Safety Concepts for Mitigation of Ionospheric Anomaly Errors in GBAS

Curtis A. Shively and Rick Niles

Abstract:	FAA and industry are developing concepts for ensuring the safety of aircraft using a GNSS ground based augmentation system (GBAS) to conduct precision approaches when relatively rare ionospheric anomalies could potentially cause unusually large aircraft vertical position errors. The paper presents two concepts for using obstacle clearance surfaces to assess the safety of a CAT I approach if an ionospheric anomaly produces vertical navigation sensor errors (NSE). In the first concept the maximum allowable vertical NSE to avoid penetration of the OCS is determined, under the assumption that the other errors also take on fixed, representative values. In consideration of the final approach OCS, a maximum vertical NSE of about 29 m (low) would be allowable at the nominal decision height (DH) of 200 ft. In consideration of the missed approach OCS (for the original direction precision approach) the allowable error high) is much smaller in magnitude. However, if at least a visual approach is protected to the opposite end of the runway, the same magnitude of error (29 m) that is allowable at DH for the final approach is also safe for the missed approach with margin. In the second concept, the risk of OCS penetration is determined for a given random distribution of the ionospheric anomaly errors, under the assumption that the other errors are also random. The allowable risk of surface penetration is 10-7 for the final approach and 10-5 for the missed approach. Observations are made based on risk results for an illustrative example distribution of the vertical NSE due to ionospheric anomalies. Again, if the requirement is met for the final approach, it will also be met for the missed approach if at least a visual approach is protected to a threshold located at the opposite end of the runway. While the above observations are based on a nominal approach with glide path angle (GPA) of 3.0 deg and threshold crossing height (TCH) of 50 ft, similar results apply to other values of GPA and TCH. It should be pointed out that the analysis and concepts presented in this paper do not depend on assumptions that are unique to ionospheric anomalies. The methodology and results apply to any source of additional vertical NSE that is assumed to be present with probability 1.0.
Published in:	Proceedings of the 2008 National Technical Meeting of The Institute of Navigation January 28 - 30, 2008 The Catamaran Resort Hotel San Diego, CA
Pages:	367 - 381
Cite this article:	Shively, Curtis A., Niles, Rick, "Safety Concepts for Mitigation of Ionospheric Anomaly Errors in GBAS," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 367-381.
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