Furuno CI-68 Operator's Manual page 63

Ship's Speed
(Here, speed is a vector value including velocity and direction.)
Depending on the base of measurement, ship's speed is expressed in two ways:
Ground tracking speed:
(Absolute speed)
Water tracking speed:
(Relative speed)
Navaid-generated speed: Ship's speed and course obtained by external navigation
(Absolute speed)
Water tracking speed
Ground tracking speed
Tide
Tide is the movement of watermass at a particular depth.
To know absolute tide (speed on ground), the following two data are required:
1) Ship's speed and course based on ground
2) Ship's speed and course based on measuring layer (A)
Absolute tide is, then, given as a difference of these two speed vectors.
Ship's speed and course relative to bottom (fixed base)
Ship's speed and course relative to water layer just below the
vessel (floating base)
equipment (GPS)
GPS satellites
Fixed
PRINCIPLE OF OPERATION
Navaid-generated speed
(Pseudo ground tracking speed)
Near-surface layer
: Base of measurement
AP-7