UDRE computation: a key issue for space based augmentation system performance

E. Sardon, J.A. de la Fuente, N. Zarraoa, J. Nieto, J. Cosmen

Abstract:	Space based augmentation systems like EGNOS, WAAS, MTSAT, will provide differential corrections (ephemeris and satellite clock corrections) and ionospheric delays to the users. In addition to that, those augmentation systems must also provide an estimation of the error associated with those corrections as the basis for providing instantaneous estimation of the protection levels (i.e. those levels that guarantee the required integrity) and the selection of the satellites to use according to the user’s RNP. Integrity is one of the most important requirements of those augmentation systems and so the estimation of those erl-ors in the corrections is critical for the systems. In particular, an estimation of the error of the computed ephemeris and clock corrections for each satellite will be represented by the UDRE (User Differential Range Error). The equivalent measurement of the error in the ionosphere estimates is the GIVE (Grid Ionospheric Vertical Error). The performance requirements of the augmentation systems are characterised by four main parameters: accuracy, integrity, continuity and availability. The estimated UDRE is directly related to integrity, continuity and availability. Due to the safety critical nature of augmentation system mission, providing precise estimations of the UDRE is even more important than providing accurate satellite corrections. The main requirements imposed on the estimated UDRE are derived from the mission ones. On one hand, integrity has to be preserved, that is the estimated UDRE must be larger than the maximum satellite errors, with a certain probability, in order to provide safety to all users. Moreover, the UDRE has to bound those errors even in cases of anomalous behaviour of either satellites (e.g. unannounced manoeuvres) or ephemeris and clock corrections computation function in the ground segment. This feature converts the UDRE computation in a cornerstone of the augmentation system service integrity mechanisms. But, on the other hand. the estimated UDRE can not be much larger than the maximum satellite errors to provide availability clf the system to users that need high precision. In this paper we discuss some approaches in the way of defining the UDRE. We will discuss several aspects associated to the UDRE such as the difficulty in its estimation and verification with regards to the adopted definitions. We will also discuss which should be the probability associated with the UDRE bound of the satellite errors and if it should be directly related with the integrity level required by the user. In particular, within the EGNOS project, the Early Test System (ETS) has been developed as a tool for system design using prototypes of the system and it is providing very useful results since 1996. One clf the main aspects of ETS is that currently it is the only possibility within EGNOS for experimentation using real data, ETS is being used in critical areas of Central Processing Facility (CPF) design, in particular, t,a test different algorithms for UDRE estimation. We will present results of the estimation of UDRE obtained using ETS, and they will be analysed in terms of derived service integrity and availability performance.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	2091 - 2100
Cite this article:	Sardon, E., la Fuente, J.A. de, Zarraoa, N., Nieto, J., Cosmen, J., "UDRE computation: a key issue for space based augmentation system performance," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 2091-2100.
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