Concept for a Dual Frequency Dual Constellation GBAS

This paper proposes one possible concept for a dual frequency dual constellation GBAS architecture. It is based on a single frequency L5/E5a mode as primary processing scheme for best standard performance, a switch to an ionosphere free combination in case of ionospheric disturbances and supporting also classical GBAS approach service types (GAST) C and D for single frequency GPS-based CAT I and CAT II/III modes. The concept is supported by a proposal of how to transmit the required corrections in the existing capacity limited VDB broadcast and is backwards compatible to legacy GBAS. A discussion about the benefits and remaining issues of the proposed architecture concludes the paper

Méthodologies de traitements optimales des mesures GPS/GALILEO GBAS avec une application à la Troposphère

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.Dans le domaine de l’Aviation Civile, les motivations de recherches sont souvent guidées par la volonté d’améliorer la capacité et l’efficacité de l’espace aérien grâce à la modernisation des moyens de navigation aérienne existants et à l’ajout de nouvelles infrastructures. Ces buts peuvent être atteints en développant les services qui permettent des opérations d’approche et d’atterrissage plus robustes et plus fiables quels que soient le lieu et les conditions météorologiques.La navigation par satellite, grâce au Global Navigation Satellite System (GNSS), a été reconnue comme un moyen performant de fournir des services de navigation aérienne [1] [2]. Depuis quelques années, les systèmes de navigation par satellites sont devenus des moyens de navigation de référence grâce à leur couverture mondiale et à leur précision. En particulier, ce système de navigation est utilisé en aviation civile à bord des avions dont la majorité est équipé de récepteurs GNSS. Le concept du GNSS requiert l’utilisation de moyen d’augmentations pour fournir une fonction de contrôle d’intégrité. Cet appui est nécessaire au vu des exigences [1] concernant la précision, l’intégrité, la disponibilité et la continuité des systèmes GNSS surtout dans les applications critiques de type aviation civile. Trois moyens d’augmentation ont alors été développés: le GBAS (Ground Based Augmentation System), le SBAS (Satellite Based Augmentation System) et le ABAS (Aircraft Based Augmentation System).Le GBAS est actuellement standardisé par l’OACI pour fournir un service de navigation incluant les approches de précision allant jusqu’à la catégorie I incluse, en utilisant les constellations GPS ou GLONASS [3]. Des travaux de recherches et de développement sont en cours pour permettre d’étendre ce service jusqu’à catégorie II/III avec un seul signal protégé (GPS L1 C/A). Cependant des contraintes limitant la disponibilité de cette solution sont apparues lors de la surveillance de la ionosphère.Grâce à la modernisation du GPS et GLONASS et à la future implémentation des constellations Galileo et Beidou, les futurs GNSS utilisant de multiples constellations et de multiples fréquences (MC/MF) sont étudiés. En Europe, les activités de recherches dans le cadre du projet SESAR se sont appuyées sur la constellation GPS et sur la disponibilité future de la constellation Galileo. L’utilisation de deux bandes de fréquences protégées permet la réduction des retards ionosphériques tout en augmentant l’impact du bruit et des multi-trajets. Cependant l’amélioration de la géométrie des satellites, grâce aux deux constellations, peut compenser cette augmentation et permettre de réaliser des approches de précision de catégorie II/III. C’est pour cela que le MC/MF GBAS devrait permettre de nombreuses améliorations telles que l’augmentation de la disponibilité du système, la meilleure robustesse face aux interférences, un meilleur modèle des retards atmosphériques et une meilleure précision due aux nouveaux signaux de meilleure qualité. Cependant, de nombreux challenges et problèmes doivent être résolus avant d’atteindre les bénéfices potentiels.Dans ce travail de thèse, deux principaux sujets en rapport avec le GBAS ont été traités, la transmission des données de