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3. Locate a fish school on a long range setting which is approaching
own ship. Note that the tilt should be kept adjusted so that the fish
school is always placed in the center of the sonar beam, i.e., so
that the fish school is displayed in strongest colors possible. Check
that the fish echo is displayed in the same color while it approaches.
If the color changes suddenly to weaker colors as the fish echo
enters FAR and NEAR areas, the TVG is improperly set. Adjust
the TVG in the scan menu to correct it. If this again produces sea
surface reflections and noise, try to remove them with the AGC
and NL controls as described later on.
Displaying Surface Fish Clearly
When you are searching for surface fish with the tilt set to a narrow
angle, sea surface reflections may disturb or mask wanted fish
echoes. In this case, in addition to the TVG adjustment described
earlier adjust the AGC control between positions “0” thru “3”.
Suppressing Seabed Tail
As noted earlier, fish schools (echoes) located near the seabed are
sometimes difficult to detect because you have to discriminate them
in the seabed reflections. The AGC control and PULSELENGTH in
the scan menu, if used properly, decrease the tail of seabed reflec-
tions, making it easier to discriminate bottom fish.
AGC Control
The AGC control functions to automatically reduce the receiver gain
only against strong echoes such as the seabed or a large fish school.
Since weak echoes remain unaffected, a small fish school becomes
easier to detect. Adjust it so that the AGC works only on seabed re-
flections. Do not turn it too far clockwise.
PL (Pulselength)
The pulselength control determines the length of the transmission
pulse emitted into the water. While a longer pulse is advantageous
for long range sounding, it has the disadvantage of being poor in
discrimination of targets, i.e., ability to separate several closely lo-
cated targets. When searching bottom fish, therefore, it is useful to
shorten the pulselength in order to separate fish echoes from seabed
reflections. Decrease the PL setting to shorten the pulselength in the
scan menu. For search of surface and midwater fish in which seabed
reflections are not so strong, the longest pulselength “ 10” should be
used.