Attitude tracking sliding mode control using extended state observer for nonparallel-ground-track imaging

A nonsingular and fast terminal sliding mode controller based on the extended state observer is designed for the nonparallel-ground-track imaging mode, whose ground targets and ground track are not parallel. First, the specific method of attitude adjustment for nonparallel-ground-track imaging satellite is studied to get the desired attitude trajectory. According to the tracking error dynamics and kinematics described by unit quaternion error, a nonsingular and fast terminal sliding mode controller is derived. Considering the disturbance rejection, an extended state observer is introduced to estimate the general disturbance which efficiently weaken the chattering inherent in sliding mode structure by reducing the switching gain. Furthermore, an adaptive fuzzy system is applied to approximate the switching term in the control law online to soften the control signal so as to effectively suppress the chattering. Finally, numerical simulations on the attitude tracking control for spacecraft in the presence of environmental disturbance and parameters uncertainties are performed, whose results show the designed control scheme can meet the requirements of control precision for the smart imaging with fast convergence speed and high control accuracy.