Ultra-low-orbit Spacecraft Attitude Control Based On Sliding Mode Control

Ultra- low-orbit spacecraft had advantages in observing the earth and other fields for its orbit altitude was as low as 120-300 km, above the earth's surface. There exist much atmosphere molecular in ultra low orbit, so the effect of atmosphere on spacecraft should not be ignored, for it may lead to instability of the spacecraft's attitude. Disturb resistance of attitude control system of was the basic qualification for ultra- low-orbit spacecraft. Sliding mode control had a strong robustness to external disturbance for its control law was incontinuous. Based on sliding mode control, this paper studied the attitude control method of ultra- low-orbit spacecraft.The first part of the work studied the attitude control model of ultra- low-orbit spacecraft. Firstly, based on description in quaternion, the error dynamics and kinematics equations of spacecraft attitude were derived. Then, the aerodynamic moment and was calculated based on its appearance to make the equations reflect the ultra- low-orbit characteristics. The second part researched on sliding mode method of ultra- low-orbit spacecraft's attitude control. Lyapunov theory was selected as the guidance of control method design. Sliding mode method for attitude control without any constraint conditions was designed. Then, attitude control method with input constraint was given. Finally, this paper put forward a terminal sliding mode method to make the attitude control system converge in a finite time.Each method's stability was proved by using the Lyapunov theory. The simulation results provided by MATLAB/Simulink verified the effectiveness of each method, indicated that the demand can be fulfilled. In conclusion, this paper's work was valuable to theory research of ultra- low-orbit spacecraft.