ADCS Preliminary Design and Simulation of a Tethered Satellite System with Tether-Torque-Based Control for Earth Observation Missions

This article presents the preliminary design of an \gls{ADCS} for a tethered satellite system, with the main objective of demonstrating that tether-induced torques can be effectively exploited as external actuation for spacecraft attitude control, by modifying the position of the tether attachment point on the spacecraft’s side. The system under consideration consists of two SmallSats connected by a flexible tether, a configuration which enables new mission concepts that would otherwise be impractical or infeasible, particularly for multi-angle Earth Observation applications, such as radar interferometry for maritime surveillance and disaster monitoring. Such missions demand high pointing accuracy, while the presence of the tether introduces strong dynamic coupling between the attitude dynamics, the tether tension and the orbital motion of the system’s centre of mass, significantly increasing the complexity of the attitude control problem. For these reasons, the ADCS represents one of the most critical and challenging subsystems for spacecraft design, as it directly affects pointing accuracy and overall mission performance and capabilities. This study implements the roto-translational motion of the tethered satellite system in Low Earth Orbit, by using quaternion-based attitude kinematics coupled with the relative dynamic equations and a tether model developed following the Kelvin-Voigt formulation in order to captures tether’s dynamics. Based on this model, a quaternion-based PID feedback control law is designed to achieve precise control and ensure convergence to the desired attitude. The torque generated by the PID law is implemented through the tether by actively shifting the tether attachment point on the spacecraft’s surface, thus generating the required control torque. A Multiplicative Extended Kalman Filter (MEKF), which is particularly well suited for quaternion-based attitude estimation, is designed for discrete-time applications. The filter processes noisy measurements generated by simulating realistic onboard sensor errors, with noise parameters derived from a state-ofthe-art study. To ensure physical realism and to model actuator saturation, a maximum velocity constraint is imposed on the attachment point in order to account for the actuator’s physical limitations and finite response capability between discrete time steps. The complete system model and control strategy are implemented on MATLAB to evaluate and to test the proposed control approach in discrete time framework. Simulation results demonstrate the feasibility and effectiveness of using tether-generated torques to control the attitude under different initial conditions, manoeuvre scenarios and system sizes