ESA GNC Conference Papers Repository

PROBA-3 is ESA's first high-precision formation flying mission. It is conceived as a small-scale technology demonstrating mission, consisting of two spacecraft flying in close formation in a highly elliptical orbit. The scientific objective is to produce a Sun coronagraph instrument with the optical payload and the external occulter disc being distributed on separate spacecraft, demonstrating all the associated technologies necessary for achieving high-precision formation flight. One of the main challenges is to develop a GNC that is able to maintain a time-varying formation during the apogee arc with very stringent attitude and relative position requirements. In addition to the scientific mission, various demonstrating manoeuvres will be performed. Among others this includes virtual structure simulation (rotating the formation, while maintaining a fixed distance), and virtual telescope focusing (resizing the formation, while keeping the Sun vector aligned). From a control point of view, the technological challenge is to utilize the state-of-the-art (structured) H8- synthesis and µ-analysis techniques in order to obtain a 6DoF multi-input multi-output (MIMO) controller which simultaneously takes care of both the attitude and the position meeting all the stringent performance specifications while guaranteeing stability and robustness. This paper presents how the formation flying control has been designed, justified and validated. In particular, it elaborates on how the H8-synthesis tools can be systematically applied to obtain the high precision controller. This includes the formulation of a fully dynamically coupled weighted MIMO plant that is compatible with the designated synthesis tools. A concise validation based on linear and nonlinear frequency- and time-domain analysis techniques will be provided to demonstrate the power and effectiveness of the tools.