B&G Network Depth - Manual (Page 10)

USING THE SIGNAL KEY
Press the SIGNAL key to display a letter of merit from A to F,
this is an indication of the return echo strength. A is excellent,
F is poor.
The sea bed can have an effect on the quality of the return
echo strength, i.e. very good echoes are returned from rock,
shale and sand, however poor echoes are returned from mud.
Therefore the merit letter could be A or B for rocks but C or D
for mud.
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