# High-Precision Tracking of Free-Space Optical Communication System on Mobile Platforms
## 2 Design of FSO Communication System for Mobile Platform  
We propose a new FSO communication system that has the high-precision tracking capability of about 1 mrad and the high data rate of about 1 Gb/s between moving platforms. Figure 1 shows the architecture of an FSO communication system based on an electro-optical pod and a fast steering mirror (FSM). The FSO system is composed of an electro-optical pod module, a precise targeting load module, and a communication processing module. The system adopts coarse-precise composite axis tracking technology. The electro-optical pod module is responsible for scanning and acquisition as the coarse sight structure. The precise targeting load module is used as the precise sight structure for pointing and tracking, which is installed inside the electro-optical pod module. The detachable communication processing module, which can receive, transmit, encode, and decode data, is located at the bottom of the electro-optical pod module. The electro-optical pod can acquire the target through image recognition in the field of view according to the scan-specified path, complete rough alignment of the visual axis, and extract light spot information. Immediately after initial capture is completed, a laser beam will pass through the upper window of the electro-optical pod module into the precise targeting load module. Precise targeting loads first collect the laser spot information through an internal sensor and calculate the offset of the visual axis. Based on the offset of the visual axis at the current moment, the offset at the next moment will be predicted. Then, coarse and precise structures are controlled to compensate for the offset every 20 ms. Finally, high-precision alignment of the visual axis is realized, and stable communication of the spatial optical link is guaranteed. Figure 2 shows the design of the precise targeting load module. The module adopts a transceiver coaxial design. As shown in Figure 3, receiving and transmitting optical paths share a primary mirror system, and they are separated and coupled through dichroic mirrors. The design of the primary mirror system is shown in Figure 4a using the Ritchey-Chretien (R-C) afocal telescope. Figure 4b shows the design of the tubular folding optical path inside the module. All optical components are folded around the R-C primary mirror, effectively reducing the volume and weight of the load. Figure 1: Design of the high-precision FSO communication system for mobile platforms. The precise targeting load module is located in the spherical turret of the electro-optical pod module. The spherical turret has image recognition, can be rotated \(\pm\)45\({}^{\circ}\) around the horizontal axis, and covers \(\pm\)45\({}^{\circ}\) of field of view in the pitch flat. (**a**) Overall appearance of the system. (**b**) Explosion view of the system. Figure 2: Design of precise targeting loads module. The R-C primary mirror is coaxial with the upper window of the electro-optical pod, which is used for optical communication (shown in Figure 3). R-C telescope system concentrates on receiving beam and diverges transmitting beam. Both FSM and planar reflectors are used to turn optical paths. In addition, FSM can adjust the direction of the optical path. Dichroic mirrors are highly reflective of the receiving beam and highly transparent to transmitting light. A beam splitter separately divides a part of receiving light to CCD for collecting spot offset. The control unit will adjust the deflection angle of FSM1 (precise sight) and the deflection angle of the electro-optical pod (coarse sight) according to this spot offset. As shown in Figure 3, inside the precise targeting loads module, the received light is first focused by the RC telescope system and then then changed direction through FSM1. After that, part of the receiving beam is split to the CCD for collecting the optical axis offset when passing through the beam splitter.