The Prototype Research Instruments and Space Mission technology Advancement (PRISMA) represents the first European technology demonstration of formation-flying and on-orbit-servicing techniques. Several hardware and software experiments, either at subsystem or system level, have been successfully conducted since the launch of the dual-satellite mission in June 2010. This paper describes the guidance, navigation, and control functionalities and presents key flight results from the so called Spaceborne Autonomous Formation Flying Experiment (SAFE) executed in September 2010 and March 2011 as one of the primary PRISMA mission objectives. SAFE is intended to demonstrate autonomous acquisition, keeping and reconfiguration of passive relative orbits for advanced remote sensing as well as rendezvous applications. As shown in the paper, the onboard GPS navigation system provides relative orbit information in real-time with an accuracy better than 10 cm and 1 mm/s (3D, rms) in position and velocity respectively. The impulsive formation control achieves accuracies of better than 10 m (3D, rms) for separations below 2 km with minimum usage of thrusters, ensuring high predictability for simplified mission operations and minimum collision risk for increased safety.
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