SISA Computation Algorithms and their applicability for Galileo Integrity

Carlos Hernandez Medel, Laura Perea Virgili, Alvaro Mozo Garcia, Juan Ramon Martin Piedelobo and Miguel M. Romay Merino

Abstract:	Taking into account the increasing importance of integrity in the context of Global Navigation, Europe is now committed to include the provision of integrity as one of the main design drivers of the Galileo system. This paper focuses on the analysis of some of the alternatives which are contemplated within the Galileo design to tackle the provision of integrity for the Galileo users. In the current Galileo baseline the integrity aspects concerning the SIS (Signal In Space) errors will be achieved by means of two parameters: Signal-In- Space Accuracy (SISA) and the Integrity Flag (IF). Together with new satellite ephemeris and clock models, the SISA is also broadcast to the users. The SISA is a prediction of the associated errors, with a certain confidence level, for the entire coverage area and valid for the applicability time interval of the models. Additionally, in order to meet the stringent integrity requirements, such as the maximum Time To Alert (TTA), the Integrity Flags are broadcast in real time to inform the users whether or not the SISA is properly bounding the SIS errors at that moment. A review of alternative integrity concepts to the current Galileo one is presented, since the implementation of the capability to provide users with integrity is one of the design drivers of the Galileo. From this point, the integrity at user level is briefly introduced before going into the details of the SISA concept and the candidate SISA algorithms. The objective is to highlight the importance of achieving a full coherency between the different elements that contribute to achieve the integrity at user level. The SISA concept and the strategy for its computation are then deeply analysed, which is necessary before developing and presenting the candidate SISA algorithms. Together with them, the hypothesis and its preliminary justification are established, including some information coming from the analysis of the Signal-In- Space Errors, obtained with real data from GPS together with prototypes for the Orbit Determination and Time Synchronisation (OD&TS) algorithms. This analysis of real data is included to support the discussion about the hypothesis included in the SISA computation algorithms. Finally, the future work to be done is presented, basically focused on the experimentation to be performed in the frame of the Galileo System Test Bed (GSTB) project, under ESA contract (see reference [3]).
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	2173 - 2184
Cite this article:	Medel, Carlos Hernandez, Virgili, Laura Perea, Garcia, Alvaro Mozo, Piedelobo, Juan Ramon Martin, Merino, Miguel M. Romay, "SISA Computation Algorithms and their applicability for Galileo Integrity," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 2173-2184.
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