ESA GNC Conference Papers Repository
Dynamics and Aerodynamic Control of a Cross-Track Tether Satellite System
The objective of this paper is to present and analyze the dynamics and control strategy of a novel type of formation flight architecture, that could enable unprecedented space mission capabilities. In particular we propose and study a tether satellite system composed by two or more satellites, flying in LEO, connected by a linear tether. That tether is maintained in tension along the cross-track direction, with respect to the orbit, i.e., perpendicular to the motion and radial directions, by exploiting the aerodynamic lift acting upon the suitably oriented satellites. Possible applications of this formation are related, in particular, to remote sensing. To carry out the study, a three-dimensional multibody model was developed, introducing the equations of relative dynamics centered in an orbiting reference frame. To simulate the conditions of low Earth orbit, several mathematical models taken from the literature were used, in order to include the Earths gravitational potential up to fourth order, solar pressure, third-body perturbations, atmospheric drag and the behavior of aerodynamic surfaces in a free molecular flow. By utilizing this model and verifying that this configuration is intrinsically unstable, it was concluded that the system needs continuous control to be stabilized. As a result, the model of the dynamics was linearized and an optimal LQR controller was introduced to calculate the force required to hold the system in place. Once the linear control law was validated by simulating its application on the full system model, aerodynamic surfaces were next introduced in order to generate the force required for control by taking advantage of the rarefied atmosphere of low Earth orbit. The proposed control strategy is tested by extensive numerical simulations. Finally, the proposed aerodynamic stabilization approach to continuously maintain a linear tether satellite system along the cross-track direction was compared to a gyroscopic stabilization alternative strategy. This strategy consists of putting the system into rotation in a direction perpendicular to the cross-track direction. The centrifugal force generated by the rotation stabilizes the system, maintaining constant oscillations in the cross-track direction throughout the orbit. Both cases have been analyzed as the size of the system and the number of satellites change.