ESA GNC Conference Papers Repository
Proba-3: Achieving Formation Flying Millimetre Accuracy
Proba-3 is the space program managed by ESA for the in-orbit demonstration of Formation Flying (FF) technology. The main demonstration goal is controlling a two small satellites formation with relative position accuracy at millimetre level. This performance will allow to fly a novel distributed coronagraph payload obtaining first class science results. At the same time FF technology demonstration will pave the road for future virtual telescope and interferometer missions. The challenging millimetre level formation control objective, drives the GNC design and binds it to the mission design. Tight performance specifications have been derived to all the system elements. Dedicated analysis, simulations and metrology tests have been performed in the frame of the project phase B and post PDR activities to guarantee the coherence of the mission design and the achievement of the objectives. Several iterations were necessary to reach a mature configuration, including the interaction with the GNC unit suppliers to check performance feasibility with components-off-the-shelf. Margins have also been assumed at system level to cope with unexpected error contributors and integration problems. The paper provides the Proba-3 design overview that includes the system and hardware configuration, modes, operation sequence, and in general the team proposed approach to achieve the required millimetre accuracy. In particular, the Proba-3 propulsion system will be sized to compensate the gravity gradient while flying a not Keplerian orbit, and at the same time minimizing the attitude perturbation. The GNC will control the relative attitude within few arcseconds accuracy, while the thrusters are operated and the companion spacecraft is moving. The FF sensors metrology chain will provide relative position measurement with micrometric accuracy, while ensuring robust formation acquisition. The payload measurements will be included in the close loop control to guarantee the final scientific performances. The thermo-elastic deformation and in-flight misalignment between the attitude sensors, the metrology sensors and the payload will be measured with high accuracy and periodically calibrated in flight. The required performances can only be met through dedicated in-flight calibration of the whole metrology chain. Review of the possible calibration methods, derivation of the calibration needs on GNC, analysis of the attainable performances and definition of an on-ground and in-flight calibration plan, is also discussed in the paper.