## 4 Discussion
### Experimental Image Analysis  
The main characteristics of the side-scan sonar images obtained by the AUV, as deduced from the experimental findings, can be summarized as follows:  
1. In the AUV's constant-altitude mode, within flat topography, the seabed profile typically follows the AUV's course, resulting in an inability to detect terrain variations. Nevertheless, these variations become significant when the vehicle encounters minor topographical undulations, as presented in Figure 26.  
(4) As the AUV alternates between submersion, surfacing, and depth adjustments, the seabed profile exhibits a funnel-like shape, similar to the changes observed in ship-borne side-scan sonar systems when vessel speed alters the diving depth, as presented in Figure 29.  
(5) During pivot steering, the outward waterfall plot of side-scan sonar imagery exhibits distortions, while the inner sections exhibit strip images that overlap and contain duplicate scans of the same geographical area, as shown in Figures 30-32. Although such images present difficulties in application, their distinct characteristics enable more straightforward interpretation.  
Figure 30: Overlapping within the inner side (toward the center) of the strip sonar image during the AUV’s pivot steering.  
Figure 29: A funnel-shaped seabed profile.  
* (6) The additional acoustic equipment onboard the AUV introduces notable interference in the side-scan sonar imagery. This interference is characterized by recurring noise patterns, as displayed in Figure 33, making it readily identifiable but challenging to eliminate.
* (7) Upon correction of slope distance, the geocoded strip image generated within the depth range that falls within the side-scan sonar's measurement capacity takes on an elliptical shape at both ends, as presented in Figure 34.  
Figure 31: Blending in the outer side (toward the periphery) of the waterfall plot during the AUV’s pivot steering.  
Figure 32: Signature deformation shown in the waterfall plot during two right-angle turns.  
Figure 33: Sources of two prominent noises (both from payloads in the same operational setting).  
As the AUV accelerates, it induces a compression effect in the images along the heading direction (Figure 35). Nonetheless, the AUV generally operates at speeds between 2 to 4 kt during side-scan sonar missions, which are slower than shipborne side-scan sonar systems. Consequently, the compression level is lower and remains below the levels observed during rapid acceleration in shipborne side-scan sonar operations.  
In the experiment conducted under the constant-altitude mode, it was theoretically expected that variations in vehicle velocity would lead to a scaling effect in the images along the heading direction. Yet, in practice, this effect is not particularly significant due to the moderate cruising speed of the AUV during side-scan sonar operations. Shipborne side-scan sonar measurements are typically carried out at speeds below 6 kt, whereas AUV-based side-scan sonar systems operate at lower speeds of 2 to 3 kt. This reduced velocity ensures the AUV can maintain image resolution and stability, even when encountering water flows. The AUV's relatively low cruising speed and minimal speed fluctuations help minimize the impact of cruising speed on the produced image.