Global / Regional Advance Autonomous Localization (GRAAL)
Clerino, A.¹; Fernet, C.¹; Peyrotte, C.¹; Damidaux, J.-L.¹; Brocard, D.^2
1Thales Alenia Space; ²CNES

What is the problem investigated?
Overview of some potential sensitivity impacting a Global/Regional Navigation System, like GALILEO and presentation of Advance Autonomous Localizations System allowing improvement on Integrity, Performance Robustness and Autonomy.

What is original about the research?
In this paper is presented a way to improve performance autonomy on GALILEO system. A global system approach is proposed, involving cross observables from satellite to satellite. Modifications on Navigation Payload architecture includes solutions to improve the Signal In Space spectral purity, integrity and robustness.

What new results have been achieved?
Signal In Space is entirely compensated for Payload disturbances, thus allowing optimized ratio EIRP to power consumption onboard for the navigation signals. Inter system (satellite&ground) Com&Ranging links solutions provides better performance autonomy and reduces ground constraints and complexity. Integrity and Robustness improvement should either consolidate or allow new safety applications.

Summary
This GRAAL system study phase 1 has been initiated by CNES in the frame of R&T CNES 2006 activities. This study has been co-financed by CNES and ThalesAleniaSpace for a duration of 8 months. The purpose of this study was to identified potential improvement of GALILEO system, in term of robustness, integrity, and performance autonomy. This study allowed to propose relevant technical solutions which could be implemented in future versions of European GNSS (improvement of GALILEO FOC and/or evolutions of 2nd generation GALILEO system).

The navigation performance offered by 1st generation GALILEO system (IOV then FOC) depends on ground segment ability to continuously realize monitoring and control of MEO satellites constellation (orbits, navigation signals, etc) in nominal conditions. In case of degraded modes of ground segment (i.e. failure or unavailability at ground station and/or ground network and/or operation levels, interferences problems at L-band at GSS level, etc) this performance can be impacted.

More precisely, GRAAL system study consists 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). This is achieved by increasing performance autonomy and functional capabilities of space segment through addition of some components mainly on-board MEO satellites.

This paper is organized in 4 chapters, that provide through a synthetic presentation the main analysis and conclusions that have been performed during CNES study.

First chapter presents an overview of GALILEO system to recall basic knowledge required as input for this study. A synthetic comparison with GPS is also detailed.

Second chapter identifies the main sensitive points of GALILEO system which either constitute limitations for the autonomy of performance of the space segment, or induce extra complexity on the ground segment. Among them, we identified three items for the Communication, two items for Ranging, two items for navigation signals and one item for attitude keeping that could be improved.

Third chapter proposes a set of functions to be implemented as potential answers to main critical points. Technical solutions are for most of them located within MEO satellites and payloads. Inter Satellite Links are proposed for communication and ranging. Signals observables are drastically improved from 1st version of GALILEO, and introduces the capability of autonomous navigation. Original technical solutions introduce new way to manage the signal within payload, allowing a perfect control of signal in space generation and propagation.

Fourth chapter presents the conclusion summarising the main results and introducing further activities to be performed. A possible way allowing to introduce some of the GRAAL enhancements onboard the early developments of GALILEO is also presented.