Verification of Low Latitude Ionosphere Effects on WAAS During October 2003 Geomagnetic Storm

The Federal Aviation Administration’s Wide Area Augmentation System (WAAS) provides high integrity GPS-based precision navigation service to users in the conterminous United States. In October 2003, only four months after WAAS was commissioned, a series of coronal mass ejections from the sun triggered one of the strongest geomagnetic and ionospheric storms of the solar cycle. WAAS user integrity was maintained during this entire time by reducing precision navigation service availability. Toward the end of this storm period a localized region of enhanced TEC that persisted for several hours was observed over Florida. GPS measurements of TEC made from the WAAS sites near the Gulf of Mexico indicate that the high density region was as much as 60 TECU (about 10 m delay at L1) higher than the surrounding ionosphere. The feature was observed in the Florida region from local evening on October 30, 2003, through midnight on the 31st. It is conjectured that this feature is not in reality so localized as to have the spatial area of roughly Florida, but may be part of a larger ionospheric structure in that longitude sector, spanning many degrees of latitude south of the sampling area of the WAAS GPS reference network. It is also thought to be the case that this larger structure may exist due to the effects of the geomagnetic storm in the South Atlantic Anomaly. We examine this hypothesis by augmenting the WAAS network with GPS dual-frequency ionospheric data over a broader geographic region including Central America and the Caribbean. These data include a network of twelve sites situated in the Caribbean islands, a network of sixteen International GPS Service (IGS) sites in Mexico and Central America, and the Continuously Operating Reference Stations (CORS) in the United States. The measurements extending south toward the geomagnetic equator allow us to trace the feature back in time to come to a better understanding of the storm-time dynamics. We show that the feature observed with the WAAS network is indeed part of a larger previously existing ionospheric structure. In addition we use JASON sea surface satellite data to demonstrate that, while other studies have shown TEC uplift above 400 km, the feature does not appear to have been lifted up beyond 1300 km altitude. The apparent extreme localization of this ionospheric anomaly poses a challenge to WAAS integrity maintenance. The WAAS reference GPS receiver network of twenty-five stations is spread over the entire U.S. including Alaska and Hawaii. Although sparse sampling is always accounted for in the error bounds that determine WAAS user integrity, prior to the October 2003 storm, a feature so localized and dense had not been observed. Such an observation may imply that undersampling error bounds be revised upward, reducing precision navigation availability even during nominal conditions unless the storm behavior is well enough understood to demonstrate with high confidence that such a localized region of high density could not exist without the detection of a storm in real-time by WAAS.