To investigate the attitude tracking control problem of rigid spacecraft under the action of external disturbances torque with known upper bound

this paper proposes a predefined-time attitude tracking control scheme

in which the tracking accuracy can be specified in advance. Firstly

a predefined-time disturbance observer which can realize high-precision online estimation for the bounded external disturbances within the specified time is designed to compensate the external disturbances using its estimated information. Then based on this observer

a quasi-terminal sliding mode and a continuous nonsingular controller is designed using terminal sliding mode control method. Lyapunov theoretical analysis results show that the upper bounds of observers

sliding modes and the controllers convergence time and the accuracy of attitude tracking all explicitly exist in the relevant parameters

so they can be easily adjusted during the design of the controller without being restricted by the initial conditions. Finally

the performance of proposed control scheme is evaluated by using numerical simulation. Simulation results show that the predefined values are loose upper boundaries

for either convergence speed or tracking accuracy

the actual performance may be far better than predefined values. Furthermore

this algorithm is also quite robust to the uncertainty of system modeling.