## 1 Introduction  
The unmanned aerial vehicle (UAV) photoelectric detection system (hereinafter, the system) is a typical type of device widely used in both military and civilian fields. Specifically, the system can be used to transfer target information and images among the fields, client-server stations (CS), and the satellite situation center (SSC). The typical application scenario is shown in Figure 1. This kind of system is especially used for detecting purposes such as target search, identification, localization, tracking, damage effect feedback, etc. Furthermore, they have been playing the roles of information transfer centers, and they are widely considered to be the most critical device for battlefield scouting based on UAV. The field of sight (FOS) can be considered to be the vision plane chosen by the UAV operator. FOS is chosen to observe the target properly, and it is considered temporally stationary relative to the ground (or to the inertia space). The line of sight (LOS) is defined as the central axis of the system camera. Ideally, to obtain clear images, LOS should always be kept vertical to FOS. However, there are various disturbances in the working process, and those disturbances will result in angular deviations of LOS in real time. Therefore, the stability of LOS is a fundamental basis for proper working. Over a certain period time of the working process, the intention of stabilizing the LOS or the system is to minimize the angular deviations of LOS relative to FOS. The index to evaluate the stability performance of the system is defined as stability accuracy. It is treated as the focus of this research, and will be defined and analyzed later in detail. On the other hand, those disturbances are considered to be casual elements, and cannot be easily predicted, because they are often caused by movements of UAV and environmental factors such as wind and noises, etc. From the perspective of product mechanism design, the system is often divided into four major parts: photoelectric and infrared detectors, which are used to capture vision images; multi-axis frame (or gimbal), for supporting physical frames and compensation movements of axial rotations; drive, control panel, and related system interfaces, for controlling the motions of motors on axes and transferring data; and passive vibration damper, for dissolving some of the disturbances. The typical composite of the system is shown in Figure 2.  
As mentioned, the stability can be affected by various unpredictable factors. Therefore, the servo-control of the system is the core for realizing stability. The overall structure of the servo-control functions shown in Figure 3 mainly consist of loops of stabilizing, tracking, and positioning. The control loops can correspond to the system's working modes of stabilizing, tracking, and positioning, respectively. The working modes can be selected and switched by the operator. Figure 1: Typical application scenario of UAV photoelectric detection system. Figure 2: Typical composite of the system. Based on the commonly used mechanism structure shown in Figure 2 and the servo-control structure shown in Figure 3, the working modes can be explained as follows: while the system is working on the stabilizing mode, the gyroscopes are used to measure angular velocities coupled to the system in real time. The velocity data will be transferred by the system interfaces. Then, the velocities will be compensated by the motor motions of the stabilizing loop, ensuring the stability of LOS to obtain clear images. In addition, the interface performance of the system is very important, both for data transmission speed and even overall system performance. The reasons for interface failure include damages or wrong assembly of hardware, incompatibilities of data formats, and inferiority of control algorithms. The failures of interfaces will directly lead to losing real-time states, and therefore the overall failures of servo-control functions.