Simrad 000-15567-001, Instruction Manual

Page: 25 / 114

OPERATION
20221511 / D
17
Speed
The GC80/GC85 gyro compass calculates bearing based on the
speed and latitude information that is input to the gyro as speed
source. Any error in speed input will therefore cause incorrect
true bearing from the gyro compass.
Press the
DISP
button until the vessel’s speed information is
displayed.
Confirm in 4 hours intervals that the displayed speed is the same
as the vessel’s actual speed.
Any discrepancy between displayed speed and actual speed is
corrected as described in
Setting the Latitude input source
,
page 53.
Speed error correction
All gyro compasses will generate a heading error caused by the
vessel speed and earth rotation. GC80/GC85 calculates this error
based on latitude and speed input information, and corrects
automatically the bearing signal output. If no speed information
is available, the gyro compass will output a heading error either
westwards or eastwards depending on the vessel’s course.
If speed information is unavailable, the figure on next page may
be used for manually calculating the heading error.
In this figure, the following values are used as example:
- Latitude: 40°
- Vessel speed: 16 knots
- Vessel heading: 30°
The heading error is found by:
1
Drawing a line between the latitude and the vessel heading
(shown with gray line on the figure)
2
Drawing a straight line (broken line in the figure) between
the vessel speed and the point where the latitude/heading
line intersects with the solid black line in the figure.
In the example above, the figure shows a speed error of appr.
1.1°, and the true bearing should then be 30°-1.1° = 28.9°.
Note!
When the course is within 270° - 0° - 90°, true heading is found
by subtracting the speed error from the compass heading.
If the course is within 90° - 180° - 270°, true heading is found
by adding the speed error to compass heading.