GPS Multipath Assessment in Real Airport Conditions to Support Category III Operations

L. Azoulai, A. Chen, Y. Cruz Cavalcanti

Abstract:	ICAO and RTCA have developed Ground Based Augmentation System (GBAS) standards [ICAO, 2009], [ICAO, 2010] and [RTCA, 2008], that are intended to support approach and landing operations down to Category III minima. Proposed standards for this new service type have been drafted and are currently being validated by international stakeholders and more particularly by SESAR partners. The 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. GBAS technology has the advantage, compared to other types of precision approaches means such as ILS or MLS, to be less sensitive to multipath. A thesis [Chen, 2010] has provided an electromagnetic-based prediction of the multipath ranging error knowing a 3D description of the airport environment, the GPS airborne antenna position and the GPS satellite position. This prediction, using a simulator developed in the frame of this thesis, has shown that an obstacle present at a distance larger than 460 m would not affect the GPS position estimate. As a consequence, the benefit expected from GBAS is the reduction and even the removal of the sensitive areas applied during ILS Low Visibility operations. These sensitive areas impose that, during a Low Visibility operation, no vehicle like an aircraft, likely to be a good reflector for ILS signal, is vacating in this area. The consequence of such clearance of sensitive areas is the increased separation between aircraft during approach, to prevent multipath disturbance from aircraft on the ground to the next landing aircraft. Thanks to GBAS technology, the absence of multipath would permit an aircraft vacating a runway or waiting to take off at a holding point, to still be present in the sensitive area while another aircraft approaches and lands on the same runway. The same scenario could occur during a guided take-off operation where an aircraft waiting to cross a runway could be closer to the runway while another aircraft is taking-off using GBAS inputs. A previous study intended to estimate GPS multipath from the ground and the airborne fuselage and resulted in the confirmation of GPS multipath error budget [ICAO, 2006]. This paper is intended to present the results of GPS multipath error estimates, based on simulations, affecting a GBAS receiver during Category III operations from approach until end of the automatic roll-out. More particularly, the objectives of these multipath simulations is to assess, for a representative set of approaches, in realistic operational conditions, the impact of reflecting obstacles, such as aircraft holding near a runway or vacating from the runway at various positions, during landing or guided take-off using GBAS. The finality is to determine whether sensitive areas currently applicable for ILS Cat III operations remain necessary or can be reduced in order to reduce separations between aircraft during low visibility operations and thus increase the access and traffic for a given runway. From the thesis results and the associated simulation tool, the first part of the study has consisted in developing the capacity to generate automatically 3D scenes of real airport environment including building and aircraft. Then, the second part of the study has developed trajectories generation and integration into the tool for landing and take-offs. After introducing the GBAS GAST D concept and the simulation tool principle and mathematical assumptions, this paper describes the ILS Cat III operational context and limitations imposed by sensitive areas. Then, several various operational scenarios of automatic approach and landing under Cat III conditions and guided take-off with one or several aircraft standing in the sensitive areas are described. In addition, airport environment and approach, landing and take-offs trajectories are described and simulated in order to be as much as possible representative of real operational conditions. In particular, two airports have been selected as they are the ones utilized in the SESAR GBAS project flight trials: Toulouse-Blagnac and Frankfurt airports. Finally, based on these assumptions, various sets of simulations scenarios are run and analyzed to identify firstly, the sensitivity of the multipath error estimate to these different conditions and secondly, potential reductions of sensitive areas assuming multipath error is low enough to not affect the aircraft automatic guidance. Finally, recommendations are provided in order to perform flight tests for validation purposes. References: [RTCA, 2008] MINIMUM OPERATIONAL PERFORMANCES STANDARDS FOR GPS LOCAL AREA AUGMENTATION SYSTEM AIRBORNE EQUIPMENT Issue C DO253 December 2008 [ICAO, 2009] ICAO NSP : Conceptual Framework for the Proposal for GBAS to Support CAT III Operations Issue 6.5 Nov. 2009 [ICAO, 2010] ICAO NSP : GBAS CAT II_III Development Baseline SARPs Proposal 100528 V2 [Chen, 2010]: "Development of a Hybrid Deterministic-Statistical GPS Multipath Simulator for Airport Navigation, PhD dissertation", A. Chen, 2010 [ICAO, 2006]: "Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications, Volume I Radio Navigation Aids", International Civil Aviation Organization, Sixth Edition, July 2006, Amendment 85
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1380 - 1394
Cite this article:	Azoulai, L., Chen, A., Cavalcanti, Y. Cruz, "GPS Multipath Assessment in Real Airport Conditions to Support Category III Operations," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1380-1394.
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