Potential for the use of GBAS at Close-by Airports

M. Felux, P. Remi, S. Circiu

Abstract:	In the most recent of a series of accidents a small business jet crashed in the year 2012 on the approach into Egelsbach airport in bad weather conditions. This airport is located just about 10km south of Frankfurt Rhein/Main airport and is mainly used for business aviation. However, it does not have an ILS to provide precision approaches in low-visibility conditions. With the first GBAS stations now operational and Frankfurt Airport being one of the candidates for a future installation, this study investigates the possible use of GBAS corrections for other airports in the vicinity of the main airport where the reference antennas are located. From a practical point of view a limiting factor in such architecture might be the availability of the VDB data broadcast. This technical issue was beyond the scope of this study and only the navigation aspects were investigated. The GBAS architecture was designed such that the transmitted correction and integrity data support approaches to the airport where the installation is located. For the GBAS approach service type D (GAST D), which is intended to support approach and autoland operations under CAT-II/III weather conditions, a maximum distance of 5km is set forth between the GBAS reference point and the runway threshold to which precision approach service is provided [1]. The main reasons for this limitation are rare ionospheric disturbances which could potentially cause differential positioning errors large enough to prevent safe operations of aircraft close to the ground. Even if a degradation of the service over distance has to be expected, a desirable level of service at a secondary airport would be enabling approaches down to a decision height of 200-300ft (ideally CAT-I or only a slightly higher decision height). This research presents the results of a feasibility study to use GBAS for approaches to other airports in the vicinity. The evaluations are based on a maximum vertical error at a desired decision height (e.g. 200ft which corresponds to the CAT-I minimum) which may not be exceeded with a certain probability. This is taken as a starting point to ensure no collisions with terrain or obstacles can occur. The tolerable vertical error is mainly determined by four parameters: The first one is the slope of the obstacle clearance surface below the approach track. As for the derivation for the GBAS criteria a standard 1:29 slope was assumed [2]. No obstacles on the approach may penetrate this surface in order to ensure that no collisions can occur. For considerations of the collision risk the total system error (TSE) is of interest. It consists of two main influencing effects. The first one is the flight technical error (FTE) that describes the aircraft’s ability to follow a predefined track. This is an aircraft specific fixed parameter which every manufacturer has to determine during the certification process of the aircraft. The second parameter is the navigation system error (NSE). In order to allocate the error budget to both systems assumptions about the FTE have to be made. For this study we used a standard deviation of 2m for the vertical FTE. The third parameter which influences the tolerable vertical error is the glide path angle of the approach. For precision approaches, values usually range from 2.5° to 3.5°. However, most of the published approaches have 3° or even steeper angles. This parameters directly influences the maximum allowable error which is the space between the approach track and the obstacle clearance surface. Steeper approaches directly result in relaxed requirements which could be exploited in the definition of approach procedures. The fourth and last parameter considered in this study is the underlying iono threat model. Different sizes of ionospheric gradients can be observed at geomagnetic latitudes. As an example, the threat model for Germany assumes a maximum slope of a gradient of 140mm/km [3] while the threat model for the CONUS region takes gradients as large as 425mm/km into account [4]. When using GBAS at large distances, there is no way for the ground system to detect possible gradients before they can affect aircraft. The largest undetected gradients which have to be considered are therefore the main contributing factor to a potential positioning error. Results from recent flight trials from January 2013 show that under nominal conditions the application of GBAS correction still yields very good results even at distances of around 60km. However, from an integrity standpoint safety of operations cannot be guaranteed over such distances to the aforementioned threats. Under the assumptions of the German iono threat model and a standard 3° approach, the maximum use distance of GBAS corrections to provide CAT-I like service is only slightly larger than 10km. However, some enhanced monitoring might have the potential to increase the service radius for applications like serving several airports in a metropolitan area. [1] RTCA DO-253C (2008), Minimum operational performance standards for GPS local area augmentation system airborne equipment, Tech. Rep. DO-253C, RTCA. [2] Murphy, Tim, et al. "Fault Modeling for GBAS Airworthiness Assessments." Navigation (2012). [3] Mayer, C., et al. "Ionosphere Threat Space Model Assessment for GBAS." Proceedings of the 22nd International Technical Meeting of the Satellite Division of the Institute of Navigation. 2009 [4] Pullen, Sam, Young Shin Park, and Per Enge. "Impact and mitigation of ionospheric anomalies on ground-based augmentation of GNSS." Radio Science 44.1 (2009): RS0A21.
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:	1403 - 1410
Cite this article:	Felux, M., Remi, P., Circiu, S., "Potential for the use of GBAS at Close-by Airports," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1403-1410.
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