What is a gyrocompass and how does it differ from a magnetic compass?

<p>A gyrocompass is a spinning-rotor instrument that settles on true north by physics alone, so unlike a magnetic compass it needs no correction for variation or deviation and is unaffected by the ship's own magnetic field.</p>A gyrocompass earns its place as the primary heading reference because it settles on true north through simple physics rather than through any external signal that can be jammed or lost. A gyroscope spinning at high speed resists any torque trying to tilt its axis, a property called rigidity in space. By mounting the spinning wheel so that gravity and the earth's rotation act on it in a controlled way, typically through a pendulous weight or a mercury ballistic that keeps the axis level, the gyroscope is made to settle with its axis pointing to true north and to hold that direction rather than oscillating.

Because it seeks true north directly, the gyrocompass needs no correction for magnetic variation, and being unaffected by the ship's own magnetic field, it has no deviation either, which is why its output can be trusted and distributed electrically to the autopilot, radar and ARPA, ECDIS, AIS, the VDR and the course recorder, giving every system on the ship the same true heading without each having to apply its own correction table.

A gyrocompass is not error-free. Speed and course changes introduce a temporary error, the ballistic deflection or speed error, because the compass reacts to the ship's own acceleration as well as the earth's rotation, and this is corrected by an internal or manual latitude and speed correction. High-latitude operation degrades performance because the horizontal component of the earth's rotation, which the compass depends on, weakens toward the poles, and most gyrocompasses become unreliable above roughly 70 to 80 degrees latitude. The gyro also takes an hour or more to settle after starting, so it must be started well before departure, and its heading is compared against the magnetic compass at intervals through the watch, with any error logged, to catch a fault before it propagates into the ARPA and autopilot.

Because so many systems depend on it, a gyrocompass failure is treated seriously: SOLAS requires ships to carry a magnetic compass as an independent, power-free backup precisely for this reason, and modern integrated bridges increasingly carry a second gyro or a satellite compass for additional redundancy. Watchkeepers are trained to notice a gyro that is drifting, hunting, or disagreeing with the magnetic compass by more than the expected error, and to report and cross-check rather than trust it blindly.