Distance Forward; Noise And Sensitivity Adjustments; Transducer Sidelobe Effect - Interphase Twinscope Operation Manual

Distance Forward

Under typical conditions, the Twinscope will show
level or shallowing bottom contours for a distance
forward of between 4X to 6X the depth below the
transducer. Obstructions in the water, such as walls,
mud banks, etc. may be seen at greater distances,
subject to the depth below the transducer and the
1,200 ft. maximum forward range.
Besides the bottom conditions, water conditions will
also affect the Twinscope's performance. For
example, surface chop, temperature inversion layers
(thermoclines) and muddy water may degrade
performance.

Noise and Sensitivity Adjustments

The Twinscope features an AUTO sensitivity feature
where it continuously adjusts the sensitivity to achieve
the optimum picture quality. The sensitivity can also
be adjusted manually to allow for a reduction in
sensitivity to minimize the effect of screen noise, or to
increase the sensitivity to show weaker targets.
In the forward scanning modes, the sensitivity setting
is especially important as too little sensitivity will
cause a loss of far forward bottom readings and too
much sensitivity will cause an increase in screen noise
and a possible display of unwanted transducer sidelobe
readings.

Transducer Sidelobe Effect

The Twinscope's transducer, like all acoustic
transducers, does not form a perfect beam of acoustic
energy. Some of the acoustic energy is contained in an
area called the sidelobes. In conventional
downlooking depthsounders, the sidelobes create little
problem except to distort the size of the actual beam
angle; however, in scanning sonars they can create
echoes that are not placed on the LCD screen in their
proper position (also known as false echoes).
As the Twinscope 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 (which is not within the main beam) strikes a
large object (i.e. the bottom) and creates a strong
return echo, the Twinscope has no way of knowing
that this "false" echo was not created by the main
beam and will go ahead and show it on the display as
if it was located within the main beam. The most
Display Depicting Forward Range
Approximately 5 X Current Depth
Display Depicting Forward
Range Approximately 8X
False
Echoes
68
Depth
Transducer
Sidelobes
Main Beam
Sidelobe
Main Beam