## 2 Materials and Methods
### Features of AUV Performance and AUV-Based Side-Scan Sonar Operations
#### 2.1.1 AUV Performance Features  
Conventional AUVs typically encompass a range of various key functions, such as depth measurement, altitude measurement, obstacle detection, self-propulsion, steering, buoyancy control, communication, and endurance. Collectively, these functionalities define their capacity to conduct side-scan sonar operations. Excluding communication and endurance, the autonomous mobility toolkit, comprised of these competencies, distinguishes AUV-based side-scan sonar operations from their shipborne counterparts. This unique suite of capabilities empowers AUVs to explore deeper depths at precise altitudes, necessitating customized operational planning for deploying side-scan sonar payloads, a process far more challenging than conventional vessel-based operations. In the hardware systems of an AUV, navigation, control, and safety systems are pivotal for underwater motion. To address the challenge of precise positioning of AUVs both above and below water, a sophisticated design combination has been implemented for the navigation system. The navigation system holds significant importance in formulating technical design solutions, and its fundamental working principle is depicted in Figure 2.  
When on the water surface, the primary hardware used for positioning and navigation in the navigation system is a satellite positioning system. However, when the AUV is moving underwater, an inertial navigation system, composed of accelerometers, gyroscopes, and heading sensors, is primarily employed in conjunction with a Doppler Velocity Log (DVL) for precise positioning and navigation. Figure 2: Working principle of the navigation system. The AUV's operational mechanism comprises the main thruster, horizontal rudder, and vertical rudder. The main thruster rotates the propeller via an electric motor, generating the necessary thrust for the AUV to move forward or backward. During AUV navigation, the horizontal and vertical rudders deflect at specific angles, allowing the rudder plate to generate lift and thereby facilitating changes in the AUV's pitch and bow states. The AUV can be controlled with four degrees of freedom: forward motion, heading adjustments, pitch changes, and vertical movement (heave). The control system is a critical hardware system that governs the AUV's vertical motion mode, and it primarily receives action responses from the depth and altitude meter. Once the depth and altitude meter measurements are obtained, response parameters are relayed to the control system, which in turn alters the AUV's current motion state. Among the most important safety systems are collision avoidance units and load shedding devices. The collision avoidance unit is employed to autonomously control the AUV, ensuring it avoids any unknown obstacles during navigation, thus safeguarding the AUV's operation in complex marine environments. It encompasses three key technologies: forward-looking sonar information processing, environmental information modeling, and real-time collision avoidance strategies. In terms of collision avoidance strategies, the AUV independently evaluates its maneuverability to determine if changes to its original operational state are necessary to optimize obstacle avoidance behavior. This project utilizes the artificial potential field method as the collision avoidance strategy, known for its fast calculation speed and excellent real-time performance. Additionally, to overcome the issue of local optimal solutions that this method may encounter, temporary false targets are employed to compensate. The load shedding device ensures that, in case of a severe malfunction or unspecified interference event, the long-range AUV triggers an emergency response by discarding a portion of its heavy load. This action enables the AUV to surface, serving as the final safety barrier for its operations. To maintain low power consumption while ensuring system safety and reliability, a mechanical load shedding structure is employed in this design.