and the convergence of the meridian lines towards the poles. It equals the course change along a great circle (orthodrome) flight path. Variations Some more expensive heading indicators are "slaved" to a magnetic sensor, called a flux gate. The flux gate continuously senses the Earth's magnetic field, and a servo mechanism constantly corrects the heading indicator. These "slaved gyros" reduce pilot workload by eliminating the need for manual realignment every ten to fifteen minutes. The prediction of drift in degrees per hour, is as follows: Although it is possible to predict the drift, there will be minor variations from this basic model, accounted for by gimbal error (operating the aircraft away from the local horizontal), among others. A common source of error here is the improper setting of the latitude nut (to the opposite hemisphere for example). The table however allows one to gauge whether an indicator is behaving as expected, and as such, is compared with the realignment corrections made with reference to the magnetic compass. Transport wander is an undesirable consequence of apparent drift. See also Acronyms and abbreviations in avionics Earth Inductor Compass Gyrocompass, a compass depending on gyroscopic precession effect instead of a basic gyroscopic effect Horizontal situation indicator Inertial navigation system, a far more complex system of gyroscopes that also employ accelerometers References Footnotes Notes Aircraft instruments Avionics