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The audio frequency modulation of the middle marker shall be keyed as follows…

  • A
    6 dots per second continuously.
  • B
    3 dashes, 3 dots and 3 dashes per second continuously.
  • C
    2 dashes per second continuously.
  • D
    a continuous series of alternate dots and dashes.

The Outer Marker, Middle Marker and Inner Marker all emit an amplitude-modulated, horizontally-polarised signal. The beacons operate at a 75 MHz carrier frequency.

Outer Marker (OM). To provide height, distance and equipment function checks for aircraft on final approach.

  • Aural identification: a 400 Hz low-pitch tone keyed in a form of 2 dashes per second (“- - - -“).
  • Visual representation (lights): Blue

Middle Marker (MM). Indicated the imminence of transition to visual guidance. Often defines the decision point.

  • Aural identification: a 1300 Hz medium-pitch tone keyed in a form of alternating dots and dashes with 3 dashes per second (“-.-.-.-“)
  • Visual representation (lights): Amber

Inner Marker (IM). Indicates the imminence of arrival above the threshold.

  • Aural identification: a 3000 Hz tone keyed continuous dots at a rate of 6 dots per second (“. . . . . . “).
  • Visual representation (lights): White

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