A gyro-compass is subject to a latitude-speed error; an error increasing with latitude and speed of the ship.
Here you'll find a table, so you'll have an idea about the value of this "latitude-speed-error".
This error has some implications, especially with integrated bridge systems.
Have an example:
After a few seconds the rudder returned to midships. This happened quite a few times during that week.
Pretty dangerous stuff; should'nt give hard over rudder with engines full ahead and a loaded ship. It's not healthy for the rudders, and the ship lists too much for ease of mind.
It was a mystery.
· The gyrocompasses switchbox gets a latitude and speed input from the GPS-receiver.
· From this input the gyro-compass system calculates automatically the lat.-speed error,
· and applies this error to the gyro-course.
· The GPS experienced irregular short blinks, with large position jumps in the GPS. GPS calculated the ship's speed as 50 knots, catching up with the faulty positions. (*)
· This data was send to the gyrocompass,
which calculated the lat.-speed error instantly as 10 ° (see table)
· the gyro applied this error, all in a matter of microseconds,
· and the autopilot gave hard over rudder, as the ship was 10 ° off course, in an instant
(*) The glitches in the GPS-position may have been caused by the mountaineous terrain, maybe blocking GPS satellite signals close by land. Who knows ?
A gyro-error can also show up during trenching projects with a trailing suction hopper dredger.
An error of one degree on the gyro-course will make an error in position of the draghead of 2 meter (with ship's length 150 meter).
Since the error goes both ways, sailing on opposite courses while dredging a trench, you are making two trenches, besides the originally planned trench. (*)
Dredging high spots is another example.
If the position of the draghead errs by a few meters; high spots will prove hard to hit.
(*) To prevent this, gyro's are calibrated before starting precision dredging jobs.
Marc Van de Velde