Marker Beacon

A marker beacon is a type of radio navigation aid transmitting on 75 MHz in a narrow, fan-shaped vertical beam. As an aircraft passes over the marker, it triggers the marker beacon receiver and produces an audible tone and visual indicator light. Markers provide fixed-position fixes on instrument approaches.

ILS Marker Beacons

  • Outer Marker (OM): 4–7 NM from runway; identifies the point at which aircraft intercept the glide slope from below; blue light; two dashes per second (− −)
  • Middle Marker (MM): Approximately 3,500 ft from threshold; indicates the CAT I decision height point; amber light; alternating dots and dashes (· − · −)
  • Inner Marker (IM): Approximately 300–500 ft from threshold; used in CAT II/III; white light; six dots per second (· · · · · ·)

Marker Receiver Settings

Marker beacon receivers have a sensitivity switch (high and low). Use high sensitivity in cruise for longer range detection; use low sensitivity on approach for more precise passage indication. Many marker receivers are automatically integrated into ILS receiver operation on modern avionics.

Related Terms

See also: Outer Marker, Middle Marker, Inner Marker, ILS.