## 3 Approach
### Lars  
The mothership carrying out this mission was equipped with a davit type of LARS. Based on this LARS, we developed a special automatic docking device that solved the problems of docking between our USV and the LARS, thereby improving the accuracy and efficiency of deployment and recovery. Traditional docking devices use a rubber band to eject a rope onto the mothership, and it is necessary to manually reset the rubber band after each ejection, which is cumbersome and dangerous. The use of the rubber band is limited, especially in the environment of high temperature and humidity. Therefore, it has to replace the rubber band frequently, which greatly increases the cost of use. In addition, the elastic force of the rubber band is fixed and the ejection distance is inconvenient to adjust. The newly designed device used an ejection mechanism of the high-pressure gas type instead of the rubber band, which could flexibly change the ejection distance by adjusting the pressure of the compressed air and automatically inflating after each ejection. Compared with the previous ejection mechanism, the new ejection mechanism greatly improved the accuracy, safety, and flexibility of the whole system. The distance of the minimum ejection was 10 m, the distance of the maximum ejection was 100 m, and the inflation time was less than 5 s. The device had both automatic and manual modes. Figure 9: The flowchart of the short-range real-time collision avoidance algorithm. When in the automatic mode, the USV automatically judged the relative position with the mothership, according to the GPS information, adjusted the ejection parameters, and completed ejection. When in the manual mode, the operator can adjust the ejection parameters of the device through the console on the mothership. In practical applications, the device can safely and quickly deploy or recover 3 tons of USV (7.5 m length) under the sea state 4. As shown in Figure 10, there were three parts in the docking device: (1) The ejection mechanism: The lead and gas cannon were ejected from the USV. (2) The PTZ (Pan/Tilt/Zoom) mechanism: Adjust the launching attitude of the ejection mechanism to ensure that the gas cannon and lead can be launched to the mother ship. The PTZ structure can be adjusted between 0-70\({}^{\circ}\) and 0-180\({}^{\circ}\) in the pitch and horizontal directions. (3) The docking mechanism: The interface between the USV and the mothership, which realized the lifting of the USV. As shown in Table 2, according to the results of the sea trials, we measured the flying height and time of the gas cannon when the ejection mechanism launched at different initial angles. We focused on the recovery process because the deployment process was simple and warranted no further discussion. The recovery process is shown in Figure 11 (in the automatic mode). Step (1): The USV sails to the vicinity of the mother ship, and the USV's GPS information is transmitted to the IPC, which calculates the recovery parameters (gas pressure, PTZ mechanism angle, etc.) based on the relative positions of the USV and mothership. Step (2): Start the ejection mechanism and the lead and gas cannon are ejected to the deck of the mothership. Step (3): Slide the connector on the boom of the mothership into the locking device through the lead. Step (4): When the sensor detects the connector has fully entered the docking mechanism, the docking mechanism quickly locks the connector and then lifts the USV. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(\mathbf{\mathbf{\mathbf{\mathbf{0}}}}\) & \(\mathbf{\mathbf{\mathbf{30}}}^{\circ}\) & \(\mathbf{\mathbf{35}}^{\circ}\) & \(\mathbf{\mathbf{40}}^{\circ}\) & \(\mathbf{\mathbf{45}}^{\circ}\) & \(\mathbf{\mathbf{50}}^{\circ}\) & \(\mathbf{\mathbf{55}}^{\circ}\) & \(\mathbf{\mathbf{60}}^{\circ}\) \\ \hline Height(m) & 7.02 & 8.72 & 10.53 & 12.35 & 15.03 & 17.02 & 19.54 \\ Time (s) & 0.93 & 0.98 & 1.06 & 1.15 & 1.26 & 1.42 & 1.61 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The test results of the ejection mechanism.