Drag-free and attitude control for long-distance, low-Earth-orbit, gravimetric satellite formation

Following the launch in Spring 2009 and operations of the GOCE satellite (Gravity field and steady state Ocean Circulation Explorer), a new mission made by a long-distance (10 km) satellite formation is under study by European Space Agency. It aims at monitoring, during a 6-year mission, Earth's gravity field fluctuations more or less in the same wavelength band of GOCE (around 100-km Earth surface grid). As GOCE is the first 'truly' drag-free satellite, the envisaged formation is expected to be the first drag-free formation, posing a suite of challenging technology and control problems under study and solution, e.g. authors have demonstrated the mission must be all-propulsion. The paper concentrates on a triad of control problems to be solved and coordinated, namely formation, drag-free and attitude, all of them being highly constrained by a long-life low-Earth-orbit mission imposing low propellant mass and therefore electric propulsion, by scarce propulsion throttability of electric thrusters and limited electric power. Requirements will be presented and how they can be met through Embedded Model Control will be outlined. Realistic simulated results are included.