Aiming DIDSON: Using Sonar Tilt
The selected display has a specified start range (the minimum range imaged) and a specified window length (distance between the minimum range and the maximum range displayed). The vertical acoustic beamwidth of the DIDSON is 14°. That means the transmitted sound in the vertical direction is contained within a 14° angle. The tilt of the sonar changes ensonified ranges as the top and bottom edges of the vertical beam hit the bottom. One wants to tilt the sonar at an angle that “fills the cone”. The correct tilt angle allows echoes to return from the minimum range to the maximum range displayed.
Figure 1 shows a drawing of the sonar tilted at a steep angle such that only a narrow band on the bottom is struck by sound.
Figure 2 shows stills and a video of an actual image as the sonar tilt is gradually changed. With the sonar tilted at a steep angle only 0.25 meter of range on the bottom in the display receives sound. When the sonar is tilted less, the band increases in width until it “fills the cone” or spreads from the displayed minimum range to maximum range. To increase the contrast between an object on the bottom and the bottom surface, lower the sonar closer to the bottom and graze the bottom at a smaller angle. This will reduce the brightness of the bottom return but an object sitting on the bottom will return large echoes from the sides struck with the sound and provide a bright return. This increased contrast between object on the bottom and the bottom makes the object stand out in the display.
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