Hierarchal PWA MPC-based Imaging Control with Eccentric Reaction Wheels and Magnetorquers for Very Low Earth Orbit Missions

Extremely Low-earth Imaging and Technology Explorer (ELITE) is a Very Low Earth Orbit (VLEO) satellite designed to innovatively achieve 0.5 m Ground Sample Distance (GSD) at 250 km (RGB, colour), which is highly sensitive to misalignments in satellite attitude control. However, ELITE's attitude control, already complex and computationally intensive due to its hybrid actuators and strong nonlinearity, will be further compromised by increasing disturbances from its eccentric reaction wheels and large solar panels, particularly as it descends to lower altitudes. Moreover, the centrifugal torques generated by the reaction wheels will render high-resolution imaging missions unfeasible. A novel cooperative attitude controller based on Hierarchal Piece-wise Affine MPC (HPWA MPC) is designed to distribute computational resources efficiently, enhance system robustness, and improve control accuracy. The cooperative hierarchal design involves different control periods for ELITE's hybrid actuator system of reaction wheels and magnetorquers to reduce computational complexity. The PWA model simplifies the complex non-linear system into highly accurate, discrete linear models for reliable linear estimation. A disturbance observer is proposed to provide real-time estimation and compensation for centrifugal disturbance torques to eliminate the uncertainties. The simulation results demonstrate that the HPWA MPC can meet the attitude control accuracy requirements for high-resolution imaging missions, even under increasing disturbances from ELITE's eccentric reaction wheels and large solar panels in VLEO. This enables ELITE to successfully complete its high-resolution imaging mission, highlighting the effectiveness of the proposed framework.