A gyro compass is a non-magnetic compass which is based on a wheel/disc that rotates at high speed about it’s spin axis as well as the rotation of the earth.
Gyro compasses have two main advantages over magnetic compasses:
- They find true north as determined by the axis of the Earth’s rotation.
- They are unaffected by ferromagnetic materials.
The gyro compass losses all horizontal directive force as the poles are approached and is thought to become useless at about 85Â° of latitude. It is generally reliable up to 70Â° but thereafter should be checked by azimuths of celestial bodies at frequent intervals (about every 4 hours and more frequently in higher latitudes).
This error is present in a compass which is damped in tilt. A compass damped in tilt always settles east of the meridian and above the horizon in NH, and vice-versa. Its magnitude depends on the observers latitude. Damping Error 1.5 (tan Latitude)
At equator, the error is nil. The error is eastward in all northerly latitudes, and vice-versa.
For the purpose of damping error, a latitude rider / adjuster is provided with the gyro compass which shifts the lubber line equal to the amount of error in the appropriate direction.
Steaming (Course, Speed and Latitude Error)
A gyro will always settle on the meridian, however when it is on a ship/vessel the gyro compass will be subjected to the movement of the earth AND the movement of the vessel. Imagine a ship steaming Northward. As it moves around the curvature of the earth the gyro, attempting to remain directed to a fixed point in space, will start to tilt upwards. The direction in which the compass settles, is therefore, different to the direction of the True North and depends on ship’s course, speed and latitude of the observer.
This error also increases as the observer’s latitude increases. The error is westward on all Northerly courses and vice-versa.
In exactly E-W courses, the error is nil. In exactly N-S courses, the error is maximum.
Manoeuvring (ballistic) error
This error occurs whenever the ship is subject to rapid changes of speed or heading. Because of its pendulous nature, the compass gravity control moves away from the centre of gravity whenever the vessel changes speed or alters course. Torque produced about the horizontal and vertical axis by manoeuvring cause the gyro mechanism to process in both azimuth and tilt. If the ship is steaming due north and rapidly reducing speed, mercury will continue to flow into the north pot, or the gravity pendulum continues to swing, making the gyro spin axis north heavy and thus causing a precession in azimuth.