Gyro Drift and Wander

Rigidity and precession in practice

Rigidity is what makes a gyro a usable reference at all: left alone, its spin axis resists any attempt to tilt it. Precession is the awkward consequence of trying to correct or move that axis deliberately, because pushing on one part of the rotor does not tilt the axis in the direction you pushed. It tilts it ninety degrees around, in the sense of rotation, from where you pushed.

This is why latitude correction mechanisms and gyro erecting systems are engineered rather than obvious: a small, continuous corrective torque applied at the right point produces the intended slow precession over time, instead of an immediate and unhelpful lurch in the wrong plane.

Real wander, apparent wander, and transport wander

Real wander is mechanical: bearing friction, mass unbalance, and manufacturing imperfections all cause the spin axis to creep away from where it was set, independent of where the aircraft or the earth are doing anything at all.

Apparent wander exists even for an imaginary, perfect gyro, because the earth rotates underneath it. A gyro that genuinely holds its orientation fixed in space appears, to an observer standing on the rotating earth, to drift steadily relative to the local horizontal and to true north. The component of that apparent drift that shows up as a heading error grows with latitude and vanishes at the equator, which is the earth-rate relationship covered below.

Transport wander is different again: it appears only once the aircraft is actually moving across the surface, because travelling east or west changes which local meridian counts as true north beneath the gyro, an effect of converging meridians rather than of the earth's rotation on the spot.

Earth rate and latitude correction

The heading-drift component of apparent wander, commonly called earth rate, is calculated as 15 degrees per hour multiplied by the sine of the latitude. At the equator the sine term is zero, so an uncorrected directional gyro sitting on the equator shows no apparent heading drift at all; at the poles the sine term reaches one, and the full 15 degrees per hour appears.

Because the size of the correction depends on where the aircraft actually is, some directional gyros include a latitude correction, or earth-rate correction, mechanism that the crew sets manually, and even where an automatic system is used, periodically resetting the gyro to a known compass heading remains standard practice precisely because real wander and any residual apparent wander both accumulate between resets.