## 2 Materials and Methods
### Features of AUV Performance and AUV-Based Side-Scan Sonar Operations
#### 2.1.1 AUV Performance Features
Impact of AUV Dimensions on Performance  
Performance Features of Small-Size AUVs  
When creating the structural design of smaller AUVs, which require efficient use of space to balance endurance and dynamic loading needs, buoyancy control is typically achieved through the coordinated action of rudders and propellers. As the AUV descends, it employs its propellers to generate forward momentum and adjusts its rudders to lift the stern. The descent primarily relies on the thrust from the propellers to submerge beneath the sea surface. Conversely, acceleration is not required during the ascent, but the AUV deploys its rudders to lower the stern before utilizing its buoyancy and propeller thrust to rise to the surface. This submersion method preserves the AUV's natural buoyancy, necessitating continuous adjustment of its angular attitude to maintain thrust equilibrium with buoyant forces. Consequently, in this configuration, all payload beams are not perpendicular to the horizontal plane, potentially resulting in inaccuracies in bottom tracking range measurements. In the context of constant-altitude navigation, the AUV will continuously adjust its pitch to regulate its altitude above the seafloor. Nevertheless, these adjustments may introduce variations in the overall attitude, potentially affecting the vehicle's ability to consistently uphold a specific depth above changing seafloor topographies in the constant-altitude mode. Compact AUVs often prioritize the use of DVLs among their payloads, as the data generated by DVLs significantly influence the AUV's intricate maneuvers. Due to the limited space available on compact AUVs, smaller DVL models are favored, covering an operational range of approximately 100 m.  
2. Performance Features of Large-Size AUVs  
In the case of larger AUVs, sufficient interior space is available to easily accommodate the installation of additional payloads. These more spacious AUVs are equipped with rudders and propellers and feature a water-permeable head and oil bladders, enabling precise control over the buoyancy of the front section by adjusting their volume. The middle section typically comprises a waterproof compartment housing a sliding rail mechanism. Skillful manipulation of the slider's position within this mechanism allows for regulating the vehicle's center of gravity, consequently controlling the overall vehicle's attitude. Simultaneously, the rudders and propellers function like AUVs that lack buoyancy control. This approach facilitates a remarkable steadiness in the AUV's overall attitude, eliminating pitch oscillations. The constant-altitude mode is suitable for sustaining the submersible at a specific depth above the seafloor. In cases of emergency, a safeguard mechanism can be activated to bring the submersible to the surface through a combination of jettisoning and bladder inflation. Large-size AUVs can accommodate an advanced iteration of the DVL, commonly referred to as an acoustic Doppler current profiler (ADCP). The ADCP exhibits similar functionality and performance to the DVL but distinguishes itself with a significantly extended operational range, typically capable of reaching depths of approximately 300 m. These instruments' operational range depends on the AUV's physical dimensions. For instance, shipborne ADCPs benefit from more installation space, enabling them to achieve significantly broader operational ranges, with some reaching depths of up to approximately 1000 m.  
The size of an AUV directly affects the quantity and variety of payloads it can accommodate, consequently having an indirect impact on the AUV's performance, particularly its underwater motion capabilities in the vertical plane. AUV Underwater Motion Control Modes in the Horizontal Plane  
1. Fixed-Point Mode  
It requires the integration of bottom tracking and satellite positioning systems. Before submersion, the AUV synchronizes with satellite signals to determine its initial coordinates.