Furuno CH-37 Operator's Manual page 24

2-8
Identifying noise source
To eliminate noise effectively, you should first identify the source
as follows:
• Operate all onboard equipment one by one while observing
the picture.
• Run the boat at various speeds to check if the noise is speed
dependent.
If neither of the above affects the picture, turn on "IR" in the
SONAR menu or adjust the NL (Noise Limiter) control as be-
low.
Rejecting noise with the interference rejector
This control is effective in rejecting random noise and sea sur-
face reflections in rough sea conditions. Pull the GAIN control
and adjust it to eliminate only noise. Reject the noise with "IR"
in the SONAR menu. Do not use an unnecessarily high setting
since it many also reject small wanted echoes.
Rejecting noise with NL control
Weak, unwanted reflections, colored light blue or green, ap-
pear when water is contaminated, or plankton layers or noise
exist. These echoes gradually become bluish as the NL control
is turned clockwise. Usually a setting from 3 to 4 provides suf-
ficient reduction.
Rejecting interference with Tx rate
When other sonars are operating nearby at the same trans-
mission interval as that of own ship's sonar, interference caused
by other sonars appears. To reduce the interference, reduce
the Tx rate setting on the SONAR menu.
Note: When the sonar is used in shallow water with the range
set between 100 m and 200 m and the Tx rate at 10,
bottom reflections caused by the transmission which is
the last but one appear on near on screen. Reduce the
Tx rate to 7 or 8 to reject them.