Alert Limits: Do we Need Them for CAT III?: Deriving GBAS Requirements for Consistency with CAT III Operations

B. Clark, B. DeCleene

Abstract:	The International Civil Aviation Organization (ICAO), RTCA and the European Organization for Civil Aviation Equipment have been developing requirements for GNSS to support precision approach and landings in Category III weather minima. Much of the effort has focused on determining the value of the signal-in-space (SIS) integrity parameter called an “alert limit.” The alert limit is a position domain error tolerance that is not to be exceeded without issuing an alert to the user. As implemented in systems supporting Category I operations, there are two primary uses of the alert limit: one for the ground station and one for the aircraft. Ground station designers use alert limits to derive ground monitor requirements for pseudorange errors or other observable parameters. This derivation can be dependent on the ground system architecture. The ground monitors run during normal operation and affect the transmitted integrity parameters. The aircraft compares protection levels to the alert limits to decide if the current satellite geometry supports the continued use of the SIS and if certain fault hypotheses are likely. The protection levels are based on the broadcast integrity parameters as well as the aircraft’s current satellite tracking performance. The intent of the protection level is to bound the SIS error in the position domain with high confidence. Current ICAO standards for Category I GNSS applications use the alert limit to define the SIS requirements. The ground system integrity requirements are written in terms of the protection level and a maximum alert limit value that may be used. There are a number of drawbacks to extending this methodology to CAT III. For example, the ground system requirements are rather high level and do not provide much insight into the pseudorange error characteristics for the failure modes of the SIS or how the selection of particular ground monitors shape these error distributions in general. However, an aircraft’s safety assessment must take into account the effects of failures of the navigation facilities. The alert limit is a rather coarse tool to standardize the SIS performance and ground station characteristics that the aircraft depends upon. Also consider that for low-visibility operations, there is a significant dependence on the aircraft capability in order to obtain operational approval. Different aircraft may have different dependencies on the external SIS to achieve a given operation. Aircraft without inertial augmentation will be much more dependent on the performance of the SIS than those that do have inertial systems. Even for aircraft with inertial systems, there are a number of different ways to integrate the inertial data with GNSS data. In defining requirements, emphasis should be placed on the SIS monitor requirements and characteristics. The standards for these monitors must be adequate for the aircraft and operational community to determine if a given navigation facility will support a Category III approach with a given aircraft. This paper outlines the drawbacks of extending this alert-limit based method to more demanding weather minima applications and provides an alternative view of requirements allocation between the ground and aircraft subsystems. Rather than standardizing the airborne receiver implementation and requiring an evaluation by the ground system, the ground system implementation can be standardized and the performance evaluated by the airborne equipment.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	3070 - 3081
Cite this article:	Clark, B., DeCleene, B., "Alert Limits: Do we Need Them for CAT III?: Deriving GBAS Requirements for Consistency with CAT III Operations," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 3070-3081.
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