Dst as an Indicator of Potential Threats to WAAS Integrity and Availability (2005)

The Federal Aviation Administration’s Wide Area Augmentation System (WAAS) provides high integrity GPS-based precision navigation service to users in the conterminous United States (CONUS). User integrity is ensured by bounding all possible GPS error sources, of which the ionospheric delay is the largest and most variable. The uncertainty in delay is characterized by the WAAS ionospheric threat model, which is based on a set of data recorded by the WAAS network since 2000. These days are chosen for their reported ionospheric activity, and WAAS network data, known as “supertruth,” has been produced post-process for those days, for further study and incorporation into the threat model. Nineteen of these days end up contributing to the threat model. As a result, a question of interest arises: to what degree do these data sets include a representative range of ionospheric disturbances when viewed over a longer time frame? A current method for choosing these days, based on observed magnetosphere-ionosphere coupling, is to examine the planetary index of geomagnetic activity Kp since 2000 for the days on which Kp reaches the higher range of its possible values. However, previous work showed that basing predictions of loss of availability due to unboundable ionospheric anomalies on the Kp index yields a higher false alarm rate for WAAS than does the geomagnetic index Dst, which measures the equatorial ring currents during storm time. This paper is a continuing exploration of how Dst performs as an indicator of WAAS-effective ionospheric activity, followed by an effort to compare the years of WAAS operation to a longer, multi-solar-cycle period. We examine the first part of this question by testing whether consideration of timing of Dst plays a role in determining whether a geomagnetic storm is WAASeffective or not. We use the coverage metric, when available, as a proxy for WAAS-effective disturbances. Coverage is measured as a daily percentage of CONUS for which precision navigation (LPV) service is available for a specified minimum level of time. A drop in LPV coverage occurs when LPV service is unavailable somewhere in CONUS. LPV service is made unavailable when disturbances in the ionosphere preclude the WAAS estimates of the ionosphere from being bounded sufficiently tightly for precision approach. For days on which there is no reported coverage number, if supertruth data has already been generated, we implement the Irregularity Detector through post-processing, and use its output to infer an effective coverage value. In total, coverage values exist or are inferable for 1079 non-contiguous days since 2000. We find that minimum daily Dst correlates more strongly with anomalous ionospheric behavior over CONUS than the hour at which Dst reaches its minimum or the rate of change of Dst. We then compare Dst from January 2000 through April 2005 to Dst records dating back to 1957. Over these several solar cycles we find that the period during which WAAS has been operational has had more than the average amount of geomagnetic activity. Although more extreme storms are undoubtedly possible, we find the metric of Dst provides a plausible argument that the WAAS threat model is representative of behavior that may be encountered in a solar cycle.