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1. OPERATIONAL OVERVIEW
1-12
How to discriminate fish echoes from bottom
1.4.4 Points to consider
• As a general rule of thumb, a vertically distributed fish school is a better sonar target
than a horizontally one, 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 bottom on the sonar screen.
• Detection of bottom fish is difficult if they are not distributed vertically.
1.4.5 Tilt angle for surface fish
Sound emitted from the sonar transducer forms a circle-shaped beam with a width of
approximately 15°* (-6dB in the vertical direction). The tilt angle is indicated by the an-
gle between 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 propa-
gates 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 effectively, 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.
*: 15° for CSH-5L, 13° for CSH-8L
Bottom
a
Case 1 Case 2 Case 3
Fish school (a) Fish school (a) Fish school (a)
Case 1 Case 2 Case 3
Bottom Bottom
(b)
a
b
a