There are three marker beacons associated with ILS equipment namely Outer Marker, Middle Marker and Inner Marker. These beacons have a rated power output of 3 watt or less and an antenna array designed to produce an elliptical pattern. For proper reception of ILS marker beacons airborne marker beacon receiver works in low sensitivity mode.
Due to advances in both ground and airborne equipment in case DME is not available the requirement of using maker beacons are as follows:
Outer Marker OM
Outer Marker is suitable substitute for the identification of Final Approach Fix (FAF) for localiser only approach
Middle Marker MM
Middle Marker indicates the position of approximately 3,500ft from landing threshold. Aircraft height is approx 200ft above touchdown at this point. MM is no longer a regulatory requirement.
Inner Marker IM
Inner Marker indicates the point at which an aircraft is at decision height on glide path during a CAT 2 ILS approach. IM is required only for CAT 2 operations that do not have a published radio altitude (RA) minimum.
|OM||– – –||BLUE|
|MM||. – . –||AMBER|
|IM||. . . .||WHITE|
|BC||. . . .||WHITE|
Note: BC – stands for back course marker