Furuno CI-68 Operator's Manual page 61

Tide calculation in NAV mode
In the NAV mode, the current indicator derives tide information from the following data:
(1) Ship's speed/course fed from navigator (GPS) (nav speed = pseudo ground speed)
(2) Ship's speed/course based on target layer (water speed)
(3) Bearing of ship's bow (heading by gyro)
Here, nav speed (1) is the ship's speed
vector sensed by the navigator, and the
moving direction is expressed with respect to
true north.
While the water speed (2) is the ship's speed
vector measured by the current indicator
using the Doppler shifts against the
measuring layer, and accordingly its direction
is expressed with respect to the ship's bow.
As the directional reference of these two
vectors are different (north vs ship's bow), we
can not simply calculate the difference of
vectors to obtain the tide vector in question.
So the direction of the water speed vector (2)
is converted to true bearing (north
referenced) by using the gyro data, and then
the tide vector is calculated.
If the gyro data contains error, the true
bearing of the water speed vector changes
(rotates) accordingly. While the bearing of the
nav speed vector does not change with the
gyro error. (The relative angle of these two
changes.)
In other words the tide vector, given as a
difference between these two speed vectors,
changes in size (speed) and bearing by the
gyro error. (Error in size and bearing means
the tide vector is unreliable and useless.)
To obtain dependable tide/tide differential
data, it is desirable to reduce the gyro error
below 1degree.
Note that the depth must be at least 40 m in
order to calculate tide.
TIDE MEASUREMENT IN NAV MODE
: True bearing of ship's bow (ref. north)
A
: Gyro reading
H
: True bearing of nav speed (ref. north)
N
: Direction of water speed (ref. bow)
W
True north
Ship's bow
=
H A
W
N
Tide Vector
Without gyro error
True north
A
H
W
Gyro
N
error
Tide Vector
With gyro error
Relative angle of water speed vector and
nav speed vector changes with gyro error.
Accordingly, the size (speed) and direction
of tide vector change.
Ship's bow
AP-5