LAAS Ionosphere Spatial Gradient Threat Model and Impact of LGF and Airborne Monitoring

The ionosphere spatial gradient and its temporal rate of change in the vicinity of a LAAS-equipped airport are likely to influence the architecture required to meet the Category II/III precision approach and landing requirements. An initial parametric analysis depicts the effects of ionosphere anomalies on the position error of users for the current Category I LAAS architecture. In the ionosphere threat model used by previous analyses, an ionosphere gradient travels towards the airport at approximately 110 m/s with a change in ionospheric delay of 6 meters over a 19-km width. When the LAAS Ground Facility (LGF) detects the ionospheric delay before it impacts the airplane, there is no error. In other cases, the accumulated error depends on the parameters of the ionospheric gradient. The previous analysis shows that, although LGF detection is extremely important, the current architecture may not be able to meet LAAS requirements [3, 11] under worst-case ionosphere conditions. In this paper, the ionosphere threat model is reexamined based on WAAS and IGS data from the 6 April 2000 ionospheric event. The analysis of additional data provides more information than was previously available. As the first step of developing a specific, clear, and bounded threat space model, the FAA Key Technical Advisors (KTAs) reached a preliminary consensus in March 2003. The resulting threat space attempts to cover a range of possible ionospheric events in CONUS extrapolated from the previous linear-gradient model. Four parameters of the ionospheric wave front were identified: gradient slope (30 – 400 mm/km), gradient width (15 – 200 km), wave front speed (0 – 1000 m/s), and angle between the wave front motion and the airplane approach direction (0 – 360°). The impact of potential ionosphere anomalies on LAAS users is simulated in range domain over this entire threat space. The dependence of the differential range error on the user-LGF separation is evaluated. The maximum differential range errors at 5 km of user-to-LGF separation are computed for four monitoring scenarios: (1) no monitoring; (2) LGF monitoring; (3) airborne monitoring; and (4) LGF and airborne monitoring. These simulations determine the fraction of the threat space in which the range error impact can be mitigated by LGF monitoring alone or by LGF and airborne monitoring together. Based on these results, the degree of importance of both LGF and airborne monitoring requirement are assessed. Data analysis of other ionosphere storms and position-domain anomaly simulations are also underway.