## 1 Introduction  
Ground-based laser communication is the communication between two or more terminals on the ground, using the atmosphere as the medium and a laser beam as the carrier. Compared with the traditional microwave communication, laser communication offers advantages of low power consumption, high bandwidth, and strong anti-jamming ability, making development of this technology an important focus in the field of information and communication technology [1; 2; 3]. The communication distance between ground-based laser communication terminals can be extensive, the divergence angle of a laser beam is quite small, and the laser can also be affected by atmospheric turbulence, presenting several challenges to this technology [4; 5]. Therefore, to meet the power requirements of laser communication, the servo control system requires micro-radian optical axis alignment accuracy. Currently, coarse and fine two-stage tracking technology is used in most ground-based laser communication systems to ensure alignment accuracy [6; 7; 8]. Objectively, the coarse tracking control system is the most important part of the control strategy, allowing the isolation of external disturbances to ensure the stability of the optical axis, and the control accuracy determines if the optical axis can be coupled into the fine tracking field of view required for a fine tracking control strategy. A servo turntable is selected here as the actuator, with nonlinear factors such as unbalanced internal torque, friction, torque ripple, model identification error of the control system, and parameter changes, which can lead to unstable operation of the system and restricting system accuracy. Traditional linear control technologies such as use of a digital proportion-integral-differential controller (PID) and lead-lag compensation are unable to meet the accuracy requirements of a coarse tracking system, therefore, a high precision and strong robustness control technology is required to restrain the influence of nonlinear factors on the system and ensure smooth operation of the servo turntable [4]. Several methods have been proposed to suppress the nonlinear disturbance and improve system accuracy, which can be divided into three main approaches. First, the control algorithm structure and the dynamic performance of the control system can be improved by adding a current loop [9], or feedforward compensators [10; 11]. Second, the traditional control algorithm can be improved by the incorporation of aspects of modern algorithms, such as fuzzy control [12; 13; 14] or adaptive control [15; 16], which can be applied to improve the robustness of traditional algorithms. Improved algorithms can restrain the influence of nonlinear disturbance on the system, but this approach cannot overcome all the shortcomings of traditional algorithms. Third, the design of a nonlinear controller based on the modern control algorithms, such as active disturbance rejection [17; 18], adaptive [19; 20], and predictive [11; 21] control strategies can avoid the shortcomings of traditional algorithms, effectively compensate for nonlinear disturbances in the system, and improve the accuracy and stability of the system. Siding mode control (SMC) is an effective approach for a robust control algorithm, and can effectively suppress the nonlinear disturbance of a system. This anti-disturbance control algorithm can ensure that the system state reaches the sliding surface in a finite time and is stable in the quasi-sliding mode domain (QSMD). Once the system state stabilizes at the QSMD, the system state is invariant to system parameter variations and other nonlinear disturbances such as friction [21]. The narrower the QSMD width, the stronger the system robustness to nonlinear disturbances [22]. However, the direct application of the SMC algorithm to ground-based laser communication servo turntable has been a challenge.