GBAS GAST D Parametric Study and Availability Simulations to Support Category III Operations

Laurent Azoulai, Fabien Souchon, Cedric Brunet, Andreas Lipp, David Duchet

Abstract:	ICAO and RTCA have developed Ground Based Augmentation System (GBAS) standards that are intended to support approach and landing operations down to Category III minima. Proposed standards of the new service level called GBAS Approach Service Type D (GAST D) have been drafted and are currently being validated by international stakeholders and more particularly by SESAR partners. The GBAS GAST D concept, aimed at covering GBAS Category III operations, introduces one important new consideration that is the partial transfer of integrity responsibility from the ground station to the airborne part compared to ILS or CAT I GBAS. In order to meet the required performance in the position domain, the airborne receiver has now the responsibility to select a satellite geometry that is adapted to its performance – this is called geometry screening. The geometry screening is the process of satellite selection according to pre-defined criteria linked to aircraft capabilities. This selection process is a real-time test to be performed prior to operation initiation. This selection process must continue during the course of operation in order to detect if the geometry does not ensure anymore the performance of the operation. This situation takes advantage of the real GBAS performance and maximizes the benefits obtained by using modern aircraft guidance systems. On the other hand, the ground station operators (ANSPs) have no a-priori knowledge of the availability of Cat III operations by assessing the signal in space performance because they would need to know details of the aircraft guidance. One consequence of this responsibility split is the fact that, potentially and given the same satellite geometry, not all aircraft would be able to land in Category III conditions using GBAS GAST D technology and secondly, that there is a need to develop a tool enabling to estimate availability of Cat III operations according to location, constellation and aircraft type. This tool could be needed to help ANSP decision makers to consider the benefit of a GBAS GAST D ground station according to the types of aircraft landing on their airports. This tool is above all needed to determine suitability of aircraft implementation in order to achieve adequate availability worldwide for Category III operations. The objective pursued by the SESAR Project is to achieve a minimum figure of availability in any location to have a positive business case for a given aircraft type. Pegasus is a simulation tool developed by EUROCONTROL. For the GBAS Category III project, an additional module has been developed in order to implement GAST-D ground and airborne algorithms, based on ICAO SARPs and RTCA DO-253C. The purpose of this tool is to provide a realistic and statistical assessment of availability of the GAST-D service at a specific location or group of locations, in order to support optimization of ground and airborne subsystems. This paper is intended to present the results of availability simulations to support Category III operations as well as to perform a parametric study. The main objective of the availability simulations is to assess, for a representative set of approaches, the impact of geometry screening thresholds on availability. In order to limit the number of simulations and assess influence of each contributing factor, a sensitivity analysis is performed with the constellation failure rates assumptions, the date using real almanac data as well as almanac assuming a degraded constellation, the number of GPS reference receivers in the ground station, the mask angle used for the ground station and for the airborne receiver. The results are compared to following availability targets, Category III availability target of 0,999 when considering probabilities of satellite outages among visible satellites (0,999 being the current availability requirements for ILS CAT III); Category III availability target of 0,99997 when not considering satellite outages (being the fault-free system availability for GBAS Cat II/III considering no more than a 2,5s time-to-alert for constellation-induced service outage causing an alert for each 24h of service). These values concur with ILS CAT III operational availability targets. Simulations have been performed on every “ILS Category III capable” airports contained in the Pegasus database. After introducing the GBAS GAST D concept and Pegasus tool principle and structure, this paper reviews the various GBAS GAST D parameters that could influence the simulation results and assesses their sensitivity. Then, global availability simulations covering Category III airports are performed with various assumptions. Results are analyzed and recommendations are given for future GBAS implementation on the ground and on-board eligible aircraft. Finally, looking at available subset geometries in case of satellite failures, a strategy is proposed with regard to the benefits obtained and the relevance of searching for an alternate geometry.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2123 - 2140
Cite this article:	Azoulai, Laurent, Souchon, Fabien, Brunet, Cedric, Lipp, Andreas, Duchet, David, "GBAS GAST D Parametric Study and Availability Simulations to Support Category III Operations," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2123-2140.
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