System Operation
Both the absorption (
α
) and spreading loss values can be changed by the opera-
tor at the Ocean Menu using the menu items ABSORB and SPREAD. When
switching between FIXED and TVG gain modes, the last gain value in each cate-
gory is restored upon selection.
3.4.7 AutoGain
The automatic receiver gain function analyzes the bottom return and automati-
cally increases or decreases the receiver gain accordingly. The AutoGain menu
item controls the level of signal amplitude threshold to which the sonar return is
compared.
Operator settings are OFF and 1 to 10. The lower the selected number, the lower
the threshold and therefore, the lower the gain setting selected by the Sonar
Processor. A typical setting is 4. However, the optimal setting will vary with bot-
tom type and other environmental conditions.
NOTE
Good bottom detect conditions must be present before the AutoGain function will
perform properly.
3.5 Ocean
Menu
The Ocean Menu allows the operator to enter various correction factors to com-
pensate for changing environmental conditions. Menu selections are:
3.5.1 Spread
This selection allows the operator to enter the amount of cylindrical and spherical
spreading loss that is expected through the ambient water medium. This coeffi-
cient value is used in conjunction with the absorption loss value (ABSORB) to
recompute the TVG curve that will be applied to the returned signal (see the
GainMode menu selection). The range available is from 0 to 60. An initial value
of 30 is recommended.
3.5.2 Absorb
This selection allows the operator to enter the amount of loss expected through
the ambient water medium. This value is used in conjunction with the spreading
loss value (SPREAD) to recompute the TVG curve as described above. The
range available is from 0 to 120 dB/km. If the exact value is not known, a value of
70 dB/km for salt water and 20 dB/km for fresh water is recommended.
3.5.3 Velocity
The displayed value is the speed of sound through the local water at the Sonar
Head (in meters per second) that the Sonar Processor will use in projector steer-
ing computations. Output data packets are not corrected for a sound velocity pro-
______________________________________________________________________
SeaBat 8101 Operator's Manual
3-7
Version 3.02
