## 4 Discussion  
### Experimental Analysis  
The data obtained from the experiments described above suggest that the technical framework of AUV-based side-scan sonar operations is not only feasible but highly effective. In the depth-directed and fixed-point experiments, it was verified that complex seabed topography and oceanic hydro-environments have a significant impact on directional movement patterns, making them unsuitable for operations in complex environments. In these experiments, two sets of tests on complex and flat seabed topography verified the stronger applicability of the variable depth depth-directed and fixed-point mode to complex seabed measurements. When using the depth-directed and fixed-point mode on flat seabed terrain, the AUV's capabilities cannot be fully utilized, resulting in non-parallel seabed profiles between left and right images. This limits the effective area of seabed images and eliminates the advantages of bathymetric line tracking and ranging corrections in data processing. Complex seabed environments require a greater emphasis on avoiding risks of collisions with underwater features and instability resulting from equipment pitching when using the constant-altitude mode. In the constant-altitude experiment, it was verified that the directional mode has poor controllability. Therefore, it is sufficient to only use the fixed-point mode with the constant-altitude mode for experiments. These experimental image data have high quality and can clearly display seabed topography, possessing a certain target detection ability,  
Figure 25: Inaccurate position information leading to information shared by two strip images. achieving the expected goal of survey line planning principles and demonstrating the applicability of the constant-altitude and fixed-point mode operation plan for detecting flat seabed areas. In vertical movement mode, the constant-altitude mode is effective in flat seabed areas. When the AUV's performance allows, it should be prioritized for use in complex seabed environments. Under the constant-altitude mode, DVL can fully leverage its advantages, while the directional mode is an unreasonable choice that sacrifices DVL's strengths. When AUV performance limitations dictate the use of constant-altitude mode, stepwise depth changes in constant depth mode are more effective than constant depth mode. In planar movement mode, the fixed-point mode is more advantageous than the directional mode. Therefore, the preferred order of mode settings in complex seabed environments is: constant-altitude and fixed-point mode \(\rightarrow\) stepwise depth transformation constant depth and fixed point mode \(\rightarrow\) constant depth and directional mode. Based on the principle of the fixed-point mode track correction algorithm, it can be observed that the survey line planning approach that prioritizes the use of the fixed-point mode possesses robustness against environmental interference such as waves and currents. Nevertheless, a closer analysis of the results from directional experiments and complex terrain constant depth fixed point experiments reveals that when operating in the directional mode, the AUV's stability against interference is reduced. Additionally, when the DVL bottom tracking is lost in the fixed-point mode, relying solely on inertial navigation data may also diminish its robustness against current interference. Thus, it is evident that for AUVs conducting side-scan sonar operations, the survey line planning method proposed in this article is highly reliant on the performance and accuracy of positioning and navigation hardware. Any malfunctions or issues in the navigation system can significantly impact its robustness. Furthermore, beyond the navigation system's influence, problems with the manipulation system and side-scan sonar payload can also result in operational failure or interruption. This highlights the crucial importance of meticulous inspections of both the AUV itself and the side-scan sonar payload during operational preparations.