Quaternion-Based Tracking Control Law Design for Tracking Mode

Various control design techniques are model dependent. They typically require knowledge of the inertia matrix. There are major challenges for each proposed controller to cope with spacecraft mission objective in terms of pointing and jitter requirements. These challenges include sensitivity to noise effects and/or modeling errors, while others are sensitive to external torque disturbances, such as torques induced by solar radiation pressure. Robust controllers have been developed to mitigate these sensitivities. In this paper, a robust nonlinear tracking control algorithm introduced previously in the open literature is modified and tolerated to be utilized with exchange momentum actuators, e.g. reaction wheels. The control law is using the commanded attitude rate, commanded attitude acceleration, attitude error quaternion and gyroscopic terms. Tracking error dynamics equivalent to satellite closed-loop time-varying nonlinear dynamic system is used alternatively to confirm that a globally stable tracking controller always exists. The proposed controller is applied to meet requirements of a tracking complex mode. Generation of the needed target attitude and attitude rate are derived in details. The motion and kinematics of the ground target relative to the in-orbit satellite is analyzed and described in orbit-referenced coordinates. The satellite dynamics are derived from first principles and reformulated also in orbit referenced coordinates. A tracking scheme for the pointing axis along the body z-axis of satellite is highlighted. Considering attitude and orbit control system (AOCS) with ideal attitude and orbit determination sensors with symmetric satellite inertia, the validity of proposed controller and target data generator is demonstrated under MATLAB/SIMULINK environment. MATLAB optimization tool is used for optimal gains selection. Robustness of the globally stable modified control law to spacecraft inertia matrix uncertainty is also discussed. Simulation results show that the proposed control law can be used successfully onboard for fast tracking and is robust enough to keep the pointing accuracy within acceptable limits with considerable inertia uncertainty