Furuno CI-68 Operator's Manual page 60

TIDE MEASUREMENT IN NAV MODE
Why gyro data is important in the NAV mode
If you consider the difference of tide vector calculations in the ground tracking mode and the
NAV mode, you can see the importance of gyro (heading) data.
Tide calculation in the ground tracking mode
In the ground tracking mode, the current
indicator derives tide information from the
following data:
(1) Ship's speed/course based on ground
(ground speed)
(2) Ship's speed/course based on target layer
(water speed)
(3) Bearing of ship's bow (heading by gyro)
Both ground speed (1) and water speed (2)
are sensed by the common transducers
mounted on the hull. Thus, they are the speed
vectors with respect to the ship's bow. No
matter what the true bearing of the ship's
heading be, the relative angle of these two
vectors does not change. The tide vector is
simply given as the difference of these two
speed vectors.
The heading data by gyro (3) is used after the
tide vector calculation just to express the
vector directions in true bearings (north
referenced).
Even if the gyro data contains error, the sizes
and relative angles of the vectors do not
change. The gyro error only affects the
bearing indication of each vector.
AP-4
: True bearing of ship's bow (ref. north)
A
: Gyro reading
H
: Direction of ground speed (ref. bow)
G
: Direction of water speed (ref. bow)
W
True north
Ship's bow direction
=
H
A
W
G
Tide Vector
Without gyro error
True north
A
H
W
Gyro
error
G
Tide Vector
With gyro error
True bearing of each speed vector contains
fixed error. But, their sizes and relative angles
are maintained despite the gyro error.
Ship's bow direction