A Study of Severe Multipath Errors for the Proposed GBAS Airport Surface Movement Application

Ground Based Augmentation Systems (GBAS), such as the U.S. Local Area Augmentation System (LAAS) can be used for both precision approach and Differentially Corrected Positioning Service (DCPS) applications. Through its support of DCPS, the LAAS Ground Facility (LGF) is required to meet the integrity requirements of all other operations that could use the GBAS VHF Data Broadcast (VDB). Our previous work [1,2,3] demonstrated that the existing DCPS integrity requirements cannot be met by CAT I GBAS without changes to both the definition of DCPS integrity [4,5] and the airborne receiver requirements [6]. One of the implications is that some future applications of GBAS that planned to use DCPS, such as airport surface movement, cannot be supported by DCPS with the CAT-I GBAS architecture. However, if airport surface movement is defined as a separate operation, it could be supported by the existing LGF geometry screening that mitigates the anomalous ionospheric threat for CAT-I precision approach and by designing Horizontal Protection Level (HPL) with increased σmultipath (or σpr_air) in airborne equipment to bound the higher multipath errors expected in the airport surface environment (as opposed to an aircraft in flight). Our previous work [7] confirmed this hypothesis and concluded that two or more times σpr_air allows to meet the current integrity requirements and achieve the Maximum Acceptable Error (MAE) of 10 meters with more than 99% availability. Note that this conclusion is derived under the assumption of no nominal error contribution to Horizontal Position Error (HPE) other than worst-case ionospheric errors. However, in the surface-movement environment, worstcase airborne multipath might be a significant fraction of HPE. Limited data for airport surface movement exists at present, so examining multipath models for ground and obstruction-influenced specular is the first step in exploring this further and is the subject of this paper. To cover higher multipath errors in the surface movement, Jahn’s multipath model for urban and suburban environments are used. If LGF geometry screening and new σmultipath using Jahn’s multipath model for optimistic suburban environment itself cannot support airport surface movement, the results in this paper include additional aircraft geometry screening proposed in [2,7] to meet the requirements and lower the MAE to a beneficial level while maintaining useful availability.