The Gyro Reference Unit (GRU), located in the doppler well aft of FS160, incorporates a very low drift characteristic and consists of a Vertical Gyro (VG) and Directional Gyro (DG). Both gyros are mounted in servoed gimbal in order to provide accurate heading indications independent of the attitude of the aircraft and throughout manoeuvring flight. The GRU also contains transmitters capable of providing pitch, roll and azimuth information. The GRU operates as a free DG corrected for meridian convergency, earth’s rate or residual transport wander. It may also be operated slaved to the magnetic flux valve signal discussed in paragraph 35.