GBAS Based Autoland System: A Bottom Up Approach for GAST-D Requirements

Abstract

Ground Based Augmentation Systems (GBAS) for the precision approach of aircraft have never been so close to support automatic approach and landings under CAT IIIc conditions as they are today. GBAS Approach Service Type D is slowly taking form with a new version of the Minimum Operational Performance Standard (RTCA-DO-253C) having been issued. Nevertheless, with the architecture of a GBAS System that will support CAT III operations almost ready in paper, the safety of the solution must be demonstrated not only for nominal conditions, but especially for worst case scenarios. As a vital requirement before certification it has to be demonstrated, that the required integrity, continuity and availability allocations are met even under these unfavourable circumstances. In the presented work, we address these definitions of worst case scenarios for an automatically landing aircraft. The bottom up threat space for GBAS can be defined as the scenario that would drive the autoland not to fulfil the JAA CS-AWO technical requirements in terms of statistical distributions of touch down parameters (evaluated using Monte-Carlo analyses). In other words, this method will provide an indicator for the quality of CAT-IIIc GBAS service mapped in satellite geometry and in ionosphere scenario that will bring the autoland system to the limits of the required performances for safety. To test these setups, we used a simulation scenario consisting of a MATLAB Simulink model of DLR’s ATTAS VFW-Fokker 614 experimental airplane including an advanced autopilot with autoland capability. This system is coupled with a current state GBAS landing system module and realistic simulated errors are injected. Thanks to more than one million simulated automatic approaches, we study the propagation of the navigation system error through the combined aircraft/autopilot system and compare the resulting total system error to a reference trajectory (approach path) and touch down parameters (actual landing). The results indicate a mitigating effect of the airplane dynamics onto the NSE due to the inertia of the aircraft and response pattern of the autopilot. A rigorous error propagation method as proposed in this paper will drive to a better knowledge of robustness of GBAS with autopilot and is a first step towards novel satellite based navigation architectures for precision approach and landing of commercial aircrafts under CAT IIIc conditions.