## 2 System Working Principle
### Alignment Principle  
Traditional wireless optical communication systems require the transmitter and receiver to be aligned on the same boresight, and the alignment principle is shown in Figure 2a. In the wireless optical communication link, the position information of the transmitting end and the receiving end is obtained utilizing the global position system (GPS) positioning system, and the position coordinates are converted into the servo execution amount of the APT system in the pitch direction and the azimuth direction, and then it is sent to the other side through the radio frequency signal for adjustment. Before controlling the alignment of the beam, the transmitting antenna needs to be controlled to scan due to the uncertain region between the transmitting antenna and the receiving antenna. When a light spot appears on the surface of the position detector (PSD) at the receiving end, the scanning is stopped, and the position information of the light spot is fed back to the transmitting end, and the transmitting end adjusts the deflection angle of the servo mechanism according to the received signal to control the beam to complete the alignment. When disturbed by atmospheric turbulence and mechanical vibration, the method of transmitting spot position information to the transmitter is still used to control the beam for tracking. It can be seen that, in the workflow of the traditional wireless optical APT system, the sending and receiving parties need to continuously transmit position signals. When the distance of the wireless optical communication link is large, it will be affected by atmospheric turbulence in the process of signal return, which is prone to signal delay, causing the sending and receiving parties to be unable to receive the spot position information at the current moment in time, and eventually lead to the interruption of the wireless optical communication link, greatly increasing the time and uncertainty required to establish the wireless optical communication link. In the airborne laser communication system, due to the strong mobility of the flying platform, it is impossible to establish a wireless optical communication link using the traditional common-view axis alignment method. The method proposed in this paper for non-common-view axis alignment using a two-dimensional mirror is shown in Figure 2b. This method decomposes the traditional long-distance alignment of wireless optical communication into two parts of the alignment system, which are the coarse alignment between the tracking camera and the two-dimensional mirror at the airborne end and the fine alignment between the two-dimensional mirror and the four-quadrant detector [16]. The transmitter keeps the drone always in the center of the camera's field of view by driving the servo mechanism. At this time, after the laser is collimated and emitted by the transmitting antenna, it covers the surface of the airborne end, and the long-distance coarse alignment from the ground end to the airborne end is completed. When the airborne end receives the optical signal, the two-dimensional mirror starts to scan and capture the program. When the light spot information appears on the PSD device, the motor decelerates and adjusts until the light spot is located at the center of the position detector to complete the fine alignment. Finally, the light beam is focused on the surface of the photodetector(APD) behind the through-hole 4-QD in the form of a point light source, the six-pointed star pattern is used to simulate the position of point light source, and the light beam is sent back from the original optical path to the ground end for reception through electro-optical conversion. During the alignment process, since the two can be operated at a single end, it effectively solves the problem that the traditional alignment method needs to transmit the position information of the sender and receiver through radio frequency signals, and the operation is simple and the complexity of the system is reduced.