Ionospheric Threat Model Methodology for WAAS

The ionosphere is the largest remaining error source affecting GPS. It also has some of the least predictable spatial and temporal variations. As such the ionosphere becomes the determining factor in system performance for WAAS. Because the ionosphere cannot be observed at all places simultaneously, the confidence bound, termed the Grid Ionospheric Vertical Error (GIVE), can only be determined with the aid of a threat model. The threat model is used to restrict the expected ionospheric behavior. It must not be too conservative or the resulting GIVEs will be too large and system availability will suffer. However, it must safely bound true ionospheric behavior in order to provide integrity. We therefore require a method that will accurately describe the limits without being overly pessimistic. Since the underlying physical processes driving the ionosphere are not entirely known, a decision has been made to base the threat model on reliable physical observation. There has been a long history of ionospheric observation dating back many decades. More recently, the data from the WAAS reference stations has been collected and processed to form some of the lowest noise and densest observations to date. The so-called “supertruth” data sets provide some of the most detailed observations of the ionosphere and therefore provide much of the basis for the determination of the threat model. This paper describes a methodology for using real data to generate worst case scenarios from which an appropriate threat model may be determined. This threat model must be coupled to a set of metrics that can distinguish wellobserved ionospheric regions.