Global/Regional Advanced Autonomous Localization System (GRAAL)

E. Sénant, C. Fernet, D. Brocard, L. Ries

Abstract:	The first phase of GRAAL system study was initiated by CNES in the frame of R&T CNES 2006 activities, laying the basic principles of Inter Satellite Links to identify potential improvements of the GALILEO system, in terms of robustness, integrity, and autonomy with the definition of associated functions (communications, ranging and data repatriation). This study was co-financed by CNES and Thales Alenia Space France for a duration of 16 months. The present article deals with the second phase of this study, started in the end of 2007. The purpose of this phase was to refine the assumptions of the first phase for the ranging and communication functions from a system point of view, to derive specifications of the proposed technical solutions for future versions of European GNSS (evolutions of 2nd generation GALILEO system). The navigation performance offered by the 1st generation GALILEO system (IOV then FOC) strongly depends on the ground segment ability to continuously perform monitoring and control of the MEO constellation (orbit, navigation signals, etc) in nominal conditions. In case of degraded ground segment (i.e. failure or unavailability at ground stations, or ground network, or operation levels, interferences problems at L-band at GSS level, etc.), this navigation performance can be impacted. The GRAAL system study consisted in conceiving, prototyping and validating a set of solutions allowing to obtain a more robust GNSS system (i.e. less vulnerable to degraded modes of ground segment) by increasing autonomy performance and functional abilities of the space segment through the addition of communication and ranging links between MEO satellites, with the aid of on-board functions such as clock and signal integrity monitoring, separately analyzed in the GRAAL study. This paper is organized in three sections, that expose the main analysis and conclusions drawn in by TAS-F concerning ISL. The 1st section briefly presents an overview of the GALILEO system to remind of the required basics as input for this study. It also identifies the main points of GALILEO system responsible for the limited autonomy performance of space segment and for the ground segment complexity. The 2nd section proposes a set of functions to be implemented as potential answers to the points identified above. Technical solutions are mainly located on board MEO satellites payloads but have strong indirect impacts on ground segment architecture. Inter satellite links are proposed for communication (navigation data broadcast, GSS data repatriation…) and ranging between satellites. Signal observables are improved from 1st version of GALILEO (due to easier signal conditions), and may allow the ability of autonomous navigation. The 3rd section presents the mains conclusions of the GRAAL study in terms of integrity, autonomy and complexity (ground segment). Finally, further activities to be performed are presented.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	1386 - 1395
Cite this article:	Sénant, E., Fernet, C., Brocard, D., Ries, L., "Global/Regional Advanced Autonomous Localization System (GRAAL)," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 1386-1395.
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