Marker beacon

From the 1930s until the 1950s, markers were used extensively along airways to provide an indication of an aircraft's specific position along the route, but from the 1960s they have become increasingly limited to ILS approach installations.

They are now very gradually being phased out of service, especially in more developed parts of the world, as GPS and other technologies have made marker beacons increasingly redundant.

The system gives the pilot a visual (blinking blue outer marker light) and aural (continuous series of audio tone morse code-like 'dashes') indication.

This alerts the pilots that they are descending through the CAT I decision altitude (typically 200 feet (60 m) above the ground level on the glideslope) and should have already initiated the missed approach if one of several visual cues has not been spotted.Similar to the outer and middle markers, a inner marker located at the beginning (threshold) of the runway on some ILS approach systems (usually Category II and III) having decision heights of less than 200 feet (60 m) AGL.

A "Z" marker was sometimes located at low- or medium-frequency range sites to accurately denote station passage.

A back course marker (BC) normally indicates the ILS back-course final-approach fix where approach descent is commenced.

[6][7] Fan-type marker beacons were sometimes part of a non-precision approach and are identified by a flashing white light and a repeating dot-dash-dot signal.