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4-3
fish echoes from seabed reflections. Decrease the
PULSELENGTH setting on the Scan menu to shorten the
pulselength. For search of surface and midwater fish in which
seabed reflections are not so strong, use the longest pulselength
“10”.
Suppressing Seabed and Sea Surface Reflections in
Shallow Waters
In shallow fishing grounds with hard or rocky bottom, seabed
reflections often interfere with wanted fish echoes and they can
not be eliminated sufficiently with the aforementioned TVG and
AGC, especially when the TILT is set to a larger angle in order
to track fish schools approaching within 400 m. In such cases
try to reduce the output power with OUTPUT in the Scan menu
without turning down the GAIN. The picture becomes clearer
when output power is reduced rather than when the GAIN is
decreased as illustrated below.
Fish echo
Fish echo
weakened
Fish echo
WRONG
METHOD
CORRECT
METHOD
OUTPUT decreased
with GAIN kept
constant
TVG and AGC
adjusted with
OUTPUT kept high
Figure 4-2 How to suppress seabed and sea surface reflections
in shallow waters