LAAS Study of Slow-Moving Ionosphere Anomalies and Their Potential Impacts

Triggered by several severe ionosphere storms that have occurred in recent years, research has been done to studying those anomalies, the physics behind them, and their potential impact on augmented GNSS users. In previous work [1-5], it was found that such ionosphere anomalies can threaten LAAS users under extreme conditions. To determine this, a spatial-gradient “threat model” was established based on ionosphere storm data observed from WAAS and IGS since 2000. Maximum differential user vertical errors were estimated based on this threat model. Although LGF monitors can detect “moving fronts”, so-called “stationary fronts” remain threatening since the LGF may never be able to observe it (e.g., if the ionosphere front stops moving at the worst possible location prior to reaching the LGF). In order to validate the threat model, a comprehensive methodology was developed to analyze WAAS “supertruth” data as well as both raw and JPLprocessed data from the IGS/CORS receiver network to search for anomalous gradients [15]. Anomalous gradients that result from this method were used to populate and validate the LAAS ionosphere spatial gradient "threat model". These data studies show that most of the ionosphere anomalies seem to move reasonably fast relative to the speed of an approaching aircraft. The few data points thought to be stationary were impossible to validate after a thorough investigation. Additional data analysis has been performed to better determine the credibility of the slow-moving segment of the ionosphere spatial anomaly threat space. In this paper, data from the Ohio/Michigan cluster of CORS stations on November 20, 2003 and from the Florida region on October 31, 2003 (UTC) are searched for slow-moving ionosphere events. One data point that stood out was verified by observation at various locations using both the dual-frequency JPL data as well as L1 code-minus-carrier estimation. A threat analysis follows to show the potential impact of this observed threat under various GPS constellation states. A sensitivity study is conducted to show how the impact relies on the upper bound of the slow-moving threat model. A “data replay” analysis is also performed to show the actual LAAS errors that would have occurred at one pair of stations in Florida. Finally, a recommendation is made in for revising the upper bound of the slow-moving threat space. 233 ION GNSS 18th International Technical Meeting of the Satellite Division, 13-16 September 2005, Long Beach, CA 1.0 INTRODUCTION The ionosphere is a dispersive medium located in the region of the upper atmosphere between about 50 km to about 1000 km above the Earth [6]. The radiation of the Sun produces free electrons and ions that cause phase advance and group delay in radio waves. The state of the ionosphere is a function of the intensity of solar activity, magnetic latitude, local time, and other factors. As GPS signals traverse the ionosphere, they are delayed by an amount proportional to the Total Electron Content (TEC) within the ionosphere at a given time. Because the ionosphere is constantly changing, the error introduced by the ionosphere into the GPS signal is highly variable and is difficult to model at the level of precision needed for LAAS. However, under nominal conditions, the spatial gradient is in the range of 2 − 5 mm/km (1σ); thus typical LAAS user errors are small (less than 10 cm, 1σ). The possibility of extremely large ionosphere spatial gradients was originally discovered in the study of WAAS “supertruth” (post-processed, bias-corrected) data during ionosphere storm events at the time of the last solar maximum (2000 − 2001). It was estimated that an ionosphere storm on 6 April 2000 resulted in a 7 m differential delay over the IPP separation of 19 km. This translates into an ionosphere delay rate of change of approximately 316 mm/km, which is two orders of magnitude higher than the typical one-sigma ionosphere vertical gradient value identified previously. Since a Gaussian extrapolation of the 5 mm/km one-sigma number planned to be broadcast by the LAAS Ground Facility (LGF) does not come close to overbounding this extreme gradient, and because it is impractical to dramatically increase the broadcast one-sigma number without losing all system availability, we must treat this event as an anomaly and detect and exclude cases of it that lead to hazardous user errors. The detailed study on the 6 April 2000 storm can be found in [1]. Several ionosphere storms of concern have occurred since the April 2000 storm. Among them, the two largest ones were on October 29-31, 2003 and November 20, 2003. Figure 1 shows a snapshot of the ionosphere delay map over CONUS on October 29, 2003 between 20:00 to 20:45 in UTC time. The subplots are “snapshots” taken 15 minutes apart. The x-axis and y-axis represent longitude and latitude, respectively. The color scale indicates the magnitude of the vertical ionosphere delay.