# High-Precision Tracking of Free-Space Optical Communication System on Mobile Platforms
## 4 Field Experiments
According to the change of horizontal coordinate and the duration of change in Figure 11a, the average speed of cart in two dynamic processes can be calculated as 2.11 m/s and 1.85 m/s, respectively. During the experiment, because the ground is relatively flat, the change of vertical coordinates is caused by the vibration noise in the movement, so its amplitude is small, and the overall change trend is not large. The spot offset obtained by CCD is within \(\pm 1\) mrad (Figure 11c,d). Compared with the experiments of single-ended swing tracking, the spot offset in the dynamic tracking experiment is significantly increased; that is, the system alignment accuracy is reduced. This is caused by the dynamic lag error and vibration error introduced in the movement of the platform. Figure 12 shows the data collected by the photoelectric detector in dynamic tracking. This value is positively correlated with the light intensity received by the system. It can be seen from the figure that the fluctuation range of the collected data is only about 3 dB, that is, the light intensity fluctuation in the dynamic tracking process is less than 3 dB, so the intensity fluctuation caused by the dynamic tracking process has little impact on the system performance. Figure 13 shows the eye diagram in dynamic tracking. The fluctuation of optical power due to the alignment error in the tracking process will lead to the deterioration of the signal-to-noise ratio (SNR). This problem can be solved by designing specific algorithms or adding filters of specific frequencies to hardware. Due to the difficulty of the design and engineering application of the adaptive Zoom FSO Communication System, the typical space-borne optical communication system is fixed focus design. Typical space-borne optical communication systems are designed with fixed focus optics. In fact, the divergence angle, communication distance, transmitting power, and signal-receiving processing algorithm of the FSOC system jointly determine the bit error rate (BER) and SNR of the system. However, theoretically, under the same conditions, the larger the divergence angle, the more serious the deterioration of the system SNR and BER. Figure 11: Horizontal and vertical data are measured simultaneously in tracking experiments between moving platforms. (**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. We also did a vehicle-mounted experiment, but it failed. On the one hand, in the tracking process, the control unit of the system will predict the position of the light spot at the next time according to the current collected spot position data to achieve low error compensation. However, in the process of the dynamic test, the lateral vibration amplitude of the vehicle is large because the vehicle cannot drill holes to install shock absorbers. The vibration in the motion of the vehicle is completely random. It cannot be compensated by prediction but also increases the error of the original spot prediction position as noise. All these lead to the deterioration of the tracking performance of the system. On the other hand, due to the limitation of the experimental site, the communication distance is relatively close, and the relative motion speed is relatively large, which makes it easy for the spot to break away from CCD's target surface and make tracking fail. We are now improving the system to achieve a longer distance tracking alignment and link communication of the FSO communication system under vehicle-mounted experimental conditions. In order to expand the application scenarios and conditions of the system in the future, we will also study the influence of the complex atmospheric environment on the performance of laser communication systems and its improvement methods.