An RBI has a compass rose with a needle displayed over it. North-South on the compass rose is aligned with the longitudinal axis of the aircraft. The end result is that wherever the needle points on the compass rose has the same angular relationship to a 360 deg circle drawn around the aircraft. Needle points to 30 deg, then the station is 30 deg to the right of the nose. Needle points to 180 deg, then the station is behind the aircraft. Needle points to 270 deg, then the station is abeam to the left.
An RMI is similar except that its North-South axis is kept aligned with the magnetic compass instead of the aircraft's longitudinal axis ie the RMI's compass rose acts like a compass. This means that when the needle moves to point at the radio station, the digits on the compass rose that the needle points to correspond to the bearing to the station, and the tail of the needle shows the bearing from the station.
Last edited by Tinstaafl; 10th Aug 2012 at 04:05.