Orbit and attitude control for gravimetry drag-free satellites

The paper outlines orbit and attitude control problems of a long-distance (>100 km) two-satellite formation for the Earth gravity monitoring. Modelling and control design follows the Embedded Model Control and shows how disturbance dynamics and rejection are mandatory to solve such problems. Orbit and attitude control can be treated separately except in the thrust dispatching law (not treated here) because of an 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 in 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 altitude and distance control to meet formation requirements. 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.