Assessment of Ionospheric Impact on LAAS Using WAAS Supertruth Data

Ming Luo, Sam Pullen, Dennis Akos, Gang Xie, Seebany Datta-Barua, Todd Walter and Per Enge

Abstract:	In order to reflect the impact of spatial ionospheric gradients on Local Area Augmentation System (LAAS) users, the LAAS Ground Facility (LGF) broadcasts a conservative standard deviation (óvert_iono_gradient or óvig) for these errors that is included in the calculation of protection levels (bounds on user position errors). However, "supertruth" data provided by the Wide Area Augmentation System (WAAS) for a severe ionospheric storm in 2000 provided an example of a sudden, sharp change in ionospheric delays that implied a spatial gradient far larger than what is typical, even during these storms. It is impractical for LAAS to increase óvert_iono_gradient to bound these rare events; thus they must be classified as failures to be detected and alerted by the LGF before the user integrity risk increases to an unacceptable level. The goal of this study is to assess the integrity risk to LAAS users posed by these sharp ionosphere gradients and to determine if additional protection is needed for LAAS users. The anomaly observed in the supertruth data has been modeled as a sharp "wave front" in which a specified linear change in vertical ionosphere delay occurs over a specified horizontal distance. This wave front is assumed to move at a constant horizontal speed and direction relative to the ground. Each of these parameters can be varied to assess the sensitivity of LAAS to a wide variety of ionospheric spatial anomalies. By simulating ionosphere wave fronts at both aircraft and LAAS Ground Facility (LGF), we have studied the resulting differential errors. We have also tested how quickly the anomaly can be detected by the Stanford LGF prototype known as the Integrity Monitor Testbed (IMT). In the scenarios examined thus far, the IMT carrier-phase measurement quality alerts within 1 – 2 seconds of the wave front crossing an IMT Ionospheric Pierce Point (IPP), whereas smaller (less hazardous) gradients are detected within several minutes by the code-carrier divergence monitor. The scenario of most concern is when the wave front "comes up from behind" an aircraft approaching a LAAS-equipped airport and overtakes one or more aircraft pierce points several minutes before the aircraft reaches the runway threshold. In this case, the differential error growth is not cut off by IMT detection
Published in:	Proceedings of the 58th Annual Meeting of The Institute of Navigation and CIGTF 21st Guidance Test Symposium (2002) June 24 - 26, 2002 Hyatt Regency Hotel Albuquerque, NM
Pages:	175 - 186
Cite this article:	Luo, Ming, Pullen, Sam, Akos, Dennis, Xie, Gang, Datta-Barua, Seebany, Walter, Todd, Enge, Per, "Assessment of Ionospheric Impact on LAAS Using WAAS Supertruth Data," Proceedings of the 58th Annual Meeting of The Institute of Navigation and CIGTF 21st Guidance Test Symposium (2002), Albuquerque, NM, June 2002, pp. 175-186.
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