# High-Precision Tracking of Free-Space Optical Communication System on Mobile Platforms
## 1 Introduction  
Free-space optical (FSO) communication has the advantages of a high communication rate, large communication capacity, and strong anti-interference ability. It is an important way of wireless high-speed communication and broadband information transmission in the future, and it is one of the most potential communication methods [1; 2; 3; 4; 5]. Because the geometric path losses increase as the beam divergence increases, FSO communication usually uses narrow beams, where the divergence angle of the transmitted beam is less than or equal to milliradians (mrad). The use of narrow beams increases the receiving power density and improves the link margin in various weather conditions, but because of the small footprint of the beam at the receiver, its alignment is also more difficult [6; 7; 8; 9; 10]. When at least one of the communication parties is mobile, it becomes more difficult to meet this demand. At present, many research studies increase the acquisition, pointing, and tracking systems to overcome the influence of relative motion and platform vibration on the alignment accuracy of the visual axis and control the tracking alignment error of the visual axis within the allowable range [11; 12; 13; 14; 15]. A mirror-based acquisition, pointing, and tracking (ATP) system is proposed. The results of field experiments show that the FSO system based on the mirror ATP mechanism can maintain the optical link between the ground and the vehicle traveling at 270 km/h for more than 6 ms [16]. Ref. [17] presents a scheme that uses a simple scan mirror and receiver with a single photo detector to achieve tracking function. This scheme can track targets at a speed of 40 mm/s within a 2 m distance in an angle range of \(\pm 30^{\circ}\). However, this study only completed the principal verificationbut did not complete the system development. A MEMS retroreflector mirror as the main component of an ATP mechanism on an unmanned aerial vehicle (UAV) is proposed for an air-to-ground FSO communication system. The ground station of this FSO communications system consists of a gimbal and an FSM. The hybrid ATP mechanism on the ground, using a gimbal and FSM, provides coarse and fine pointing. It is capable of positioning the beam at 0.0064\({}^{\circ}\) resolution to track the UAV [18]. However, the research has only completed the development of the subsystem, and the test of the integrated system has not been carried out. In addition, a unique hierarchical two-level technique of pointing control system is proposed in Ref. [19] for an intersatellite laser communication and ranging link. The lower level controls the gimbals of the optical head of the electro-optical transceiver; the higher level is a fast closed loop that simultaneously controls the beamwidth and direction. Representative simulation results are given in this paper, but there is still a long way to go before the scheme is actually put into use. In order to reduce the volume and weight of the FSO communication system, a ground-to-train FSOC system based on a mirror actuator is designed [16; 20]. Thanks to the beacon light and quadrant photo detector (QPD), it can track the target within \(\pm\)20 degrees. This scheme removes the mechanical stabilization system and camera tracking system but still reserves the beacon system. Thus, the volume finally reduces to a certain extent. Ref. [21] proposes a relatively simple system with about 20 degrees receiving angle, which removes the beacon system and reserves servo motors. Compared with Ref. [16], this scheme has a wider tracking range but slower tracking speed. With the development of free-space optical communication technology, its application range and market are more and more extensive. This increase in applicability is particularly evident on mobile platforms. Therefore, a flexible new FSO system is needed that can maintain the optical path connection between these transceivers on mobile platforms. In this paper, an FSOC system based on electro-optical pod and fast mirror is proposed and implemented.