CDF Overbounding in the Galileo Experimental Off-Line Analysis Tool (E-OATp)

M. Sánchez-Gestido, J.F. Martín-Albo, J. Arranz-Richart

Abstract:	As a result of the increasing demands for improvement in Global Navigation Satellite Systems, integrity has been identified as one of the key concepts in the design of the Galileo mission. New applications of satellite positioning rely on the provision of high precision values along with the continuous availability of a reliable signal. Integrity performances required for the Galileo system are defined in terms of confidence levels on the determination of several accuracy related parameters. The actual computation of an accuracy parameter and its related confidence level can be performed, in general, by using a large number of samples or by applying an inflation factor to the estimation calculated with a reduced ensemble of measurements. As long as those accuracy parameters are eventually related to the service availability for the user, they are limited by values that depend on the requested system availability. This condition poses an additional requirement on the allowed inflation factor. Accuracy parameters can be associated to a number of values describing the statistical behaviour of a distribution that bounds, in a mathematical sense, the distribution of the experimental error. For its simplicity, a gaussian distribution is generally considered for the over-bounding of the cumulative probability of the distribution (CDFoverbounding). Several alternatives, based on that approach but not fully satisfactory, have been defined over years. This paper describes the options considered for the CDFoverbounding in the prototypes implemented, as part of the Galileo system, within the Experimental Off-Line Analysis Tool (E-OATp). This tool computes two calibrations for accuracy parameters: The historical Signal-in-Space-Accuracy (SISA), used to compute the broadcast SISA value under nominal conditions, and the Pre-processing Residual Error Calibration (PREC), used to inform about the quality of the input at the integrity check. A trade-off analysis is presented in order to address the dependence of the results on the relevant variables of the problem. Its implications on the performance of the system are also addressed. The purpose of the E-OATp, developed by Deimos Space under Alcatel Alenia Space contract, is to prototype a subset of algorithms to be eventually implemented in the Off-Line Analysis Tool (OAT), which is part of the Galileo Mission Support Facility (MSF). The scope of the MSF is both to provide with inputs (calibrations) to its real-time processes and also to provide the GMS with a certain level of performance analysis capabilities. The EOATp will be integrated in the Engineering Tool Environment (ETE), where the algorithm prototypes will run against real or simulated scenarios (GPS, Galileo or a combination of both).
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	563 - 574
Cite this article:	Sánchez-Gestido, M., Martín-Albo, J.F., Arranz-Richart, J., "CDF Overbounding in the Galileo Experimental Off-Line Analysis Tool (E-OATp)," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 563-574.
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