Constellation Availability Concepts for Galileo

G. Salgado, S. Abbondanza, R. Blondel and S. Lannelongue

Abstract:	The European Space Agency (ESA) has initiated the GalileoSat Definition Phase. The GalileoSat system [1] includes the Galileo Space Segment, the associated Ground Segment (i.e. Control/Command and all means associated to the Navigation Mission), and the infrastructure required to elaborate and broadcast the user integrity data. Within the GalileoSat industrial consortium, Alcatel Space Industries is in particular responsible for the System Analysis, Trade-offs and Validation Tasks. Special emphasis has been devoted to the system performance definitions to be proposed for the constellation dimensioning. The purpose of this paper is the description of the concept proposed for the constellation availability performance characterization and associated specification. Up to now the approach used has been the computation of a “mean” availability over the system lifetime assuming that the system is in a steady state. This approach is not adapted to the specification of a new system such as Galileo, where the objective is to build up a system that allows to warranty a service to the end user. The users want to know what they will get from Galileo during periods of time that are meaningful to them such as one month or maximum one year. Additionally, they want to make sure that this "short term" performance will be maintained during the system lifetime. For end users, "mean performance" warranty on the system lifetime has little sense. This requires to go through a modification of the availability concept, as it is understood currently by the “navigation community”. The constellation nominal and degraded states availability and the probabilities to be in nominal and degraded states during the system lifetime substitute the concept of "mean" availability. This approach has proved to be necessary in order to better characterize the type of service warranty to the user and the risk associated to this warranty of service. It has also advantages when faced to the specification of a new system. This paper describes in detail the approach proposed to compute and specify the Galileo constellation availability. First results of the performance obtained with the recently proposed Galileo constellation are also included as examples during the performance presentation. The impacts of the new performance concept on the constellation maintenance and renewal strategies as well as satellite design (long and short-term failures specification) will also be described in the paper. The work presented in this article has been done in the frame of a European Space Agency contract. Responsibility for the contents resides on the authors.
Published in:	Proceedings of the 2001 National Technical Meeting of The Institute of Navigation January 22 - 24, 2001 Westin Long Beach Hotel Long Beach, CA
Pages:	778 - 786
Cite this article:	Salgado, G., Abbondanza, S., Blondel, R., Lannelongue, S., "Constellation Availability Concepts for Galileo," Proceedings of the 2001 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 2001, pp. 778-786.
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