GBAS Alert Limits Compared to ILS Collision Risk Model Navigation System Error Assumptions

T. Murphy

Abstract:	The international community is now engaged in the development of requirements for GBAS to support Cat II and Cat III operations. RTCA and others have developed proposed requirements. A very significant requirement which has yet to be agreed to by the ICAO Navigation Systems Panel (NSP) is the value of the Alert Limits required for a level of service from GBAS to support CAT II/III operations. An important issue related to the validation of proposed requirements for the alert limit is the compatibility of the proposed requirements with existing obstacle clearance criteria and other procedure design criteria. The ICAO Obstacle Clearance Panel (OCP) has developed a Collision Risk Model (CRM) for ILS to support procedure design. The ILS CRM has long been a standard within ICAO and is used routinely to support ILS procedure designs. This paper reviews the Navigation System Error (NSE) assumptions used in the definition of the ILS CRM and compares these assumptions to assumptions that have been used in derivation of GBAS accuracy and Alert Limit requirements. The paper considers the error probability distributions defined for the synthetic ILS used in the development of the CRM. Error bounding based on this synthetic ILS model is compared to the error bounding provided by the GBAS alert limits. The paper shows that proposed values for GBAS alert limits are consistent with, or more conservative than, the assumptions underlying the ILS CRM. Therefore, use of the ILS CRM in operational procedure design for GBAS approaches should be conservatively safe. The work described in this paper is expected to be an important step towards the final validation of a standard service level for GBAS to support CAT II/III operations.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	327 - 341
Cite this article:	Murphy, T., "GBAS Alert Limits Compared to ILS Collision Risk Model Navigation System Error Assumptions," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 327-341.
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