26
Interpreting The Forward Vertical
Display
The Interphase iScan 180 provides a display which shows
acoustic echo returns from the underwater area beneath and
ahead of the vessel. The Phased Array Transducer steers an
acoustic beam over an arc which can be adjusted from
approximately 12 to 90 degrees. As the iScan 180 steers
the beam to different positions, it transmits a pulse of
energy and then waits a defined period of time (depending
on the range selected) and listens for any return echoes. As
the energy from this acoustic beam strikes underwater
objects or the bottom, a small portion of the energy is
reflected as an echo back to the transducer. When the echo
is received at the transducer, it’s converted into a small
electrical signal, processed and displayed on the iScan
180’s LCD.
The unit knows the direction in which it sent the transmit
pulse and the time it took to receive the return echo, so it
can determine the location of the object or bottom that
created the return echo. As the iScan 180 sequentially
steps the acoustic beam from directly forward to the
bottom, the LCD display shows a continuously updated
display of the return echoes in their approximate position in
relation to the vessel.
Because the LCD display is only showing the acoustic
echoes that are returned to the transducer, it can not show
forward bottom conditions that are hidden from its field of
view or are hidden due to obstructions in the acoustic
beam’s path through the water. In addition, smooth bottom
conditions far forward of the vessel are difficult to see as
very little of the acoustic energy is reflected back as an
echo. See the sketch at left.
Bottoms that are rough and rocky or are sloping upwards
will reflect more acoustic energy back to the transducer and
will show up better far-forward of the vessel than bottoms
that are very smooth or slope downward. However, even
though the bottom may be smooth and does not show up far
forward, large obstructions (sea wall, large rocks,
underwater shelves, etc.) will typically send back strong
echoes that can be seen far forward, as the sketch at left
indicates.
Strong Return from
Up-sloping Bottom
Weak Return From Far-Forward
Strong Return from Wall Far-Forward
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