Fault Modeling for GBAS Airworthiness Assessments

A new type of service has been proposed for Ground Based Augmentation System (GBAS) that is intended to support approach and landing operations down to the lowest minimums (i.e. CAT IIIb). Proposed standards for this new service type have been drafted and are currently being validated. This so called GBAS Approach Service Type D (GAST D) includes new low level requirements for monitoring as well as a requirement for additional geometry screening in order to protect the user from failures of several types. This paper discusses how the proposed requirements can be interpreted in order to develop a fault model that describes the magnitude and dynamics of malfunction induced navigation systems errors that are undetected or prior to detection. Such a fault model can be used to demonstrate acceptable airplane system level responses to malfunctions as part of airworthiness approvals. The paper includes a review of the types of malfunctions that are anticipated and the monitoring requirements that limit the impact of those malfunctions. Then a dynamic error model is proposed and the parameters of that model are presented for each type of failure. The relationship between the largest undetected errors and the user defined geometry screening is explored for each type of malfunction. Some discussion of how this model is anticipated to be used in the context of airworthiness demonstrations is included. This work represents an important step towards development of airworthiness requirements needed in order for GBAS to support CAT III operations in the future.