## 2 Overview of Autonomous Decision-making System  
With the aim of improving the AUVs' autonomous ability, this study proposed and developed the autonomous decision-making system (ADS) of AUVs. As shown in Figure 1, the proposed ADS comprises five closely related components, which includes the decision layer, the navigation layer, the control layer, the monitoring unit, and the communication unit.  
The decision layer is responsible for advanced decision-making and task management, such as task scheduling, emergency control, and permission management. The navigation layer provides path planning, environment mapping, and situation awareness to support autonomous navigation. In the navigation layer, the AUV uses the electronic chart data to conduct the online path planning under the multi-constraints, and according to the real-time data obtained by various sensors to construct the environment map and re-plan the path. The control layer is used to generate control commands that control the AUV traveling along the planned path or uses reactive behavior to obtain real-time responsiveness of the AUV system. The communication unit is a bridge for data exchange between the AUV and the command center system.  
The navigation layer can plan the voyage paths according to the AUV's missions and the environmental information. At present, many available AUVs still need to download the workflow for a certain task to the control computer with a pre-programmed form before the start of the task. At the stage of task execution, human intervention is required at any time to ensure the security of the AUV and the reliability of the task execution. This traditional operation procedure cannot handle large-scale, highly dynamic and complex marine environments, and has low adaptability to new environments, which can easily lead to the loss of the AUV or the failure of mission's execution. Advanced AUVs are often equipped with expensive sensors so that the price and maintenance costs of AUVs are relatively high, and the loss or failure to complete tasks is unacceptable.  
Therefore, AUVs must have the ability to adapt to the large-scale, highly dynamic marine environments to ensure their own safety and the accuracy of mission's completion. Planning is an effective way to deal with complex environments. Path planning and path re-planning generate the trajectory of the AUV navigation to ensure collision-free, safe and energy-saving and to adapt to dynamic and complex marine environments. The path planning achieves the autonomy of the AUV, ensuring the security of the AUV and the reliability of task execution. The following is an analysis of the constraints that need to be considered for AUV navigation path planning.  
**Trajectory dynamics constraints.** When the actual AUV travels, there are dynamic constraints, such as the maximum radius of gyration, the maximum speed, the maximum acceleration, etc. The planned path should satisfy these constraints, which requires adding corresponding constraints in the algorithm to limit the generation space of the track. The path generated by the local path planning module should satisfy the dynamic constraints, and there should be no turning radius which is too small or other conditions that do not satisfy the dynamic constraints.  
Figure 1: The system architecture of autonomous decision-making system (ADS).  
**Obstacle and current constraints.** Obviously, the path planned by the path planning algorithm needs to meet the collision-free constraint. In addition, the motion of the AUV should also take into account the influence of the water flow. The tangential water flow will cause the AUV to deviate from the original path and result in motion errors. The reverse flow will increase energy loss. Therefore, the path planning algorithm should plan as far as possible to avoid the collision and meet the path of certain water flow constraints.