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 2 dashes or 6 dots 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