Ionospheric Estimation and Integrity Threat Detection

A full realization of the Wide Area Augmentation System (WAAS) is intended to provide aircraft guidance throughout the en route, terminal, non-precision and precision approach phases of ight. The most demanding phase is precision approach where vertical positioning accuracy of ones of meters is necessary. Integrity Supported by FAA Grant 95-6-005. requirements ensuring safety of life specify that any vertical position errors greater than the Vertical Protection Limit be enunciated to the ight crew within six seconds. The ionosphere is the foremost impediment to such a guarantee. Stanford, as a member of the National Satellite Test Bed (NSTB), is developing techniques for estimating the ionosphere in real-time to provide high accuracy position corrections. Paramount in the estimation process is the detection of ionospheric integrity threats. Modal decomposition of the ionosphere is the foundation of that process. Ionospheric modes which cannot be observed by the sparse WAAS network are integrity threats if they produce large vertical position errors at the aircraft. Likewise, observable modes may threaten integrity if they either cannot be conveyed to the aircraft or decorrelate too quickly in distance or time. We present a thought experiment for measuring the integrity of wide-area ionospheric corrections. This experiment suggests a theoretic lower bound on the ionospheric component of the vertical protection level the WAAS system can provide to the aircraft. The results of this analysis provide a point of departure for developing an integrity metric for both the WAAS system provider and aircraft on precision approach.