Evaluating Fault-Mode Protection Levels at the Aircraft in Category III LAAS

This paper presents a new approach for assessing faultmode risk in a Ground Based Augmentation System (GBAS) such as the Federal Aviation Administration’s Local Area Augmentation System (LAAS). According to current GBAS standards, the ground system is burdened with the full responsibility for protecting users from hazardous navigation errors that might result from a satellite fault. Unfortunately, the current architecture penalizes most users, since the ground system must protect a hypothetical worst-case aircraft. Given the aggressive performance requirements of next generation (Category III) LAAS, this paper proposes an alternative architecture, which would transmit additional information to airborne users and thereby eliminate performance penalties associated with worst-case assumptions. Specifically, the proposed alternative GBAS architecture would shift the evaluation of fault-mode error bounds, also known as protection levels, away from the ground station to the user avionics. Fully backwards-compatible, the modified architecture would continue to serve legacy users while providing significant benefits to users with newer equipment. By evaluating fault-mode protection levels at the airborne user, the new architecture leverages information unavailable to the ground station including the user position and velocity, the subset of satellites selected by the user for navigation, and the magnitude of user-receiver errors. By leveraging knowledge of these details, the new architecture eliminates worst-case assumptions and results in tighter error bounds with higher availability.