Integrity Based on MT28 for EGNOS: New Algorithm Formulation & Results

T. Authié, S. Trilles, J-C Fort, J-M Azaïs

Abstract: The purpose of a Satellite Based Augmentation System, such as EGNOS or WAAS, is to decompose all range error sources and to distribute them to the civil aviation user community with reliable navigation services for different flight phases. Integrity refers to the notion of trust that the user may have in the positioning. Integrity includes the ability of the system to provide confidence thresholds as well as Alarms in case of anomalies. Considering satellite integrity, two ways are possible for an SBAS. The first is to broadcast per satellite the same UDRE value applicable for every user located inside the service area defined by the message type 27 content. As these UDREs are dimensioned to protect the worst user location – generally at the border zone – it penalizes automatically the users having lower residual errors. The second is to broadcast per satellite a covariance matrix of residual errors through the message type 28. This matrix contains all the structure of the orbitography and synchronization residual errors. It can be seen as a protective ellipsoid around the computed satellite position and clock containing the true – but unknown – satellite position and clock at a certain level of confidence. Using the MT28 message each user is able to reconstruct the integrity value by projecting this protective ellipsoid along its line of sight. EGNOS operational system is implementing the MT27 solution whilst the WAAS is based on the MT28 one. The MT28 approach is the current baseline for EGNOS V3. Starting from recent R&D activities, Thales Alenia Space together with CNES (French Space Agency) has developed a dedicated module providing the message type 28 based on orbitography and fast synchronization variance-covariance matrices combined with satellites residual measurement errors. The MT28 formulation has been designed in collaboration with Statistics and Probabilities laboratories (Paris Descartes and Toulouse Paul Sabatier) in France. The algorithm contains also a mechanism that reacts immediately to orbit or clock satellite feared events such as a clock jump. This new concept for EGNOS provides a drastic improvement with respect to the first studies provided during EGNOS V3 phase A & B and HISTB V2 by the Thales consortium. The new MT28 module is integrated into a SPEED platform, the SBAS Operational Test-bed that fully represents EGNOS performances in terms of accuracy, continuity, availability and integrity for Safety Of Life services. The performance evaluation shows a very good level of EGNOS LPV200 availability with respect to the MT27 current approach. The integrity is constantly maintained on the Geostationary Broadcast Area with a good level of integrity margin. This paper provides a high level architecture description of this new EGNOS algorithm as well as a set of performance figures showing the achieved improvements.
Published in: Proceedings of the 30th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2017)
September 25 - 29, 2017
Oregon Convention Center
Portland, Oregon
Pages: 1077 - 1088
Cite this article: Authié, T., Trilles, S., Fort, J-C, Azaïs, J-M, "Integrity Based on MT28 for EGNOS: New Algorithm Formulation & Results," Proceedings of the 30th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2017), Portland, Oregon, September 2017, pp. 1077-1088.
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