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

Besides the bottom conditions, water conditions will also
affect the Color Twinscope's performance. For example,
surface chop, temperature inversion layers (thermoclines)
and muddy water may degrade performance.

Noise and Sensitivity Adjustments

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

Transducer Sidelobe Effect

The Color 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 Color 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 Color
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 typical display of the sidelobe echoes
appears as an arc at the same distance as the bottom depth ,
and in the worst case, from the bottom below to the surface
ahead. After using the Color Twinscope in different
situations, with different gain settings, you should become
proficient in identifying the bottom echoes caused by the
transducer's sidelobes. To minimize the sidelobe effect, the
gain should be reduced.
Transducer
False
Echoes
31
Sidelobes
Main Beam
Sidelobe
Main Beam
"False" Bottom
Echoes Caused by
Sidelobe Returns.