What to check on a Gyrocompass.
A gyrocompass derives true heading rather than magnetic heading, either from a spinning mass that precesses to align with the earth's rotation axis, or, in newer fibre-optic gyro (FOG) designs, from the Sagnac effect measured by laser light travelling in opposite directions around a coiled fibre. Both approaches are immune to magnetic deviation, which is the reason gyrocompass output, not the magnetic compass, feeds radar, ECDIS, autopilot and AIS. SOLAS Chapter V allows a satellite compass as an alternative heading reference on some vessel classes, but the spinning-mass and FOG…
What Makes This Type
A gyrocompass derives true heading rather than magnetic heading, either from a spinning mass that precesses to align with the earth's rotation axis, or, in newer fibre-optic gyro (FOG) designs, from the Sagnac effect measured by laser light travelling in opposite directions around a coiled fibre. Both approaches are immune to magnetic deviation, which is the reason gyrocompass output, not the magnetic compass, feeds radar, ECDIS, autopilot and AIS. SOLAS Chapter V allows a satellite compass as an alternative heading reference on some vessel classes, but the spinning-mass and FOG gyrocompass remain the dominant fit because they keep working with no line of sight to satellites and are harder to spoof or jam than a GNSS-derived heading.
Main Components
- Master compass / sensor unit: the spinning rotor and its follow-up system in a conventional design, or the fibre-optic gyro assembly and processing electronics in a FOG unit.
- Repeater compasses: transmit heading to bridge wings, steering stand and other positions from the master unit, historically by step-transmission, now often by serial data.
- Speed and latitude correction input: conventional spinning-mass gyros need speed and latitude entered or fed automatically to correct for speed error and damping error, since the rotor's apparent north-seeking is distorted by the ship's own motion.
- Power supply and standby battery: gyrocompasses take time to settle after a cold start, so continuity of power during a blackout matters more here than for most bridge equipment.
Selection and Sizing
Settling time after start-up, typically a few hours for a spinning-mass unit against a shorter interval for FOG designs, is a practical factor on vessels that shut the compass down in port. Accuracy is normally specified within about 0.5 to 1 degree of true heading depending on latitude and vessel speed, degrading at high latitude as the horizontal component of earth rotation the mechanism relies on shrinks. Interface requirements matter as much as the sensor itself: the unit has to output the heading data format that ECDIS, radar and the integrated bridge system on board actually expect.
Regulations and Class
SOLAS Chapter V, Regulation 19 requires vessels of 500 GT and above engaged on international voyages to carry a gyrocompass or equivalent heading device, with a heading repeater at the emergency steering position. Class rules require periodic testing of heading accuracy against an independent reference and functional testing of alarms for power loss and excessive heading error. Type approval under the relevant performance standard is required before the unit can be fitted as the primary compliant heading source.
Typical Faults
| Fault | Consequence |
|---|---|
| Speed/latitude correction not updated or wired incorrectly | Steady-state heading error that grows with speed, feeding a wrong course into autopilot and radar |
| Follow-up system wear on spinning-mass units | Repeater compasses drift out of agreement with the master, confusing cross-checks on the bridge |
| Power interruption during a blackout | Full re-settling period needed after restart, leaving the vessel without a true heading reference for hours at the worst possible time |
| FOG fibre coil degradation with age | Slow accuracy drift that is easy to miss without a scheduled comparison against GPS-derived course |
What to Look for in a Supplier
- Type approval certificate valid for the vessel's flag and trading pattern.
- Confirmed data output protocol compatibility with the existing ECDIS, radar and autopilot rather than a unit needing a signal converter bolted on afterward.
- Documented settling time and standby power arrangement suited to how the vessel actually operates in port.
Cross-check gyro heading against GPS course made good on a steady leg at least once a watch - a slow drift is invisible on the repeater alone but obvious the moment you compare it to an independent source.