Journal of Global Positioning Systems (2004) Vol. 3, No. 1-2: 251-258 Performance Evaluation of the Wide Area Augmentation System for Ionospheric Storm Events S. Skone, R. Yousuf and A. Coster Department of Geomatics Engineering, University of Calgary, 2500 University Dr. N.W., T2N 1N4 Calgary, Canada e-mail: sskone@geomatics.ucalgary.ca Tel: + 01-403-220-7589; Fax: +01-403-284-1980 Received: 15 Nov 2004 / Accepted: 3 Feb 2005 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 Wide Area Augmentation System (WAAS) users is the feature called storm enhanced density, which is associated with large TEC gradients at mid-latitudes. This effect is a significant source of error in the WAAS correction models. The Canadian GPS Network for Ionosphere Monitoring (CANGIM) consists of three GPS reference stations in western Canada, augmented by two additional sites in the northern United States. In addition to measures of ionospheric activity, WAAS messages are collected continuously at these sites and decoded (post-mission) at University of Calgary. Localization schemes have been developed to compute WAAS ionosphere corrections for any location in North America. In this paper, performance of the broadcast WAAS ionosphere model is quantified through comparison with truth data from over 400 GPS reference stations in North America. WAAS ionosphere model accuracies throughout North America are evaluated for intense storm events, and compared with WAAS Grid Ionosphere Vertical Error (GIVE) bounds. Limitations in the WAAS ionosphere model are identified for enhanced ionospheric activity and, in particular, the storm enhanced density phenomenon. Key words: Ionosphere, WADGPS, WAAS, GPS, positioning, geomagnetic storm 1 Background 1.1 Storm enhanced density Storm enhanced density (SED) was originally recognized in the early 1990’s with the Millstone incoherent scatter (IS) radar (Foster et al., 2002; Foster and Vo, 2002) and has been studied in detail with satellite data from the DMSP and IMAGE satellites, and with TEC data collected from multiple GPS receivers located across the US and Canada (Coster et al., 2003a; Coster et al., 2003b). Analysis of the GPS TEC data shows that during geomagnetic disturbances, ionospheric plasma is transported from lower latitudes to higher latitudes, redistributing plasma across latitude and local time. Fig. 1 An example of storm enhanced density over North America during a geomagnetic storm event March 31, 2001