# High-Precision Tracking of Free-Space Optical Communication System on Mobile Platforms
## 4 Field Experiments  
We did field experiments in 2023 to prove that our system is feasible. Before the tracking experiment of the moving platform, we first did the preliminary experiments of single-ended swing tracking under static platform conditions. The two sets of FSO communication systems are placed at two ends 20 m apart. Under the conditions of initial alignment, the transmitter device is controlled to swing periodically between two points with different horizontal and vertical coordinates. At the receiving end, the CCD of the system collects spot off-target position information at a frequency of 200 HZ; FSM1 and the electro-optical pod compensate this spot offset at a frequency of 200 HZ and 50 HZ, respectively. Figure 9 shows the results of tracking experiments under single-end swing conditions. When the transmitter device swings periodically between two fixed orientations, the  
Figure 8: (**a**) Camera positioning error without correction. (**b**) Camera positioning error after matrix correction. Figure 7: Spot coordinates obtained when FSM1 scans the entire camera field of view in periodic motion; below the surface is a contour line of spot coordinates. (**a**) Spot x-coordinate surface plot. (**b**) Spot y-coordinate surface plot. coordinates of the receiver device also swing periodically between two fixed orientations under the control of the tracking program in order to align with the transmitter all the time (Figure 9a,b). In the whole process mentioned above, the value of spot off-target collected by CCD at the receiving end is within \(\pm\)0.6 mrad (Figure 9c,d), that is, the receiver points to the transmitter correctly during the entire single-end swing experiment. In addition, the value of spot off-target is stable within \(\pm\)0.3 mrad most of the time, and the value greater than \(\pm\)0.3 mrad appears in the form of periodic burrs. This is because when the position of the transmitter changes, the position of the spot collected by the CCD of the receiver will also appear to have a large offset. In addition, an overshoot phenomenon will occur when FSM1 and the electro-optical pod are controlled to compensate for the offset. Therefore, with the periodic position of the transmitter, the spot-off-target position collected by CCD also appears periodic burrs. After the static tracking experiment, we carried out a dynamic experiment with the cart. Figure 10 shows the dynamic experiment on the cart. Two sets of the FSO communication systems were placed on carts 100 m apart. The signal sent by the transmitter is a pseudo-random code generated by the FPGA and encoded using 8 B/10 B code to prevent channel vacancy duration. After the systems are turned on, the carts are kept still, and the system at both ends will automatically scan and align with the opposite system under the control of the tracking program. When the optical power at both ends is stable and the system can communicate normally, it indicates that the two devices are successfully aligned. After that, the cart at the transmitter remains motionless, and the cart at the receiver is pushed back and forth to test the dynamic tracking performance of the system. Figure 10: Dynamic tracking experiment using cart. Figure 9: Horizontal and vertical data are measured simultaneously in single-ended swing tracking experiments. (**a**) Horizontal direction of receiving device. (**b**) Vertical direction of receiving device. (**c**) Horizontal coordinates of spots collected by CCD. (**d**) Vertical coordinates of spots collected by CCD. Figure 11 shows the test results of the receiver system in the dynamic tracking experiment on the cart. It can be seen from the figure that due to the movement of the cart, the FSOC system must change the direction of the beam to ensure alignment of the optical link at both ends, so the coordinates of the receiver system change with the movement of the cart. The change trend of the horizontal coordinate is consistent with the movement trend of the cart first pushing forward and then pulling back.