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Optimal GPS/GALILEO GBAS methodologies with an application to troposphere

Alizé Guilbert 1 
1 SIGNAV - ENAC Equipe TELECOM-SIGNAV
TELECOM - ENAC - Equipe télécommunications
Abstract : In the Civil Aviation domain, research activities aim to improve airspace capacity and efficiency whilst also tightening safety targets and enabling new more stringent operations. This is achieved through the implementation of new Communications, Navigation, Surveillance and Air Traffic Management (CNS/ATM) technologies and processes. In the navigation domain, these goals are met by improving performance of existing services whilst also expanding the services provided through the development of new Navigation Aids (Navaids) or by defining new operations with existing systems. One such developmental axe for enabling expansion towards new such operations is the provision of safer, more reliable approach and landing operations in all weather conditions. The Global Navigation Satellite System (GNSS) has been identified as a key technology in providing navigation services to civil aviation users [1] [2] thanks to its global coverage and accuracy in relation to conventional Navaids. This global trend can be observed in the fitting of new civil aviation aircraft since a majority of them are now equipped with GNSS receivers. The GNSS concept includes the provision of an integrity monitoring function by an augmentation system to the core constellations. This is needed to meet the required performance metrics of accuracy, integrity, continuity and availability which cannot be met by the stand-alone constellations. Three such augmentation systems have been developed within civil aviation: the GBAS (Ground Based Augmentation System), the SBAS (Satellite Based Augmentation System) and the ABAS (Aircraft Based Augmentation System). The Ground Based Augmentation System (GBAS) is currently standardized by the ICAO to provide precision approach navigation services down to Category I using the GPS or GLONASS constellations [3]. Research and standardisation activities are on-going with the objective to extend the GBAS concept to support Category II/III precision approach operations with a single protected signal (GPS L1 C/A), however some difficulties have arisen regarding ionospheric monitoring that threaten to limit availability of this solution. With the deployment of Galileo and BeiDou alongside the modernization of GPS and GLONASS, it is envisaged that the GNSS future will be multi-constellation (MC) and multi-frequency (MF). European research activities within the SESAR program have focused on the use of GPS and Galileo. The service commitments for this last constellation is expected to be in place in the medium term. The use of two protected frequency bands enables the mitigation of ionospheric errors at the expense of multipath and noise inflation, whilst the improved geometry of two constellations may be used to counter this resulting inflation and enable Cat II/III for worse performing aircraft. Therefore the MC/MF GBAS concept should lead to increased availability, stronger robustness to unintentional interference (due to the use of two protected frequency bands), better ground segment monitoring capabilities, better modelling of atmospheric effects and improved measurement accuracy from modernized signals. However, several challenges and key issues must be resolved before the potential benefits may be realized. This PhD has addressed two key topics relating to GBAS, the provision of corrections data within the MC/MF GBAS concept and the impact of tropospheric ranging biases on both the SC/SF and MC/MF GBAS concepts. Due to the tight constraints on GBAS ground to air communications link, the VHF Data Broadcast (VDB) unit, a novel approach is needed when expanding to a MC/MF corrections service [4]. One of the proposals discussed in the PhD project for an updated GBAS VDB message structure is to separate message types for corrections with different transmission rates. Furthermore, This PhD argues that atmospheric modelling with regards to the troposphere has been neglected in light of the ionospheric monitoring difficulties and must be revisited for both nominal and anomalous scenarios. The thesis focuses on how to compute the worst case differential tropospheric delay offline in order to characterize the threat model before extending previous work on bounding this threat in order to protect the airborne GBAS user. This previous work led by Ohio University for assessing differential tropospheric delays [5] [6] [7] is based on GPS data collection which is inherently subject to undersampling. Furthermore, the bounding methodology was constrained by restricting the scope to SF GPS GBAS and an already defined data message format. In the scope of MC/MF GBAS development, an alternative approach was needed. Therefore, in this PhD project, Numerical Weather Models (NWMs) are used to assess fully the worst case horizontal differential range component of the troposphere (differential tropospheric delay between aircraft and ground assuming aircraft and ground are at the same altitude). An innovative worst case horizontal differential range tropospheric gradient search methodology is used to determine the induced differential ranging biases impacting aircraft performing Cat II/III precision approaches with GBAS. This provides as an output a worst case differential ranging bias as a function of elevation for two European regions (low-elevation coastal and high-elevation mountainous). The range vertical differential component (differential tropospheric delay between aircraft and ground assuming aircraft and ground are not at the same altitude but are at the same latitude and longitude) is also modelled by statistical analysis by comparing the truth data to the GBAS standardized model for vertical tropospheric correction up to the height of the aircraft. A model of the total uncorrected differential ranging bias is generated which must be incorporated within the nominal GBAS protection levels. In order to bound the impact of the troposphere on the positioning error and by maintaining the goal of low data transmission, different solutions have been developed which remain conservative by assuming that ranging biases conspire in the worst possible way. Through these techniques, in order to protect the user against tropospheric ranging biases, it has been shown that a minimum of 3 parameters may be used to characterize a region’s model. The main contributions of this thesis are firstly the development of an optimal processing scheme for meeting Cat II/III performance requirements with the MC/MF GBAS trough the derivation of the error budget degradation when using lower frequency corrections than the current GBAS message correction rate of 2Hz, and the validation of these theoretical analysis with real data. Other contributions deal with the determination and bounding of differential tropospheric ranging biases in the horizontal and vertical directions. Finally, other contributions include the validation of the differential tropospheric ranging biases computation, and the comparison of tropospheric gradients between U.S. data and European data as well as between low relief and high relief regions.
Keywords : GNSS Galileo
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Submitted on : Thursday, July 21, 2016 - 5:06:57 PM
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Alizé Guilbert. Optimal GPS/GALILEO GBAS methodologies with an application to troposphere. Signal and Image processing. INP Toulouse, 2016. English. ⟨tel-01347791⟩

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