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Attaining microradian control accuracy within seconds is essential for the coarse pointing assembly (CPA) to establish and maintain the intersatellite laser link under multiple disturbances. In response to the control requirements and challenges, this article proposed a fixed-time refined disturbance observer-based composite control (FTRDOBC) scheme. First, a periscope-type CPA model is established, incorporating gimbal coupling torque, friction, and satellite-transmitted vibration to characterize the operational environment. Second, by utilizing the partially known information of the multiple disturbances, a fixed-time refined disturbance observer (FTRDO) is presented, featuring a state-dependent variable exponent coefficient auxiliary input. Combining the FTRDO with the presented time-varying fixed-time controller, an FTRDOBC scheme is proposed, aiming for high precision control in predefined time while circumventing the singularity problem. Experimental results are provided to demonstrate the effectiveness of the proposed method.

The high-precision alignment of spacecrafts subject to multiple composite disturbances [e.g., axle frictions and back electromotive force (EMF)] and mandatory pointing constraints is an extremely challenging issue for both theoretically and practically. To address this challenge, a refined deep coupled alignment control scheme is proposed in this article. First, a deep coupled alignment model of spacecraft based on the dynamics of the reaction wheel actuator is established, which reveals the influence of the composite disturbances, such as the axle frictions and the back EMF on the system, and separates the composite disturbances into mismatched and matched disturbance according to the affected channels. Second, in view of the characteristics of the composite disturbances, heterogeneous sliding mode disturbance observers (SMDOs) are designed to estimate them accordingly. Then, in order to cope with the mandatory pointing constrains, barrier functions are introduced to convert the alignment errors to limit their variation range. Finally, combined with heterogeneous SMDOs and conversion errors, a composite refined controller is constructed to finely compensate the composite disturbances in the alignment system. High-precision control performance is achieved and the mandatory pointing requirement is satisfied. Simulations and experiments verify the effectiveness of the proposed scheme.