## 3 Experimental Results and Analyses
### Experiment in Waters with Complex Seabed Topography
#### 3.2.1 Experiment in Constant-Depth and Fixed-Direction Modes  
The fixed-depth directional experiment aims to verify the applicability of the two modes, constant depth and direction, as well as their combinations, in complex terrain sea areas, and to establish the effectiveness of the survey line direction planning principle. This experimental design takes into account the unique characteristics of complex terrains, including variable oceanic conditions, diverse seabed features, and irregular bathymetry. In this experiment, the AUV was stationed on the western boundary of the survey area, and it conducted scanning along a survey line that extended for 2500 m from north to south. The entire survey line was deployed at an approximate depth of 100 m. The AUV operated using a 500 kHz frequency, with a single-sided range of 150 m. Throughout its operations, the AUV consistently maintained a depth of 80 m below the sea surface. The experimental parameters are shown in Table 6.  
Regarding the overall course, it displayed a slight deviation of roughly 2\({}^{\circ}\) to the south by east, as illustrated in Figure 19. The AUV submerged from depths that exceeded the operational capabilities of the DVL and adhered to the specified heading and timeline. One notable limitation of this operational mode is its independence from DVL-derived data. The position information within the data acquired by the side-scan sonar system is derived from inertial navigation. In cases where the AUV's heading angle deviates from the predefined course angle, the AUV calibrates its heading angle to align with the predetermined  
\begin{table}
\begin{tabular}{c c} \hline Repeat Group Number & 1 \\ Depth (m) & 80 \\ Range (m) & 150 \\ Survey Line Spacing (m) & 150 \\ Survey Line Length (m) & 2500 \\ Survey Line Quantity & 1 \\ Navigation Speed (kn) & 3 \\ Survey Line Direction (\({}^{\circ}\)) & 180 \\ Experimental Equipment Model & Small AUV \\ Main Purpose of the Experiment & Verify the applicability of constant depth and direction \\ \hline \end{tabular}
\end{table}
Table 6: Parameter settings for the fixed-depth directional experiment in complex terrain.  
Figure 18: Complex seabed topography.  
course. In situations with lateral water flows, there may be an angular disparity between the planned course and the actual path taken. Due to inherent inertial navigation errors, disparities can occur between the actual and recorded trajectory, resulting in variations in the position information along survey lines compared to the initially configured parameters.  
Figure 19(a) illustrates the variation in the AUV's depth relative to the sea surface over time, measured in meters (m). Figure 19(b),c depicts waterfall plots produced by the AUV's side-scan sonar during its submersion. From Figure 19(b), it is noticeable that as the AUV descends, the seabed profile lines generated by the side-scan sonar gradually converge, ultimately leveling out as the submersible reaches a specific depth. As the AUV descends further, the gap between these lines progressively increases, as illustrated in Figure 19(c). Eventually, the seafloor details become invisible from the diagrams due to surpassing the detection range of the side-scan sonar.  
Figure 19: Course angle and image data in the experiment under constant-depth and fixed-direction modes. (**a**) Course angle. (**b**) SSS band image.  
Figure 20: Depth data and waterfall plots. (**a**) Course angle. (**b**) Waterfall chart submersion segment. (**c**) Constant depth segment.