Furuno CI-68 Operator's Manual page 67

In the water tracking mode, watermass just below the transducer (near-surface layer) is
taken as the base of all measurements (virtual ground). Therefore, the ship and tide speeds
in the water tracking mode are not absolute but relative to this near-surface layer.
Vw = 9 kt (Ship's speed based on near-surface layer)
Vwa = 8 kt (Ship's speed based on layer A)
Vwb = 5 kt (Ship's speed based on layer B)
Start of
trip
Near-
surface
1
layer
Layer A
Layer B
1 kt
2 kt
10 NM (1 hour trip)
2 3
:Base of measurement
5 kt
End of
trip
Assume that the ship and all measuring layers
are moving in the same direction, but at different
speeds.
PRINCIPLE OF OPERATION
AP-11