Architecture and Performance of the Long Loop Algorithm for EGNOS V3 NLES Stations

Sergi Locubiche-Serra, Marc Solé-Gaset, Miguel Ángel Suárez-Gopar, Nasser Zaidi, Arnault Sfeir, G. Secon-Granados, J.A. López-Salcedo

Abstract: The third version of the European Geostationary Navigation Overlay Service (EGNOS) is a project that envisions to provide Satellite-Based Augmentation System (SBAS) coverage across Europe in the coming years. It arises as part of the modernization program promoted by the European GNSS Agency (GSA) to address obsolescence problems of previous EGNOS versions, improve robustness and enhance the performance of safety-of-life (SoL) services. In EGNOS V3, the termed Navigation Land Earth Stations (NLES) are ground stations in charge of generating the SBAS signals and further transmitting them to a geostationary (GEO) satellite at C band, which then performs down-conversion to L band and broadcasts the resulting signals back to the Earth. Consequently, EGNOS signals experience both uplink and downlink propagation effects if no further action is taken on them. One of the key elements for successful operation in EGNOS V3 (and, in fact, in any SBAS system) is to accurately control signal emission at the output of the GEO satellite transmit antenna. More particularly, the objective is that the downlink SBAS signals be synchronized with the Global Navigation Satellite System (GNSS) signals commonly used for ranging purposes, which experience only the downlink effects from the satellites to the Earth. To this end, the Long Loop Algorithm (LLA) is a software integrated into the NLES stations that has been designed with a twofold purpose. First, to control GEO signal emission through the NLES signal generator and perform real-time steering of SBAS signals to virtually remove the uplink propagation effects from the NLES to the GEO output antenna. This is achieved by precisely determining the effects and delays to be eliminated, and further commanding the signal generator accordingly. And second, to do so while keeping coherence between code and carrier measurements, in order to be compliant with performance requirements stated by the International Civil Aviation Organization (ICAO). The purpose of this paper is to present the LLA that has been designed for EGNOS V3 NLES stations. We describe the features of the software and the architecture of the algorithm, highlighting the use of more than one signal path to ensure proper LLA operation and the ability to operate in prediction mode in the absence of receiver measurements due to hardware malfunction for some seconds. In addition, the core of the algorithm is briefly introduced, highlighting the use of two consecutive processing blocks, namely the estimation and control modules. The correctness and performance of the proposed LLA and the fulfillment of ICAO performance requirements at this step of development is shown through MATLAB simulation results and experimentation tests with real hardware and a GEO payload simulator.
Published in: Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021)
September 20 - 24, 2021
Union Station Hotel
St. Louis, Missouri
Pages: 311 - 334
Cite this article: Locubiche-Serra, Sergi, Solé-Gaset, Marc, Suárez-Gopar, Miguel Ángel, Zaidi, Nasser, Sfeir, Arnault, Secon-Granados, G., López-Salcedo, J.A., "Architecture and Performance of the Long Loop Algorithm for EGNOS V3 NLES Stations," Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 311-334.
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