Ionosphere Threat to LAAS: Updated Model, User Impact, and Mitigations

Several severe ionosphere storms have occurred in recent years that tripped the WAAS storm detector and caused partial WAAS service shutdowns. Under extreme conditions, these spatial gradients can threaten LAAS. In previous work [1-4], a “linear spatial gradient front” model was established and a threat space was extrapolated based on data from the 6 April 2000 ionospheric storm. User vertical error was estimated based on this threat model. The mitigating impact of LAAS Ground Facility (LGF) and airborne monitoring was also analyzed. Although those monitors can detect a “moving front” scenario, the so-called “stationary front” scenario remains 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.) It was shown that a ground-based Long Baseline Monitor (LBM) is able to mitigate such a threat if the baseline is set appropriately [5]. However, the cost and complexity of LBM deployment would be severe. Although the worst-case ionosphere anomaly poses a serious concern, it is unclear what the prior probability of these extreme events may be, how credible the boundary of threat space is, and to what extent the threat model captures possible ionosphere storm behavior. In order to answer those questions, additional data analysis has been performed to better determine the credibility of the ionosphere spatial anomaly threat space. Recent CONUS ionospheric storms (using WAAS and JPL IGS/CORS data during October and November 2003) were studied thoroughly. The ionosphere threat model has been modified based on this new data. Instead of being extrapolated from a single observed anomaly as the previous model, the revised threat space is populated with many more observed data points. In this paper, the threat of ionosphere spatial anomaly to LAAS is analyzed based on the revised model. The worst-case user vertical error and tolerable threat space are simulated with LGF and LGF-plus-airborne monitoring under various satellite constellations. The effectiveness of airborne monitoring is examined. When monitors are not sufficient to mitigate the potential threat, geometry screening is introduced as the final resource to protect integrity (the result is a loss of availability). Three methods are described and compared for geometry screening: real-time ionosphere error simulation, Lmax screening, and VPLH0 screening. Availability assessment is performed for both monitoring conditions with various Vertical Alert Limits (VALs). Finally, a possible solution is suggested to protect integrity under ionosphere threat without changing the current CAT I LAAS architecture.