Performance Evaluation of the Wide Area Augmentation System for Ionospheric Storm Events

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.