## 3 Experimental Results and Analyses
### Experiment in Waters with Complex Seabed Topography
#### 3.2.2 Experiment in Constant-Depth and Fixed-Point Modes  
The objective of this experiment is to demonstrate the advantage of the variable depth constant depth mode over the constant depth constant depth mode and to establish the superiority of the fixed-point mode over the directional mode. Additionally, it aims to demonstrate the applicability of the survey line planning principle with regards to survey line direction, spacing, overlap, extension lines, and auxiliary lines. Compared to the experiment under constant-depth and fixed-direction modes, the experiment carried out under constant-depth and fixed-point modes leverages DVL positioning data, which provides more precise position information. The specific requirement for the water depth positioning in the positioning mode fundamentally supports this approach. This necessitates that the distance between the vehicle and the seafloor consistently falls within the DVL's measurement range, considering the side-scan sonar's detection range. In this research, the AUV descended from a water depth of 40 m following a guiding line to the beginning of the first survey line. Then, it covered a distance of 2500 m from north to south along the first survey line, executed a 180\({}^{\circ}\) turn and ascended. Following this, it proceeded to the starting point of the second survey line, 75 m away from the first line, and traversed 2500 m from south to north along the second survey line, as illustrated in Figure 21. The experimental parameters are shown in Table 7. \begin{table}
\begin{tabular}{c c} \hline \hline Repeat Group Number & 1 \\ Depth (m) & 90–97 \\ Range (m) & 150 \\ Survey Line Spacing (m) & 75 \\ Survey Line Length (m) & 2500 \\ Survey Line Quantity & 2 \\ 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 fixed point \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Constant depth fixed-point experiment parameter settings. Figure 21: Schematic diagram of the course in the experiment under constant depth and fixed-point modes. Setting the AUV's submersion depth is critical in this experiment. To align with the operational ranges of the DVL and side-scan sonar, the depth configuration must be approached on a stepwise basis. It is essential to ensure that any discrepancies in the empirical depth data fall within the prescribed ranges of the side-scan sonar and DVL while avoiding activation of the bottom-off altitude alarm. The stepwise depth profile for each segment was established using empirical data. The experiment was divided into two submersion stages. In the initial stage, referred to as the \"guidance stage\", the AUV descended from the sea surface (0 m) to a depth of 90 m. After completing the first survey line, it executed a right-angle turn to transition to the second survey line. In the second stage, the AUV descended from a depth of 90 m at the start of the second survey line to 97 m and then initiated its ascent, as illustrated in Figure 22. The right graph in Figure 23 visually represents the AUV's surfacing process, accompanied by the gradual expansion of the seabed profile. The experiment further confirms the strong applicability of the survey line direction, spacing, overlap, extension line, and auxiliary line setting principles for complex seabed terrains. In combination with the flat terrain constant depth experiment, it is evident that  
Figure 23: Mosaic diagram and waterfall plot of experimental results. (**a**) Course angle. (**b**) Waterfall chart. Figure 22: A 3D schematic diagram of submersion depths. the constant depth mode combined with the bathymic contour direction survey line planning principle significantly outperforms the combined parallel survey zone edge line setting principle.