Orals Revision: Gyro Compass — Principles & Errors

Orals revision card — MIN 653 (M) Amendment 1, same competence as the magnetic compass card: 'Determine and allow for compass errors.' The syllabus explicitly also expects knowledge of the control of the master gyro and repeaters, and knowledge of the main types of gyro-compass — not just the error theory below.

Why a Gyrocompass Points North at All

A gyrocompass isn't magnetic — it uses a fast-spinning rotor's gyroscopic inertia (resists changes to its spin axis orientation) combined with a control system exploiting the Earth's rotation and gravity, so the spin axis precesses until it settles pointing toward True North, not Magnetic North. That's the headline advantage over a magnetic compass: no variation, and (ideally) no deviation from the ship's own magnetism.

The Four Errors You Need to Know

• Speed (course and speed) error — also called steaming error. The gyro senses the combined rotation of Earth and the ship's own movement over the Earth's surface, not Earth's rotation alone. Depends on speed, course and latitude. Largest on northerly/southerly courses, zero on easterly/westerly courses — a genuinely testable, specific fact, not just "it has an error."
• Latitude (damping/settling) error — the spin axis settles slightly off true north due to eccentricities in the damping mechanism itself. Distinct from speed error — this one is about the compass's own settling behaviour, not the ship's motion.
• Ballistic deflection error — temporary, occurs during rapid changes in speed or course (e.g. a hard turn, a sudden acceleration). Disappears once the ship settles back onto a steady course and speed.
• Ballistic damping error — also temporary, also triggered by abrupt speed/course changes, but arising specifically from how those changes interact with the damping mechanism rather than the deflection itself. Easy to conflate with ballistic deflection error in an oral — know that they're related but distinct, and be ready to say so if pressed.

What Examiners Actually Push On

• The speed-error direction rule (max on N/S, zero on E/W) is a clean, specific fact examiners like to test — know it cold, don't hedge
• Be ready to explain why a gyrocompass needs a settling/damping period after being switched on, tying back to the latitude/damping error above
• Know the practical answer too: how do you actually check gyro error on watch? (Comparing against an azimuth/amplitude of a celestial body, or against the standard/master compass, is the expected real-world answer.)

Quick Self-Check

• On which courses is speed error at its maximum, and why is it zero on E/W courses?
• What's the practical difference between ballistic deflection error and ballistic damping error?
• Why does True North from a gyrocompass not require a variation or deviation correction the way magnetic compass readings do?

Test yourself: Navigation Knowledge Checks.

Related Reading

• Magnetic Compass — Parts, Coefficients A–E & Corrections — the companion card for the other half of this MIN 653 competence