corrections dans le contexte du MC/MF GBAS et l’impact des biais de mesures troposphériques dans le cadre du SC/SF GBAS et du MC/MF GBAS. Dû aux strictes contraintes portant sur le format des messages transmis à l’utilisateur via l’unité de VHF (Very High Frequency) Data Broadcast (VDB) [4], une nouvelle approche est nécessaire pour permettre l’élaboration du MC/MF GBAS. Une des solutions proposée dans cette thèse est de transmettre les corrections et les données d’intégrité à l’utilisateur dans des messages séparés à des fréquences différentes. De plus ce travail de thèse remet en question la modélisation de l’atmosphère. En effet, au vue de la difficulté de surveiller les retards ionosphériques, ceux relatifs à la troposphère furent en partie négligés et doivent être réévalués aussi bien dans des conditions nominales que non-nominales. Cette thèse se concentre d’abord sur les moyens de calculer le pire gradient troposphérique pour caractériser la menace troposphérique avant de développer les précédents travaux pour borner cette menace dans le but de protéger l’utilisateur.Les précédentes études faites par l’Université d’Ohio pour traiter les retards troposphérique différentiels [5] [6] [7] sont basées sur la collecte de données GPS qui est intrinsèquement liée à du sous-échantillonnage. De plus, dans le cadre du SF GPS GBAS, la méthode pour borner l’erreur fut contrainte par le format du message transmis. En vue du futur MC/MF GBAS, une nouvelle approche s’est avérée nécessaire. C’est pour cela que dans ce projet de thèse, des modèles météorologiques numériques (NWMs) sont utilisés pour estimer intégralement la composante horizontale du pire retard différentiel troposphérique (retard différentiel dû à la décorrelation horizontale entre l’avion et la station sol). Une méthode innovante pour rechercher les pires retards différentiels troposphériques horizontaux est utilisée pour déterminer les biais de mesures qu’ils induisent impactant les avions visant une approche de Cat II/III avec le GBAS. Un modèle de ces pires biais de mesure troposphériques différentiels horizontaux dépendant de l’élévation des satellites pour 2 régions européennes (une région côtière à bas-relief et une région montagneuse à haut relief) est alors développé. La composante verticale du pire retard différentiel troposphérique (retard différentiel dû à la différence d’altitudes entre l’avion et la station sol) est aussi modélisée grâce à une étude statistique qui compare les données réelles au modèle standard établi pour le GBAS. Un modèle du biais de mesure différentiel total non corrigé est développé et doit être introduit dans le calcul des niveaux de protections sous des conditions nominales.Pour borner l’impact de la troposphère sur l’erreur de position tout en se focalisant sur le souhait d’avoir un nombre de données transmises à l’utilisateur faible, différentes solutions ont été développées. Elles restent conservatives en supposant que les biais de mesures se combinent pour engendrer la pire erreur de position verticale. Avec ces méthodes, au minimum 3 paramètres, définis selon leur région géographique d’utilisation, doivent être transmis à l’utilisateur pour le protéger contre ces biais de mesures troposphériques.Les principales contributions de cette thèse sont le développement d’un modèle optimal de traitements pour répondre aux exigences liées à la Cat II/III d’approche avec le MC/MF GBAS. Ceci a été effectué tout d’abord grâce à l’analyse théorique de la possibilité d’avoir des messages transmis à une fréquence plus faible que celle standardisée actuellement à 2 Hz puis par la validation de cette possibilité grâce à l’analyse de données réelles.Ensuite, les autres apports de cette thèse portent sur les solutions permettant de déterminer et de borner les biais de mesures troposphériques différentiels dans les directions horizontales et verticales. Enfin, d’autres contributions incluent la validation du calcul des biais troposphériques et la comparaison entre les gradients troposphériques apparaissant dans les données américaines et européennes pour une région côtière à bas-relief et une région montagneuse à haut relief

