Furuno CHS-5LMARK-2-55, Operator'S Manual

Page: 24 / 90

1. OPERATIONAL OVERVIEW
1-12
1.4.3 How to discriminate fish echoes from the bottom
The following figure illustrates how two schools of fish (a) and (b) are displayed on
screen using three different tilt angles.
How to discriminate fish echoes from bottom
1.4.4 Points to consider
• As a general rule of thumb, a vertically distributed school of fish is a better sonar
target than a horizontally one, since it reflects the transmitted pulse back toward to
the source.
• In case 3, both schools of fish (a) and (b) are presented. Generally speaking, how-
ever, midwater schools of fish tend to be larger than bottom schools of fish 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
Case 1: Tilt angle 30 to 40 degrees
School of fish is obscured by the bottom echo.
Case 2: Tilt angle 10 to 20 degrees
School of fish is located above the bottom echo (midwater).
Case 3: Tilt angle 0 to 5 degrees
School of fish is located close to the bottom echo.
Bottom
a
Case 1 Case 2 Case 3
School of fish (a) School of fish (a) School of fish (a)
Case 1 Case 2 Case 3
Bottom Bottom
(b)
a
b
a