A compass using a gyroscope to find geographic north, unaffected by magnetic fields; mandatory on most large vessels.
Quick facts
Regulation
SOLAS Chapter V
In practice
The OOW must be aware that a gyrocompass exhibits a small latitude-dependent error and a speed error, both of which are corrections applied to the indicated heading. More critically, after a power interruption the compass requires several hours to settle and re-align with north; during this period, steering and bearing-taking must be performed on the magnetic compass. Regular comparison of gyro and magnetic compass readings—at least once per watch—remains a mandated safety practice that detects developing gyro error before it leads to a navigational incident.
Regulatory detail & full definition
A gyrocompass uses the principles of gyroscopic inertia and precession to seek and maintain alignment with the earth's true geographic north, rather than magnetic north. Unlike a magnetic compass, it is unaffected by the ship's own magnetic field, making it accurate for steering and for feeding heading data to radar, ECDIS, autopilot, and ARPA without requiring correction for variation or deviation. SOLAS Chapter V, Regulation 19 requires a gyrocompass or equivalent on ships of 500 GT and above on international voyages, and the Admiralty Manual of Navigation describes its settling time and latitude errors in detail.
The OOW must be aware that a gyrocompass exhibits a small latitude-dependent error and a speed error, both of which are corrections applied to the indicated heading. More critically, after a power interruption the compass requires several hours to settle and re-align with north; during this period, steering and bearing-taking must be performed on the magnetic compass. Regular comparison of gyro and magnetic compass readings—at least once per watch—remains a mandated safety practice that detects developing gyro error before it leads to a navigational incident.