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1. OPERATIONAL OVERVIEW
1-13
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 angle 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 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 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
Tilt angle 0°
Sea surface
Tilt angle 5-6°
Sea surface
15°
15°
Tilt angle and surface fish (example: CSH-5L)