## 3 Multi-Ship Collision Avoidance Decision Making Based on Full Communication
### Intention-Based Ship Collision Avoidance Processing  
Due to their huge inertia and under-actuated characteristics, the CA process of ships presents a distinctive feature compared with railway and road traffic. First of all, the huge inertia of ships determines that all emergency and rapid maneuvers made by the encountered parties during the CA process will be affected and cannot achieve the expected goal. Secondly, the under-actuated characteristics of ships determine that all behaviors in the CA process need to be maneuvered in advance and achieve the desired state through  
Figure 3: Blockchain data structure design for ship collision avoidance communication. time accumulation. This leads the bridge team to draw up a reasonable CA decision as the first step of the CA process and then make the ship reach the planned state through a series of CA maneuvers. If the CA decision is reasonable and the CA maneuver is effective, the CA will be successful. Otherwise, ship collision accidents will occur due to various reasons. In practice, the CA process can be summarized into a logical structure with internal and external layers, that is, the hidden CA intention and the shown layer of CA decision/maneuver. The content of the communication between the encountered ships is mostly the general CA intention rather than the detailed CA decision. Then, the encountered ships conduct specific CA maneuvers according to their respective CA intentions. However, this may cause a problem where ship collision accidents are very common [17], especially due to the deviation or complete misunderstanding of each other's CA intention during communication. A previous study [15] studied 50 serious ship collision accidents around the world and found that the main or important reason for 37 of the accidents was communication failure among the ships (for 17 cases, this was the main reason, and for 20 cases, this was one of the important reasons). The process of ship CA often lasts for a long time. During the whole process, the movement of the ship is manifested as the accumulation of small changes in the slow-motion state. This often involves multiple CA communications and changes to the own ship's (OS's) CA decision according to the situation of the other ship. Therefore, the process of ship CA shows the characteristics of highly coupled CA decision-making between the encountered ships. In short, the observable CA maneuver behavior and the hidden CA intention together constitute the internal and external logical structure of the ship CA process. In the repeated communication and exchange of opinions between the encountering ships, they influence each other's CA intention and CA decision-making and finally avoid collision. Figure 4 illustrates the process of path target determination and the two-stage CA process in a two-ship encounter situation. Generally, the ship CA process can be divided into two stages, i.e., the extra trajectory to avoid collision and the stage of returning to the original course after passing the encounter region. According to COLREGs, the give-way ship (Ship A in Figure 4) needs to turn right to clear the region ahead of the stand-on ship (Ship B), while the stand-on ship should keep its course and speed to pass the encounter region from the foreside of the give-way ship and keep vigilant in case the give-way ship does not take measures in time. Therefore, the path target of the giving-way ship is set outside the ship domain of the stand-on ship at the CPA. When the give-way ship passes through the path target, it will enter the second stage and return to its original course. The stand-on ship should pass the encounter region at the original speed and heading. During the CA process, the repulsion field (SRF in Section 3.2) around the ships plays an insurance role in avoiding encountering distances that are too close. Figure 5 illustrates the process of path target determination and the two-stage CA process in a multi-ship encounter situation.