Interphace ULTRASCAN PC90, Operation Manual, Page: 41 / 56

41
Ultrascan PC90 will show level or shallowing bottom
contours for a distance forward of between 4-times
and 8-times the water depth beneath the transducer.
Obstructions in the water may be seen at much
greater distances (within the Ultrascan PC90’s 1200-
foot maximum range).
Smooth bottom conditions far forward of the vessel
are difficult to image as very little of the acoustic
energy is reflected back as an echo (see the diagram
at top left). Bottoms that are rough and rocky or are
sloping upward will reflect more acoustic energy
back to the transducer and will show strong far-
forward returns (see diagram at bottom left).
It is important to note that even if the bottom is
smooth and does not show up far-forward,
acoustically reflective obstructions in the same far-
forward region (sea walls, large rocks, underwater
shelves, submerged buoys, etc.) will typically send
back strong echoes, as the diagram at right indicates.
Regardless of the actual vertical dimension of such
far-forward obstructions, they will often appear on
the Ultrascan PC90 display as a vertical line or arc.
Transducer Sidelobe Effect
The Ultrascan PC90’s transducer, like all transducers,
does not form a perfect beam of acoustic energy.
Some of the energy is contained in areas outside of
the main beam called sidelobes (see diagram at
right). In some situations, sidelobes can lead to
echoes that are not placed in the proper position on
the display.
As the Ultrascan PC90 sends off its acoustic beam in
a specific direction, it assumes that any return echoes
are within the main beam. However, if the sidelobe
energy strikes a reflective object (such as the
bottom), creating a strong echo, the Ultrascan PC90
will display the “false” return as if it were located
within the main beam.
The most typical display of sidelobe echoes appears
in the vertical display as an arc at the same distance
as the bottom depth (see diagram on following page).
After using the Ultrascan PC90 in different
situations, with different gain settings, you should
become proficient in identifying the bottom echoes
caused by the transducer’s sidelobes. A gain
reduction will help to minimize the sidelobe effect.
Strong Return from Wall Far-Forward
NOTE
Poor water conditions such as surface
chop, temperature inversion layers and
muddy water may degrade the Ultrascan
PC90’s performance
Transducer
Sidelobes
Main Beam
False
Echoes
Main Beam
Sidelobe