Abstract: An attitude layered control strategy based on supertwisting observer was presented to address the noncollocated attitude control problem caused by spacecraft payload sensors and base actuators. The dynamics model of spacecraft/Stewart/payload and the equivalent dynamics model in Stewart joint space were built. The supertwisting observer was employed to estimate the spacecraft platform angular velocity. The observer inputs and output are the Stewart strut lengths and relative angular velocity，respectively. The smooth integral sliding mode control law was designed to realize a high precision payload pointing control. The payload attitude information was used by the smooth sliding mode control law. The spacecraft platform controller was designed to achieve a steady spacecraft platform attitude control. The Lyapunov stability analysis shows that the designed controller and observer can guarantee the convergence of the closedloop system. Numerical simulation results illustrate that while gyrosfree in platform，the payload attitude pointing accuracy is up to 0.1″，which is equal to that acquired by platform gyros attitude control.

Key words: ultraquiet spacecraft, observer, attitude layered control, noncolocated attitude control, pointing control