Magnetic and Gyroscopic Compasses
Both compasses seek north, but for different reasons: the magnetic compass follows the earth's field, the gyrocompass follows the earth's rotation. The exam tests the north-seeking mechanism, the construction that supports it, and the errors each instrument carries.
Magnetic compass: field-driven
- A card carrying magnets sits on a pivot and jewel in a liquid-filled bowl with an expansion chamber, gimbals and a binnacle, read against the lubber line.
- The magnets align with the horizontal component H; directive force is greatest near the magnetic equator and falls to zero near the magnetic poles.
- The fluid damps oscillation and relieves the pivot of most of the card's weight.
- It indicates magnetic north, so deviation and variation must be applied to reach true.
Gyrocompass: inertia-driven
- A rotor spinning at high speed in an assembly free to precess; the earth's rotation tilts the spin axis and a pendulous element produces the precession that settles the axis in the meridian.
- North-seeking and true-referenced: no variation or deviation applies, only gyro error.
- It needs settling time and inputs of latitude and speed; errors include latitude error, speed or course error, and ballistic deflection during turns.
Worked example
The sun's true bearing is 135.0 degrees and the gyro repeater reads 132.5 degrees. Gyro error = true minus gyro = +2.5 degrees, that is 2.5 degrees E, with the gyro reading low. Remember 'compass least, error east'. Reversing it: true = gyro + easterly error = 132.5 + 2.5 = 135.0 degrees. No variation is applied to a gyro bearing; that correction belongs only to the magnetic compass.
Traps
- Applying variation or deviation to gyro bearings.
- Assuming gyro error is identical on all headings; speed error changes with course and speed.
- Believing the magnetic compass points to the geographic pole rather than the magnetic pole.
- Trusting a gyrocompass that has not settled or has no latitude and speed input.