The European Space Agency is currently developing the PROBA spacecraft, an Earth-observation mini-satellite whose mission objective is to demonstrate the feasibility and benefits of autonomy in space. Potential benefits include the reduction of ground-station operating costs and a better management of limited on-board resources. One of the core building blocks of an autonomous spacecraft is the attitude control and orbit navigation system (ACNS) which enables the autonomous acquisition of orbit and attitude knowledge, the prediction of Earth target overflys, the plannign and scheduling of spacecraft attitude operations, the execution of manoeuvres required by on-board payloads and the detection of non-nominal events. This paper presents the design, oepration and predicted perfomrance of the PROBA ACNS during the critical imaging manoeuvres, with special emphasis on the autonomous attitude control functions of the satellite: the quaternion-based determination of the attitude and the on-board generation of the reference profile. It also outlines the software development approach which relies on the use of advanced Computer-Aided Software Engineering (CASE) tools for model development, analyses, simulations and automated flight code generation.
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