Development of Signal in Space Performance Requirements for GBAS to Support CAT II/III Landing Operations

Tim Murphy

Abstract:	The standards and requirements for Ground Based Augmentation Systems (GBAS) to support CAT I operations are finalized. 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. These previous efforts at deriving requirements have been based on a so-called “ILS-look-alike” philosophy where the requirements are extrapolated by linearization of various performance requirements for ILS or MLS. This approach is flawed because the basic error characteristics and failure modes of GBAS are fundamentally different than those of ILS or MLS. Consequently, the extrapolation of angular ILS requirements has resulted in inappropriately tight proposed requirements for GBAS. This paper documents a proposed alternative methodology to determine the navigation system performance required to protect the safety of the CAT II/III operations. The proposed methodology is based on providing equivalent levels of safety for the operation. A key driving requirement for a GBAS to support CAT II/III operations is the magnitude of the Alert Limit. A methodology for determining the appropriate values for the Alert Limits needed to support CAT II/III operations is presented in this paper. This methodology has been presented to the ICAO GNSSP and is also being considered by RTCA and EUROCAE as a method for determining the appropriate Vertical Alert Limit (VAL) for CAT II/III operations. This paper documents the efforts of the author and others in developing and maturing the methodology. Also, analysis based on the methodology is presented.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	20 - 28
Cite this article:	Murphy, Tim, "Development of Signal in Space Performance Requirements for GBAS to Support CAT II/III Landing Operations," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 20-28.
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