Originally Posted by grounded27
...Yes but it relies on an IRU for heading information on many type aircraft.
I find that hard to believe. An aircraft as small as a new production Twin Otter uses a separate, dedicated magnetometer to provide the ESIS (Emergency Standby Instrument System) with heading information, and both the ESIS and the ESIS magnetometer are powered from their own battery, which has 6 hours endurance (same as the aircraft fuel tanks) and is independent of the aircraft electrical system.