Providing guidance and navigation to a landing aircraft is one of the many challenging uses of the Global Positioning System (GPS) because both high accuracy and user safety are required during the procedure. The largest and most unpredictable source of GPS positioning error for a single frequency user is the ionosphere. The goal of a differential-GPS-based Augmentation System (AS) in mitigating ionospheric errors is two-fold. First, the AS broadcasts ionospheric error corrections to its users for improved positioning accuracy. Moreover, the system provides a service that GPS alone does not; by broadcasting confidence bounds on the corrections, the AS ensures user position estimate integrity, which is crucial to safety-of-life applications. Although its reference stations are not in general collocated with the user’s GPS receiver, the AS must protect users from possible ionosphere density variations occurring between its measurements and the user’s. I present observations of an ionosphere disturbance as a case study of the scenarios of interest for Augmentation Systems. The disturbance was remarkable for its apparent extreme localization of about 700 km across, its persistence over one geographic region for several hours, and its magnitude compared to the surrounding areas. This extremely localized region of high total electron content (TEC) was observed over the southeastern U.S. at local evening and nighttime during one of the most severe geomagnetic storms of the current solar cycle. I develop a regional three-dimensional electron density model that is horizontally piecewise continuous by identifying the latitudinal and longitudinal regions of continuity based on TEC maps from the dual-frequency ground-based CORS and IGS network data in the United States from the October and November 2003 storms. I assign and test various vertical density profiles to each of the horizontal regions. Since the observability of vertical density variation is limited with ground-based data only, I use TEC measurements made by space-borne GPS receivers onboard the SAC-C and GRACE satellites and the JASON satellite dual-frequency altimeter as they pass through the region of anomalous ionosphere to help constrain the altitudinal variation of the electron densities. This and similar studies serve as evidence that the ionosphere threat models developed for the Federal Aviation Administration’s Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS) are sufficiently conservative to maintain user integrity even under such extreme storms.

4 May, Friday RRB–227, 10 am