SeaBat UI - User Manual
Operation
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The pulse type CW uses a frequency with a continuous wave centered
around the selected frequency.
The pulse type FM uses a frequency modulation within a frequency range
that is for instance for 200kHz 185-215kHz and for 400kHz 381-411kHz.
The system will search for the frequency to use that have the least
disturbance.
An echo sounder with a frequency of 210kHz will disturb the sonar signal
with a pulse type CW at 200kHz, while the pulse type FM has no or very
little disturbance from the echo sounder.
4.3.8 Absorption
The absorption allows the operator to enter the amount of loss that is
expected through the ambient water medium. The absorption loss is
proportional with the salinity of the water and the sound frequency: the
higher the salinity or frequency, the greater the loss. Also an increased
temperature will increase the absorption loss.
If the exact absorption loss value is not known, a value of 110dB/km for
salt water and 70dB/km for fresh water is recommended for a 400kHz
operation, and a value of 50dB/km for salt water and 20dB/km for fresh
water is recommended for a 200kHz operation.
This value is used in conjunction with the spreading loss coefficient (see
below) to compute the TVG (time varied gain) curve that is applied to the
returned signal (see Gain on page 19).
Figure 4-25
Absorption
4.3.9 Spreading
The spreading allows the operator to enter the amount of cylindrical and
spherical spreading loss that is expected through the ambient water
medium. Spreading of sound through water may be the greatest
contributor to the loss of sound. As sound moves away from its source in
all directions, the acoustic pressures will decrease with the range.
This value is used in conjunction with the absorption loss value (see
above) to compute the TVG (time varied gain) curve that is applied to the
returned signal (see Gain on page 19).
In practical situations the transmission loss will lie between 20 and 40dB;
in other words, the spreading loss will neither be perfectly spherical nor
perfectly cylindrical. Finally, if the sonar operates very close to the seabed
or very close to objects in general, near-field effects may yield a
transmission loss less than 20dB.
If the exact spreading loss value is not known, an initial value of 30dB is
recommended.
Figure 4-26
Spreading