Orbit and attitude control for gravimetry drag-free satellites: When disturbance rejection becomes mandatory

The paper outlines orbit and attitude control problems of a long-distance two-satellite drag-free formation for the Earth gravity monitoring. Modeling and control design follows the Embedded Model Control and shows how disturbance dynamics and rejection are mandatory. Orbit and attitude control can be treated separately except in the thrust dispatching law of the all-propulsion actuation. Orbit and attitude control split into three sub-problems to be designed in a hierarchical way. In both cases the inner loop is a wide-band drag-free control aiming to zero the linear non-gravitational accelerations of the orbit control and the total angular acceleration in the attitude case. Drag-free demands for disturbance measurement and rejection by means of a specific disturbance dynamics and observer. The orbit outer loops are the altitude and formation distance controllers. The attitude outer loops are in charge of rejecting the residual drag-free bias and drift, which demands a narrow-band control suitable for star tracker measurements, and of aligning the optical axes of each satellite, which demands accurate sensor and wide bandwidth. Simulated and experimental results are provided.