Protecting Against Unsampled Ionospheric Threats

Nominally, the mid-latitude ionosphere sampled by WAAS is smooth and slowly varying. However, during ionospheric storms, small-scale irregularities may form. These irregularities cause rapid changes in Total Electron Content (TEC) over comparatively short distances. WAAS has observed changes greater than 20 meters of vertical delay at the GPS L1 frequency over a few hundred kilometers. Such features are very difficult to model with the single frequency SBAS correction scheme that uses a 5x5 degree grid. Some of these features exhibit significant curvature and may even fit entirely within a single grid cell. Worse yet, some of these features may exist, but not be sampled by any of the ground-based WAAS reference Stations (WRSs). In this case, they would be invisible to the system. The WAAS ionospheric algorithms must protect users under all conditions. The protection bound broadcast to the user, termed the GIVE, must be sufficiently large to guard against such undetectable features. Therefore, it is essential to understand the magnitude of such threats given the gaps in our observations.