## 3 Experimental Results and Analyses
### Experiment in Waters with Flat Seabed Topography
#### 3.1.2 Experiment in Constant-Depth and Fixed-Point Modes  
The purpose of this experiment is to test the applicability of fixed-depth and variable-depth fixed-point modes on flat terrain, as well as to verify the planning principles for survey line direction, spacing, and overlap. The differences between this experiment and the constant-altitude experiment are:  
1. The survey line direction follows the principle of being parallel to the edge of the survey area to analyze the effect of the fixed-depth mode on violating the principle of parallel isobaths on flat terrain;  
Figure 13: AUV’s altitude, course, and image data under constant-altitude and fixed-point modes in the experiment. **(a)**. AUV altitude and coordinate information. **(b)**. Side-scan sonar band image displayed by overlapping. **(c)** Waterfall chart including model information. \begin{table}
\begin{tabular}{c c} \hline Repeat Group Number & 2 \\ Altitude (m) & 20 \\ Range (m) & 75 \\ Survey Line Spacing (m) & 75 \\ Survey Line Length (m) & 500 \\ Survey Line Quantity & 4 \\ Navigation Speed (kn) & 3 \\ Survey Line Direction (\({}^{\circ}\)) & 45\({}^{\circ}\) \\ Experimental Equipment Model & Small AUV \\ Main Purpose of the Experiment & Verify the applicability of constant-altitude mode \\ \hline \end{tabular}
\end{table} 
Table 3: Flat terrain constant-altitude fixed-point experiment parameter settings. 2. Controlled experiments with different navigation speeds are established to observe the impact of speed on images;
3. A dense control group is established to analyze the applicability of survey line layout methods in terms of spacing and overlap. The conditions for this experiment are twofold. Firstly, the AUV maintained a constant depth of 20 m and operated at a low frequency, equipped with a measuring range of 150 m. Four survey lines were deployed, each spanning 500 m and spaced 75 m apart. The vehicle cruised at a speed of 2 kt, with the experiment conducted in horizontal (0\({}^{\circ}\)) and vertical (90\({}^{\circ}\)) survey directions, resulting in one experimental group for each direction. Furthermore, the experiment involved a mid-sized AUV with one experimental repetition, a small-sized AUV equipped with a 75 m range capability, which operated at 3, 4, and 5 kt in the horizontal survey direction. Second, to enhance the depth mode measurements, the AUV maintained a constant depth of 20 m, using a low-frequency setting with a 75 m measuring range. Six survey lines were deployed, each measuring 160 m long and spaced 30 m apart. The AUV maintained a constant speed of 3 kt, and the experiment was performed in both horizontal and vertical directions, with one experimental group designated for each direction. Moreover, an additional experiment was conducted using the AUV equipped with a 50 m measuring range, involving the traversal of four survey lines, each stretching 200 m in length, in addition to one experimental repetition undertaken by a small-sized AUV. Experimental parameter settings are presented in Table 4. The vehicle's depth, course, and image data under the initial experimental conditions are illustrated in Figure 14, whereas the second experimental conditions are presented in Figures 14-16. Through the image analysis of Figure 14a,b, it can be observed that the seabed line in Figure 14c is not parallel. When combined with the survey area's northwest shallow seabed and southeast deeper seabed, as well as the movement direction of the AUV from north to south, it can be inferred that the seabed line gradually expands from bottom to top, which is correct. However, due to the small depth change, the expansion trend is not particularly evident.