Operation Manual
FarSounder, Inc.
F31552 (Rev. 2.6.1)
Page 33 of 56
important part of FarSounder's signal processing and image display chain. Different target categories are
processed differently utilizing
a priori
information pertaining to the target category type. For example, the
sea floor is generally comprised of many small reflectors as part of one larger continuous surface, while
in-water targets are typically comprised of fewer target points and often have many angles and surfaces
which face the sonar. By exploiting these and other differences, FarSounder's products are able to operate
effectively in shallow water and at navigationally significant ranges.
To produce the most complete pictures, some parameters of the processing controls may need to be
modified to suit the environment. These parameters are controlled by the Squelch slider. This control can
fine-tune the processing system in the event that too few targets are displayed, or too many targets are
making the view difficult to understand.
To effectively use the Squelch control, it is important to understand how different sonar targets reflect sound
waves. The reflections from different targets are detected by the system as a signal with some energy level
relative to the energy level of the ambient noise in the water and the electronics. This relationship between
the signal level and the noise level is called the Signal to Noise Ratio (SNR). The larger the SNR, the better
the system will detect a target and separate it from the noise.
SNR is generally controlled by two metrics: the acoustic reflectivity of a target and the distance to the target.
Targets that are physically larger, have a large gas content, have large surfaces facing the sonar, or are
very hard will generally be more reflective than targets that are physically small, have high water contents,
have surfaces not facing the sonar, or are very soft; the higher the reflectivity of the target, the higher the
SNR from the reflected signal. Additionally, a target at close range will have a higher SNR than the same
target at far range.
Because of these two controlling metrics, the system must filter out targets that have low signal levels at
varying ranges. These signals may be from targets that are too far away or physically too small to detect.
Larger, more reflective targets will be detected at longer ranges than smaller, less reflective targets.
Note that when changing any of the processor settings (ie. Squelch levels, processing ranges, etc.) changes
will not take effect until the next complete processing cycle.
This typically means 2 pings after adjusting
the control, you will see the change, since generally when you see an image that needs adjusting there is
already another ping processing with those same settings internal to the software. Changes will be applied
to the next "fresh" ping.
Squelch Control Slider
Variations in target characteristics also have effects on how "noise" is related to in-water targets. Generally,
in-water targets are "louder" (more reflective) targets than the sea floor, and require a different target size
(reflectivity) cutoff. The Squelch slider controls the target size cutoff for in-water targets. In general, setting
this control to a lower level will detect smaller, less reflective in-water targets.
For best operation, the user should typically set the Squelch until clutter from in-water debris is filtered
effectively.
A value of 165-175 is typical for most conditions.
Like with radar, it is recommended that the
squelch be set so that occasional false alarms are detected. This will help to ensure that real targets are
not missed due to squelch levels set too high.
When making Squelch adjustments, typically a change of a few dB is all that is needed. Remember that
changes generally take 2 pings to become visible.
5.8. Control Settings Examples
When learning to use the sonar, it may be useful to practice adjusting the settings in a simple environment
where you have a good idea of what is actually under the water. Like any navigation tool, you should
