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Processing and Integrity of DC/DF GBAS for CAT II/III Operations

Giuseppe Rotondo 1 
1 SIGNAV - ENAC Equipe TELECOM-SIGNAV
TELECOM - ENAC - Equipe télécommunications
Abstract : In Civil Aviation domain, to cope with the increasing traffic demand, research activities are pointed toward the optimization of the airspace capacity. Researches are thus ongoing on all Civil Aviation areas: Communication, Navigation, Surveillance (CNS) and Air Traffic Management (ATM). Focusing on the navigation aspect, the goals are expected to be met by improving performances of the existing services through the developments of new NAVigation AIDS (NAVAIDS) and the definition of new procedures based on these new systems. The Global Navigation Satellite System (GNSS) is recognized as a key technology in providing accurate navigation services with a worldwide coverage. A symbol of its importance, in civil aviation, can be observed in the avionics of new civil aviation aircraft since a majority of them are now equipped with GNSS receivers. The GNSS concept was defined by the International Civil Aviation Organization (ICAO).It, includes the provision of an integrity monitoring function by an augmentation system in addition to the core constellations. This is needed to meet all the required performance metrics of accuracy, integrity, continuity and availability which cannot be met by the stand-alone constellations such as GPS. Three 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). GBAS, in particular, is currently standardized to provide precision approach navigation services down to Category I (CAT I) using GPS or Glonass constellations and L1 band signals. This service is known as GBAS Approach Service Type-C (GAST-C). In order to extend this concept down to CAT II/III service, research activities is ongoing to define the new service called a GAST-D. Among other challenges, the monitoring of the ionospheric threat is the area where the integrity requirement is not met. Thanks to the deployment of new constellations, Galileo and Beidou, and the modernization process of the existing ones, GPS and Glonass, the future of GNSS is envisaged to be Multi-Constellation (MC) and Multi-frequency (MF). In Europe, research activities have been focused on a Dual-Constellation (DC) GNSS and DC GBAS services based on GPS and Galileo constellations the robustness of the entire system against unintentional interference thanks to the use of measurements in two protected frequency bands, • the robustness against a constellation failure, • the accuracy improvement by using new signals with improved performance, and more satellites. any of the information included herein. Reprint with approval for publisher and with reference to source code only • improved detection of ionosphere anomalous condition thanks to the use of DF measurements. • mitigation of the residual ionospheric induced range error These last points in particular are considered as one of the biggest benefits brought by the DF GBAS. To overcome the problems experienced by Single-Frequency (SF) GBAS due to ionosphere anomalies, the use of two frequencies (Dual Frequency, DF) has been selected as a mean to improve ionosphere anomalies detection and to mitigate ionosphere residual errors. Advantages in using a DC/DF GBAS (GAST-F) system are, however, not only related to the integrity monitoring performance improvement. However, the use of new signals and a new constellation, does not bring only benefits. It also raises a series of challenges that have to be solved to fully benefit from the new concept. In this thesis, some challenges, related to DC/DF GBAS, have been investigated. One of them, rising from the use of new GNSS signals, is to determine the impact of error sources that are uncorrelated between the ground station and the aircraft and that induce an error on the estimated position. Using two frequencies, there is the possibility to form measurement combinations like Divergence-free (D-free) and Ionosphere-free (I-free) for which the errors impact has to be analyzed. In this thesis, the impact of the uncorrelated errors (noise and multipath as main sources) on ground measurements is analyzed. The aim is to compare the derived performances with the curve proposed in (RTCA Inc.; DO253-C, 2008) for the ground correction accuracy and derived for GPS L1 C/A. Another issue raised by the use of DC/DF GBAS is the increased number of satellites and the presence of a second frequency. This leads to the constraint of having a big number of channels in GNSS receivers to track all available signals. Moreover, to broadcast a bigger number of corrections from the ground to the aircraft, the messages capacity has to be increased with respect to the current SF/SC GBAS. To solve this problem, some solutions have been proposed, one of these is the implementation of a satellite selection algorithm. In this PhD, the impact of some algorithms proposed in literature has been analyzed on a simulated DC GBAS system. The last analysis performed in this thesis regards some of the challenges in the integrity monitoring domain. GBAS has been validated, nowadays, only for GAST-C to provide CAT I service. Although almost the same architecture has been used to provide CAT II/III service within GAST-D (but with new monitors on the ground architecture), the concept to derive the airworthiness between the two services is totally different. This major difference is justified by the fact that for CAT III operations, requirements are more stringent than for CAT I. Despite all the efforts done, GAST-D for CATII/III has not been validated. The cause of the non-validation of GAST-D is the lack of integrity performances in monitoring the ionosphere anomalous activity with the proposed monitoring scheme. Even if for GAST-F, relying on DF combinations, the monitoring of the ionosphere could not represent the main issue and the integrity performances of current monitors may be sufficient to meet the requirements, two considerations have to be done • In case of loss of frequency, for GAST-F, the ionosphere monitoring presents the same condition as for GAST-D. If the latter is not validated, the fallback mode is GAST-C, limiting the availability of CAT II/III operations. • Improving the integrity performances of GAST-D will permit to create a system with an enhanced CAT II/III operations availability thanks to the use of GAST-D as fallback mode in case of frequency loss whenever GAST-F is used as primary mode. In case of GAST-D as primary mode, GAST-F can be considered the fallback mode for cases of ionosphere anomalous conditions. Considering previous conditions, the work done in this thesis has focused on the monitoring of the ionospheric conditions that are impeding GAST-D to be validated. A solution combining RAIM SC/SF GBAS differential corrections and foreseen GAST-D monitors is proposed. The combination of these integrity monitoring functions permit to get closer to GAST-D requirements for particular ionospheric scenarios where the maximum ionospheric induced range error can be assumed. The consideration of dual constellation in the same mix of integrity monitoring functions has also been studied, as a possible fallback mode of GAST-F when one of the two frequencies is lost. The ionosphere is not the only integrity issue for GAST-F. Other analysis have been done considering the impact of new signals or new processing modes on the existing monitors. Concerning this, the impact of a lower update rate, for the PRC and RRC, on the Excessive Acceleration (EA) monitor has been analyzed. The aim is to verify the feasibility of the monitor in extending the current update interval from 0.5 seconds up to a proposed value of 2.5 seconds.
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Submitted on : Tuesday, January 10, 2017 - 2:41:17 PM
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Giuseppe Rotondo. Processing and Integrity of DC/DF GBAS for CAT II/III Operations. Signal and Image processing. INPT, 2016. English. ⟨NNT : 2016INPT0130⟩. ⟨tel-01430980⟩

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