## 3 Methods
### Airborne End Fine Alignment Experiment  
As shown in Figure 11, it is the change of the output voltage of each quadrant of the four-quadrant detector during the flight state of the UAV during take-off, movement, and hovering. It can be seen from the figure that at the 0 sampling point, the voltage values of the first and fourth quadrants are the highest, that is, after the capture is completed, the light spots fall on the first and fourth quadrants. At sampling points 50-200, after the motor decelerates and executes the fine alignment command, the voltage changes in each  
Figure 10: Ground terminal tracking curve. (**a**) Azimuth direction; (**b**) Pitch direction. Figure 9: Schematic diagram of ground terminal alignment. (**a**) Transmitting terminal; (**b**) Upper computer interface. quadrant are relatively gentle. During the flight, the motor continuously adjusts the pitch angle and azimuth angle to complete the tracking, and the voltage value of each quadrant changes greatly at this stage; after the sampling point 3800, the UAV is in the hovering stage, and the voltage value of each quadrant changes gradually. Figure 12 shows the variation of the voltage difference in the pitch and azimuth direction of the through-hole four-quadrant detector, in which Figure 12a shows the voltage difference variation curve in the azimuth direction, and Figure 12b shows the voltage difference variation curve in the pitching direction. It can be seen from the figure that the voltage difference is too large in the range of sampling points 0-3700. At this time, the UAV is in the mobile flight stage, and the adjustment range of the two-dimensional mirror is relatively large; after the sampling point of 3700, the voltage difference is close to 0. At this time The UAV is in a hovering flight state, the adjustment range of the two-dimensional mirror is small, and the voltage difference curve gradually tends to be stable. Figure 11: Four-quadrant detector voltage variation curve for each quadrant. Figure 12: Four-quadrant detector x-y axis voltage change curve. (**a**) X-axis direction; (**b**) Y-axis direction. As shown in Figure 13, the spot position distribution calculated according to the voltage difference on the four-quadrant detector has a total of 6300 data points. As shown in Figure 14, since there is a hole in the center of the four-quadrant detector, there is no voltage data at the hole after alignment, so there is a hole in the middle of the spot position distribution, and the closer to the central through hole, the higher the alignment degree. After calculating the distance from the coordinates of each spot to the center of the detector, the pitch angle and azimuth angle errors of the two-dimensional mirror are obtained through calculation, and the tracking error of the system is calculated to be 13.98 \(\upmu\)rad(3\(\sigma\)) according to Formula (9). When the communication link distance is 10 m, the light beam is captured 10 times by using the two-dimensional mirror on the airborne end, all of which are captured successfully and the average capture time is 31.2 s. In the tracking process, the tracking errors of the spot in the azimuth direction and the elevation direction of the through-hole four-quadrant detector are calculated respectively. From the test results in Figure 15, it can be seen that the tracking mean square error of the system in the azimuth direction is 19.02 \(\upmu\)rad(3\(\sigma\)), and the tracking mean square error in the pitch direction is 22.35 \(\upmu\)rad(3\(\sigma\)). When the communication link distance is different, the signal optical power received by the airborne end is also different. As shown in Figure 16, when the communication link distance is 10 m, the optical power change curve is measured by the APD at the airborne end.