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2316 - 2327
Cite this article:	Yousuf, R., Skone, S., "WAAS Performance Evaluation under Increased Ionospheric Activity," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2316-2327.
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