Furuno CI-68 - Manual (Page 63)

PRINCIPLE OF OPERATION
AP-7
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: Ship’s speed and course relative to bottom (fixed base)
(Absolute speed)
Water tracking speed:
Ship’s speed and course relative to water layer just below the
(Relative speed) vessel (floating base)
Navaid-generated speed: Ship’s speed and course obtained by external navigation
(Absolute speed) equipment (GPS)
Fixed
GPS satellites
Navaid-generated speed
(Pseudo ground tracking speed)
Water tracking speed
Ground tracking speed
Near-surface layer
: Base of measurement
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.