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Points to Consider
• As a general rule of thumb, a vertically distributed fish school is a
better sonar target than the seabed, since it reflects the transmitted
pulse back toward to the source.
• In case 3, both fish schools a and b are presented. Generally
speaking, however, midwater fish schools tend to be larger than
bottom fish schools and they are often displayed near the seabed
on the sonar screen.
• Detection of bottom fish is difficult if they are not distributed ver-
tically.
Tilt Angle for Surface Fish
Sound emitted from the sonar transducer forms a circle-shaped beam
with a width of approximately 13 degrees in the vertical direction
(vertical beam width). The tilt angle is indicated by the angle be-
tween the center line of the beam and the horizontal plane. Then, if
the tilt angle is set to 0 degrees, the center line is parallel with the sea
surface and one half of the emitted sound goes upward toward the sea
surface.
This causes a half of the emitted sound to be reflected back toward
the transducer and displayed on the screen as sea surface reflections.
When the sea is calm, since the sound is reflected just like a light
hitting a mirror at a narrow incident angle, it propagates away and the
sea surface reflections become negligible.
However if the sea is not calm enough, they will become dominant
and will interfere with observation of wanted echoes. To minimize
these sea surface reflections and to search surface fish schools effec-
tively, the tilt angle is usually set to 5-6 degrees so that the upper
portion of the beam becomes almost parallel with the sea surface.
When the sea is rough, it is often set to a little larger angle.
TILT 0°
13°
TILT 7°
13°
SEA SURFACE SEA SURFACE
Suitable Tilt Angle
The figure on the next page illustrates the relationship among tilt
angle, depth and detection range. Refer to it to find out the suitable
tilt angle for a given depth/detection range.