On-orbit evaluation of variable-shape satellite attitude dynamics under atmospheric drag torque and gravity gradient torque

Abstract

In low-Earth orbit, aerodynamic orbit control can be achieved by changing a satellite’s cross-sectional area or angle of attack. For this purpose, the satellite is required to maintain a prescribed attitude with respect to the atmospheric flow. However, aerodynamic forces can cause perturbing torques if the satellite’s attitude is not aligned with the atmospheric flow, which can make it difficult to maintain the target orientation. This issue is particularly pronounced for satellites with large cross-sectional areas or those in low-inclination orbits, where the directional limitations of magnetic torquers are more severe. To address these limitations, we propose an attitude control system that utilizes a variable-shape satellite and equilibrium points. This method enables the equilibrium point to be shifted to an arbitrary attitude by dynamically changing the satellite’s shape. Consequently, disturbance torques can be minimized at the desired attitude, expanding the applicability of aerodynamic orbit control to a wider range of satellite sizes and orbital conditions. This study evaluates attitude dynamics around the equilibrium point to demonstrate the feasibility of the proposed system in an actual orbital environment. The proposed strategy is demonstrated on orbit via the HIBARI satellite, a previously developed 50-kg-class microsatellite, and the experimental results are discussed. By changing the satellite’s shape and observing its attitude motion without active control on orbit, the effects of shape change on attitude dynamics are evaluated and disturbance-related parameters are estimated. Furthermore, the residual magnetic moment is identified as the primary factor that disturbs the ideal oscillatory motion around the equilibrium point. Under the assumption that the residual magnetic moment is properly managed, the attitude dynamics around the equilibrium point are evaluated based on on-orbit data. The results show oscillatory motion around the equilibrium point, demonstrating the applicability of the proposed system in orbit.