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Modeling and monitoring of new GNSS signal distortions in the context of civil aviation

Abstract : Global Navigation Satellite Systems (GNSS) is used nowadays in various fields for navigation and positioning including safety-of-life applications. Among these applications is civil aviation that requires a very high quality of service for the most demanding phases of flight. The quality of the GNSS service is typically based on four criteria (integrity, accuracy, availability and continuity), that have to meet International Civil Aviation Organization (ICAO) requirements. To meet these requirements any source of potential service degradations has to be accounted for. One such example is GNSS signal distortions due to the satellite payload which can manifest in two ways: Nominal signal distortions generated by healthy satellites due to payload imperfections. This type of perturbation can limit the accuracy of the GNSS measurements and result in the unavailability of the service for some very stringent phases of flight. To mitigate their impact, a precise characterization of these distortions and a knowledge of their effects on civil aviation GNSS receivers are necessary. Non-nominal distortions that are triggered by a satellite payload failure. Non-nominal distortions, also called Evil WaveForms (EWFs) are rare events that may pose an integrity risk if the signal remains used by the airborne receiver. The strategy proposed by ICAO to deal with the EWF challenge is to characterize threatening distortions by the definition of a Threat Model (TM) and to build an appropriate monitor, referred to as Signal Quality Monitor (SQM) that will be able to detect any distortion from the TM that could lead to a position integrity failure. This task is performed by GNSS augmentation systems including Ground Based Augmentation Systems (GBAS) and Satellite Based Augmentation Systems (SBAS). The current monitors are based on the analysis of the correlation function. Supported by the groundwork performed by civil aviation on signal distortions for the GPS L1 C/A signal, this dissertation aims at proposing new distortions models associated to the new Global Positioning System (GPS) and Galileo signals that will be used by civil aviation after 2020. One important characteristic of GNSS signal distortions is that although they impact all users of the distorted signal, the consequence on the estimated pseudorange is dependent upon the GNSS receiver setting. This makes arduous the estimation of the impact of signal distortions on a GNSS user. The receiver parameters that have an influence on the pseudorange measurement estimated from distorted signals (nominal or non-nominal distortions) are listed. In addition illustrations to show the influence of these parameters on the GNSS receiver signal processing are proposed. The thesis first looks at the nominal distortions through GPS L1 C/A and Galileo E1C signals. Different types of observations are used based on correlation or chip domain visualization, and using high-gain and omnidirectional antennas. This investigation allows to: compare results with the state-of-the-art to validate the receiver processing software developed for this study, confirm published results and provide new results, make a comparison between nominal distortions observed from measurements collected with a high-gain dish antenna and with an omnidirectional antenna. The conclusions of the analysis are that the nominal distortions are relatively constant over years and that a precise characterization of nominal distortions is difficult notably because it is challenging to isolate signal distortions induced by the satellite from distortions induced by the receiver. After the observation of nominal distortions, the dissertation investigates the non-nominal distortions due to the payload failure. In particular, new TMs for new signals (GPS L5, Galileo E5a and Galileo E1C) are proposed. To define these TMs, the same parameters as the ones used to define the ICAO TM for GPS L1 C/A are used. The main work then consists in defining the range of the TMs parameters for the new signals. The limitation of the range of these parameters is based on two criteria: the impact of a distortion on a reference station and the impact of a distortion on differential users. It is noticeable that the new proposed TMs are larger than the GPS L1 C/A ICAO TM, resulting in an increase by a factor 100 of the number of considered threats. Then, the dissertation investigates the SQM that would be necessary to protect a civil aviation user against the TMs for new GNSS signals. The new SQM is based on current receiver technologies, in particular the ability to use many correlator outputs from the same signal. The main contribution is to propose an innovative representation to test and compare the SQMs performance whatever the received signal C⁄N_0 is. This representation is based on several assumptions but a strategy is exposed to still be able to use this representation if all assumptions are not fulfilled. From this representation, new SQMs (for each signal) are designed, their performances are assessed, and optimization processes are described to reduce their complexity. The concluding chapter of the dissertation reviews the main contributions of this Ph.D.. In addition perspectives for future works that could be conducted from the study performed in this Ph.D. are exposed.
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Submitted on : Monday, May 29, 2017 - 11:05:16 AM
Last modification on : Wednesday, November 3, 2021 - 5:37:54 AM
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  • HAL Id : tel-01528481, version 1



Jean-Baptiste Pagot. Modeling and monitoring of new GNSS signal distortions in the context of civil aviation. Signal and Image processing. INPT, 2016. English. ⟨tel-01528481⟩



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