# Research on Bandwidth Improvement of Fine Tracking Control System in Space Laser Communication
## 1 Introduction  
Currently, the demand for satellite observation data is on the rise, and satellite networking is being widely discussed. Traditional satellite communication methods are constrained in terms of device size, weight, volume, and communication rate, and the radio spectrum is becoming more congested due to the increasing number of satellite launches. As a result, space laser communication is considered an effective solution for satellite communication [1, 2, 3]. For example, SpaceX's Starlink project is a satellite Internet service that aims to provide high-speed Internet connectivity worldwide by deploying a large network of satellites. In September 2021, the Starlink project was launched from Vandenberg Space Force Base in California aboard a Falcon 9 rocket. It carried Internet satellites equipped with laser links for satellite communications into orbit. The Tokyo University of Aeronautics and Astronautics also proposed the concept and conducted research on laser communication between a 100 kg low-earth-orbit (LEO) satellite and a geostationary-earth-orbit (GEO) satellite. They demonstrated the feasibility of transmission from a 100 kg LEO satellite to a GEO satellite through link budget analysis [4]. Therefore, laser communication is a highly researched topic in the field of satellite communications.  
To ensure the stability of the laser communication link, it is necessary to accurately align the beams of the two communicating sides. The required alignment accuracy is typically in the order of \(\upmu\)rad. Laser communication usually employs a compound-axistracking system, which combines a coarse tracking system and a fine tracking system to meet the accuracy requirements. The coarse tracking system is based on a servo turntable and possesses characteristics of a wide dynamic range; it primarily suppresses the disturbance of low frequency and high amplitude of the beam. The fine tracking system is based on an FSM and exhibits fast response characteristics. It is primarily used to compensate for the residual errors of the coarse tracking system and to suppress disturbances in the middle- and high-frequency ranges. The higher the bandwidth of the fine tracking control system, the better the dynamic target tracking ability and the stronger the stability of the compound-axis system. It has been proven that in order to fully take advantage of the compound-axis system, the closed-loop bandwidth of the fine tracking system should be six to ten times the closed-loop bandwidth of the coarse tracking system, or even higher [5]. Therefore, improving the bandwidth of the fine tracking system is beneficial for enhancing the overall dynamic performance of the compound-axis system. PZT FSM is the core component of the fine tracking system, possessing a high resonant frequency and excellent dynamic performance. Consequently, PZT FSM is also widely used in other dynamic target tracking fields. However, the piezoelectric actuator of PZT FSM exhibits nonlinear characteristics such as hysteresis, which can constrain its performance. Therefore, this paper primarily focuses on studying hysteresis compensation for PZT FSM and its impact on improving the bandwidth of the fine tracking control system.