About Tvg Gain - Simrad ES70 - REFERENCE MANUAL REV A Reference Manual

Echo sounder theory
The biomass value is also used by the researches to calculate how much fish there is in
the ocean. The final result is by far the value provided solely by the echo sounder, as
the numbers are subject to numerous calculations and adjustments made by experienced
scientists. Other means to establish the final result are trawling and catch data from the
fishery community. You may have seen that this biomass value can be very large.
Note
If you have other echo sounders or sonar running asynchronous with the ES70, the ES70
will also measure the transmit pulse from the secondary system. That will effectively
ruin the biomass calculations. You can remove the interference using the
Ping Filter
function in the dialog, but the ES70 receiver will still pick up the transmit
Echogram
pulse and implement it in the calculations. A full synchronization of the various acoustic
instruments is required. If your own vessel produces excessive noise this will also be
taken into the biomass calculations and provide you with inaccurate information.
Related topics
• Biomass information pane on page 51

About TVG gain

TVG means Time Varied Gain. When TVG is used in an echo sounder, we also some
times refer to it as depth variable gain.
Because the strength of the echoes will become weaker with increasing depth, the echo
sounder will automatically amplify the deepest echoes more than the shallower echoes.
In fact, the gain will increase proportional to how long the echo sounder "waits" for the
echoes. When you choose the TVG setting you can either switch it off (which we do NOT
recommended), or you can choose settings or .
School Gain (20 log R) Fish Gain (40 log R)
The various settings control the gain algorithms, how much gain to be applied when
the depth increases. When you choose the setting, you will apply
Fish Gain (40 log R)
more gain than if you choose the setting. This is simply because
School Gain (20 log R)
individual fishes emit smaller echoes than a school, and this makes them more difficult to
detect. In the equation the character R means "Range".
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