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It is important to understand that the PC/180 will
often see the bottom and will show it on the
forward display. This display information can be
used to help understand the structure of the
bottom ahead of the boat, but the bottom echoes
can also mask or be confused with other desired
targets. In the diagram at left, for example, a bait
ball or school of fish is lost in the bottom echoes.
However, as the vessel approaches the target, it
will become distinct from the bottom echoes as it
moves into the nearer ranges.
Special Situations
With experience you should be able to use the fact
that the forward beam will show the bottom, to
your advantage. You will learn to recognize
bottom structure such as varying water depths
across the sector, drop-offs, ledges, and rocky
patches on the bottom ahead. Two examples of
changing bottom conditions are shown in the
diagrams at right.
In the upper diagram, a shallow bottom in the
Deeper water
starboard direction is clearly imaged in the
to port (left)
horizontal scan. In the lower diagram, a channel
of boat.
is imaged. The deepest point of the channel
ahead of the vessel is indicated by the gap in the
bottom echoes.
Forward Imaging Capabilities
If the horizontal range setting is greater than the
forward distance to the bottom, the PC/180 can
detect targets at distances beyond where the beam
hits the bottom. This is because the acoustic
beam can bounce off the bottom and keep going
forward until it strikes a solid object and then
returns as an echo.
Even in shallow water it is possible to see several
hundred feet ahead by using this "bounce"
technique. When looking far-forward in shallow
situations such as this, it is advisable to adjust the
gain manually. Otherwise the Auto Gain feature
will reduce the gain because of the strong close-in
bottom reflections. Refer to page 30 for a
discussion of gain settings.
Sides of Underwater
Trench
43
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