Troposphere Reassessment in the scope of MC/MF Ground Based Augmentation System (GBAS)

International audienceIn civil aviation, there is currently a demand for greater airspace capacity and efficiency. In order to meet these long term goals, services must be expanded to provide more reliable and robust approach and landing operations in all weather conditions, globally. One potential application would be to use the Ground Based Augmentation System (GBAS) to enable Cat II /III precision approaches, the most stringent operation currently defined and with the lowest separation minima. Whilst a GBAS solution using a single frequency of the Global Positioning System (GPS) is under the late stages of development and standardization to meet CAT II/III performance requirements, some open questions remain and availability will not be assured for installations worldwide and all of the time. This paper details the activities related to the measurement processing techniques under investigation for the Multi-Constellation (MC) and Multi-Frequency (MF) Ground Based Augmentation System (GBAS) within the SESAR (Single European Sky ATM Research) Framework Work Package 15.3.7. In this scope several research threads are being undertaken to improve the performance of Ground Based Augmentation System (GBAS) to support CAT II/III precision approaches. Several challenges and key issues must be solved including those related to atmospheric modelling. Previous work principally undertaken at Ohio University [1] [2] [3] highlighted the need to consider the troposphere as a possible source of positioning failure. GBAS activities in Europe have followed the approach of validating the values of the protection levels, which include a component relating to ionospheric gradients. Therefore the position error induced by tropospheric failure should be safely bounded by validating that the combination of atmospheric errors does not exceed the assumed models. However, there are a number of arguments for revisiting this topic and specifically addressing the tropospheric threat. Firstly, recent observations [4], reported at last ICAO NSP (International Civil Aviation Organization – Navigation System Panel) meeting, showed unexpected atmospheric behaviour. These observations have been confirmed by the FAA (Federal Aviation Administration) and Boeing and have shown that significant spatial gradients with no link to ionosphere activity are likely to appear mainly during warm and sunny days. The root could be related to a non-modelled behaviour of the troposphere. Even if the range errors induced by this phenomenon are around 9 cm and are not significant compared to those due to ionospheric gradients, the combination of these “troposphere” gradients with ionospheric gradients could lead to missed detection or false detection of the ground subsystem’s ionospheric monitor, thus impacting integrity and continuity. Secondly, in the advent of dual-frequency GBAS, the ionosphere may feasibly be removed through the ionosphere-free smoothing technique. In this case, the main contributor to the atmospheric error will come from the tropospheric delay. Under such a scenario, the troposphere threat model must be defined and a means for bounding the potential errors derived. This paper presents an initial analysis with the aim of evaluating the impact of non-nominal troposphere on Vertical Protection Level (VPL) for different scenarios. The goal of this comparison is to ascertain the extent to which the proposed tropospheric bounding methodology increases the VPLs used at the aircraft. Finally, this paper has initiated the process of assessing the impact of modelling the non-nominal troposphere on GBAS VPLs. Indeed a new methodology is proposed and seems to improve performance in terms of availability while respecting some constraints on a low data requirements for the VDB transmission