Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsa11b..04w&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SA11B-04
Physics
2435 Ionospheric Disturbances, 2439 Ionospheric Irregularities
Scientific paper
In 2003, the FAA commissioned the Wide Area Augmentation System (WAAS). This new navigation system is capable of guiding aircraft to within a few hundred feet of the ground. Although the system accuracy is typically better than 2 m (95% in the vertical direction), there exist rare fault modes that can create errors more than ten times as large. Because these rare faults may be unobservable, the system availability is limited to times when they can be guaranteed to be small. The dominant source of these errors is the ionosphere. The WAAS ground segment consists of 25 dual-frequency reference stations that sample the ionosphere at discrete locations throughout the service volume. The users, however, only have access to a single frequency and cannot estimate the ionosphere directly. The ionospheric delay that they experience must be estimated from the ground station measurements. More importantly, the uncertainty from each estimate must be rigidly bounded as these corrections are part of a safety-of-life system. In addition, the users may be anywhere within the service volume; WAAS needs to protect all possible ray paths based on its discrete samples. The vast majority of the time Total Electron Content (TEC) ionospheric delays are smoothly varying functions of time and space. Given even just a few measurements, the surrounding ionosphere can be predicted accurately for tens of minutes. Unfortunately, this is not always the case. During the last solar peak, several major storms were observed that created highly localized disturbances in the ionosphere. These disturbances are not easily modeled by the low bit rate message format used to communicate to the users. Even worse, they may not be sampled at all by the ground stations. Thus, during such storms users may suffer very large correction errors. WAAS therefore includes three protection factors against this threat: a storm detector, and spatial and temporal threat models. The storm detector is an internal consistency check to determine if the observations are consistent with nominal (quiet) or disturbed ionosphere. The spatial, or undersampled, threat model characterizes the magnitude of disturbances that can escape detection given current sampling, while the temporal threat model characterizes the amount it may change over the lifetime of the correction. This paper will describe most significant ionospheric TEC disturbances observed by the WAAS network. It will further explain why these features are so problematic to navigation and what is being done to reduce their impact.
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