The received signal intensities, in the regions of top and bottom sides of the figure (i.e., the edge part of scanning), were relatively low because of the longer distance to the target and smaller solid angle for the receiving optics. There are some intensity ripples on the top and bottom sides. We supposed that this phenomenon was caused by the weak internal reflection of the transmitted pulse (at the optical devices in the sensor such as the dome lens, mirror, and lens) and the nonideal characteristics of the peak hold circuit. The current peak hold circuit is the simple capacitor type, as described in Sec. 2, and keeps the held voltage with a relatively long time (order of microseconds, corresponding to a distance of a few hundred meters). The pulse width is very short (1 ns), but the voltage holding continues for microseconds. This causes signal accumulation with (1) internal reflection, (2) backscattering in water, and (3) target signal. The influence of this accumulation becomes relatively distinct when the level of the target signal becomes low. If the level of the internal reflection has an angle dependence, this causes the above-mentioned ripples in the intensity map.