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

Optimal Processing Scheme for meeting Cat II/III with the MC/MF GBAS

International audienceTo meet the long term goal of greater capacity in aviation, services must be expanded to provide more reliable and robust approach and landing operations in all weather conditions, globally using modernised navigation systems. This paper details the measurement processing techniques under investigation for the Multi-Constellation (MC) and Multi-Frequency (MF) Ground Based Augmentation System (GBAS) within the SESAR Framework Work Package 15.3.7. It deals with the performance improvements obtainable for CAT II/III precision approaches, the most stringent operation currently defined. GBAS has the potential to provide CAT II/III services without the need for expensive and regular maintenance and flight testing that comes with the current Instrument Landing System (ILS). Furthermore, in the case of ILS, multipath problems may restrict separation criteria in some conditions thus limiting capacity. SESAR WP15.3.7 is investigating a potential change in the message correction update rate. With the current correction rate of 2Hz it will not be possible to send multiple constellations and correction types beyond the bare minimum. This paper presents analyses relating to the error budget degradation when using lower frequency corrections

Characterization of Tropospheric Gradients for the Ground-Based Augmentation System Through the Use of Numerical Weather Models

International audienceIn the scope of the Single European Sky Air Traffic Management Research (SESAR) Work Package 15.3.7, a number of research threads are being undertaken to improve the performance of multi-constellation multi-frequency ground-based augmentation system to support CAT II/III precision approaches. Several challenges and key issues must be solved including those related to atmospheric modeling. However, there are a number of arguments for revisiting this topic and specifically addressing the tropospheric threat. First, recent observations, reported at the last International Civil Aviation Organization Navigation System Panel meeting, showed unexpected atmospheric behavior. The source could be related to a non-modeled behavior of the troposphere. Second, in the advent of dual-frequency ground-based augmentation system, the main contributor to the atmospheric error will come from the tropospheric delay. That is why this paper explains a methodology to reassess the tropospheric threat within two main steps: an analysis of European meteorological data and an analysis of a new bounding methodology for dealing with the troposphere threat and its impact on vertical protection levels

Development of processing models for Multi-Constellation/Multi-Frequency GBAS

National audience The primary aim of this thesis is to determine an optimum processing methodology for Dual Constellation (GPS and Galileo) and Dual Frequency GBAS, thus resolving the challenges this presents. The thesis is performed within the scope of SESAR WP 15.3.7 and more precisely sub-task 3.6 of this work package on the subject of measurement processing. SESAR WP 15.3.7 is a SESAR Joint Undertaking funded project consisting of major European industry and research institutes.</p

Evolution of Corrections Processing for MC/MF Ground Based Augmentations System (GBAS)

International audienceThe Ground Based Augmentation System (GBAS) is currently standardized at the International Civil Aviation organization (ICAO) level to provide precision approach navigation services up to Category I using the GPS or LONASS constellations. Current investigations into he use of GBAS for a Category II/III service type known as GAST D are ongoing. However, some gaps in performance have been identified and open issues remain Multi-frequency and multi-constellation solutions are being explored within the European SESAR program (WP 15.3.7) to address these issues. The addition of a secondary constellation provides many advantages such as better geometry, robustness against signal outages, relaxing of demanding constraints. Furthermore, new signals offer the potential to combine measurements on multiple frequencies to mitigate the effects of the ionosphere including during disturbances and helps the stringent continuity and availability requirements to be met.However, whilst the advantages of using many more signals is clear, there exists a major constraint with respect to the available space for message transmission from the GBAS VHF Data Broadcast (VDB) unit [5]. Currently, corrections and their integrity are provided in combined messages broadcast every half second (2Hz). However, extending this approach to multiple correction types, based on the different signals and observables for two or more constellations will not be possible. Furthermore, if the need arises to include future signals from the modernized constellations or expand further than two constellations then no additional transmission space would be available. It is for these reasons that the authors have investigated the possibility of providing corrections at a lower rate than the current 2Hz, with a separate message type dedicated to providing the integrity status of each correction in a manner akin to the Satellite Based Augmentation System (SBAS) [6].In order to justify this approach and to select the ideal correction message rate, a number of items must be addressed