Gyrocompasses

How a gyrocompass finds north

A mechanical gyrocompass spins a rotor inside a gyrosphere or sensitive element. Gravity control and damping make the spin axis settle on the meridian. A cold start takes hours to settle. Speed and latitude errors need corrections, which modern units take automatically from GNSS and the speed log.

A fibre-optic gyrocompass (FOG) measures the earth's rotation rate with three fibre-optic gyros using the Sagnac effect, and finds north from it. It has no moving parts and settles much faster than a mechanical unit. Many FOG units also output roll and pitch, and some add heave or full inertial navigation.

Key specifications

Standards and what they ask of a used unit

IMO resolution A.424(XI) is the gyrocompass performance standard and ISO 8728 its test standard. A heading device that sends heading to other equipment is covered by MSC.116(73) and ISO 22090-1. SOLAS chapter V regulation 19 requires a gyrocompass, or other non-magnetic means of finding heading, with heading and bearing repeaters on ships of 500 GT and up, and a rate-of-turn indicator from 50,000 GT. IEC 60945 covers the environmental tests.

For DP, the class rules and the FMEA count the heading references. DP class 2 and 3 units normally carry three gyrocompasses so the controller can vote out a bad one. A FOG unit that supplies both heading and motion to DP is a common-mode point, since one fault removes two sensor types at once, and the FMEA has to treat it that way.

Failure modes and inspection

On a mechanical gyro:

• The gyrosphere wears with running hours and is exchanged at OEM intervals.
• Support fluid degrades and must be changed.
• Follow-up motors, pumps, fans and power supplies fail.
• A unit stored on its side or shocked in transit can damage the sensitive element.

On a FOG, the light source and electronics age, and calibration drifts. Repairs go back to the OEM, which makes OEM support status decisive.

A survey compares heading against an azimuth observation or a reference GNSS compass alongside, logs the settle time from cold, reads the error log and checks every output channel under load.

Documentation

• Gyrosphere serial number, running hours and last exchange date.
• Last OEM service report, with the measured heading error.
• Type-approval certificate for the configuration offered.
• For FOG units, the last OEM calibration certificate.

Integration with your unit

A heading management or switchover unit selects which gyro feeds which consumer and alarms on disagreement. Check that the used gyro's outputs suit your consumers. An older vessel may need step or synchro signals for repeaters, while a DP controller wants IEC 61162 at a high update rate. Speed and latitude inputs must come from your GNSS and log. Mounting must align with the fore-and-aft line, and any offset is corrected at commissioning.

Refurbishment, used or surplus

Gyrosphere exchange programmes from the OEMs are the normal way to keep a mechanical gyro in service, and an exchanged sphere with a service report is a sound spare. A complete used gyro suits a unit already running the same model, so repeaters and switchover units carry over. For a new DP heading reference, unused surplus or new FOG units remove the gyrosphere cost altogether.

Related equipment

Heading feeds radar systems, and DP setups pair gyros with motion reference units. Naventr can arrange an inspection of a used gyro on request, at the Seller's site.