# High-Precision Tracking of Free-Space Optical Communication System on Mobile Platforms
## 3 Prototype of FSO Communication System for Mobile Platform
In addition, there are sensors at the receiving end for collecting light intensity information. The control module will calculate the FSM2 control instructions in real time according to the collected light intensity information [17], so as to avoid the constant drift of FSM2 and ensure the coaxial axis of the receiving end. Finally, there is translation and rotation when the camera is assembled, so there is rotation deviation \(\theta\) and translation deviation \((a,b)\) between the camera coordinate system \(xoy\) and the precise targeting load coordinate system \(x^{\prime}oy^{\prime}\) (that is, the FSM1 coordinate system). This relationship can be expressed by the following formulas:  
\[x^{\prime}=x\times cos\;\theta-y\times sin\;\theta-a \tag{1}\]  
\[y^{\prime}=x\times sin\;\theta+y\times cos\;\theta-b \tag{2}\]  
Therefore, we can use the parameter array \((sin\;\theta,\;cos\;\theta,\;a,\;and\;b)\) to describe this deviation. In addition, since the spot deforms at the edge of the camera, the deformation can also be corrected according to the coordinate translation and rotation. Hence, we block the camera's field of view, and each module uses a different array of parameters to correct for errors caused by translation, rotation, and distortion. In the case of an external light source, we control the FSM1 to carry out two-dimensional periodic motion in small steps to scan the entire field of view of the camera. As shown in Figure 7, we will obtain the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Elements** & **Parameter** & **Specification** \\ \hline \multirow{4}{*}{Laser communication device} & Wavelength & 1540.56/1563.05 nm \\  & Transmitted optical power & +33 dBm \\  & Received optical power & \(\geq\)\(-\)30 dBm \\  & Field of view of FSM & \(\pm\)26.2 mrad \\  & Field of view of electro-optical pod & \(\pm\)785.4 mrad \\  & Laser divergence angle & 0.5 mrad \\ \hline \multirow{4}{*}{R-C afocal telescope} & Primary mirror diameter & 60 mm \\  & Secondary mirror diameter & 12 mm \\  & Multiple of primary mirror & 10 \\ \hline \multirow{4}{*}{Camera} & Field of view & 3 mrad \\  & Spot diameter on target surface & 100 um \\  & Pixel size & \(5\times 5\) um \\  & Resolution & 320 (H) \(\times\) 320 (V) \\ \hline \multirow{4}{*}{FSM} & Mirror dimension & \(\varphi\)20 \(\times\) 3 mm \\  & Angular resolution & \(\leq\)2.4 urad \\ \cline{1-1}  & Angular acceleration & \(\geq\)10,000 rad/s\({}^{2}\) \\ \cline{1-1}  & Mirror reflectivity & \(>\)99.5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Prototype specifications. spot coordinates that correspond one-to-one to the FSM1 coordinates, which are calculated from the spot information collected by the camera. The parameter array of error correction can be obtained after nonlinear curve fitting for the data of different modules. During the tracking process, we will use the FPGA in the control unit to calculate corrected spot coordinates in real time through the correction array, which will correct the positioning bias of the camera. As shown in Figure 8, after correcting the spot coordinates collected by the camera, the positioning error of the camera edge is greatly reduced, and the positioning overall error of the camera is reduced by one order of magnitude. In the above way, we completed the coaxial calibration of the camera system for beam positioning. Thus far, the coaxial debugging of the whole system has been completed.