Geometry Screening for GBAS to Meet CAT III Integrity and Continuity Requirements

摘要

Standards are being developed to enable Ground Based Augmentation Systems (GBAS) to support Category II and Category III (CAT II/III) operations. An example of GBAS is the FAA’s Local Area Augmentation System (LAAS). As a part of this process, new requirements are being formulated to better define the performance of monitoring to mitigate various fault conditions. Several monitoring methods are used to enable GBAS to meet the integrity and continuity requirements for levels of service that support landing operations. To ensure that monitoring done by the ground station is sufficient to support adequate operations performance, GBAS requirements include airborne functional requirements designed to limit the impact of various fault modes as observed by the user in the position domain. Geometry screening through the use of protection levels and critical satellite checks are two such functional requirements already defined in the standards. As more detailed, lower level monitoring requirements are developed, additional geometry screening requirements are being considered for implementation in the standards. This paper describes some of the geometry screening requirements under consideration, explains why these new functional requirements may be needed, and explores the potential impact of such requirements on availability of GBAS for various levels of service. rnThe paper shows that the effectiveness of ground monitoring may be greatly enhanced by some modest additional geometry screening in the airplane. The interaction and relative impact of critical satellite checks, maximum single satellite error projection checks and basic protection level checks are discussed. Availability of GBAS service levels as a function of these variations is observed. Finally the effect of additional geometry screening on the mitigation of the ionospheric anomaly threat is discussed