EGNOS CPF Check Set - Stand Alone Performance

R. Wolf, U. Rossbach, W. Werner, S. Lannelongue, J. Michelon and P. Thomas

Abstract:	One of the main objectives of the European Geostationary Navigation Overlay System (EGNOS) is the protection of the user by offering not only position accuracy but also service integrity. For this reason the Central Processing Facility (CPF) is divided into two sets: the Processing Set (PS) and the Independent Check Set (CS). While the task of the PS is to generate ionosphere as well as ephemeris and clock corrections with corresponding error bounds, the CS is in charge of verifying the correctness of this information by checking the whole set of NOF (Navigation Overlay Frame) messages independently. The main checks, satellite correction (UDRE) Check and ionospheric correction (GIVE) Check, follow total different strategies. While the UDRE check applies the corrections and monitors the residual error, the GIVE check re-computes the correction and compares it to what has been broadcast to the user. The CS is neither allowed to use any data produced by the Processing Set, nor to use the same input, i.e. measurement data. Many processing steps done by the PS have to be redone by the Check Set, using different algorithms to avoid common failure modes. This includes independent estimation of the RIMS inter-frequency biases and clock errors as well as estimating a Check Set internal ionospheric model. The actual integrity checks are just the last (and not even most demanding) step in a long processing chain. The observations used to evaluate the NOF content can themselves be affected by errors and therefore have to be carefully monitored before delivered to the checks. A major challenge during the Check Set Prototype development by IfEN has been the requirement of providing high integrity and being at the same time robust against data gaps, corrupted observations and other feared events at the input. Therefore, an observation has to pass many detection and exclusion stages from reception of raw data to a fully validated smoothed pseudo range. This includes mechanisms for the single line of sight, as well as cross checks for common events per satellite and per RIMS. The Check Set is intended to ensure integrity of the EGNOS message. Although the current design can be tuned to provide nearly arbitrary levels of integrity & continuity, the price would be a significant reduction of availability. Who wants a system preserving integrity, but being not available most of the time? The current tuning tries to balance all requirements to provide just enough integrity & continuity to meet requirements, while keeping availability as high as possible. The Check Set Prototype has not only undergone intensive Stand Alone Testing, but also test together with the prototype Processing Set on the so called integrated platform. These test have manly been conducted by the CPF contractor Alcatel. Those results (included in this paper) demonstrate the real added value of the Check Set in case of Processing Set failure. While IfEN has finished algorithm design and prototyping work, Logica is currently implementing the operational EGNOS Check Set. The last part of the paper describes the industrialisation of the complex integrity algorithms, which have been independently specified and prototyped, into a real-time, high availability service. The design of the system, in particular the partitioning of the overlapping augmentation footprints into semi-independent processing 'lanes' will be described. The performance constraints on the system will be outlined, and the operational solution will be described. The validation of the safety-critical Check Set will be based on scenarios that collectively cover all nominal and foreseen extreme operating conditions. An outline of the validation tests will be given, and initial performance results for the operational software on the target hardware will be presented.
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:	1736 - 1745
Cite this article:	Wolf, R., Rossbach, U., Werner, W., Lannelongue, S., Michelon, J., Thomas, P., "EGNOS CPF Check Set - Stand Alone Performance," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1736-1745.
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