Definition
Low-power VHF radio transmitters along the approach course of an Instrument Landing System (ILS) that radiate a narrow signal straight up. As the aircraft passes overhead, the airborne marker beacon receiver triggers a coded audio tone and a colored light in the cockpit, telling the pilot a specific distance from the runway threshold. The standard ILS markers are the Outer Marker (blue light, slow Morse dashes), the Middle Marker (amber light, alternating dots and dashes), and the Inner Marker (white light, rapid dots).
Plain English
Small ground transmitters that send a beam of radio signal straight up. When the aircraft flies over one during an instrument approach, a light flashes and a tone plays in the cockpit so the pilot knows exactly which point along the approach they have just crossed.
Context Anchor
Seen during ILS approach discussions, especially when identifying the outer marker, middle marker, or inner marker.
Derivation
Marker comes from the idea of marking a position. Beacon comes from Old English beacen, meaning a sign or signal. Together: a radio signal that marks a specific spot along the approach path.
Why Pilots Care
They give precise position fixes during low-visibility approaches, helping pilots maintain awareness and decide when to land or go around.
Grounding Statement
As the aircraft flies through the radio signal, the cockpit indication tells the pilot that a known point on the approach has just been crossed.
Intuition Check
Do not think of marker beacons as runway lights or physical signs the pilot looks for outside. In this context, they are radio signals received in the cockpit.
Example Sentence 1
Crossing the outer marker, the pilot heard the slow tone and saw the blue light flash, confirming the aircraft was established on the ILS at the correct distance from the runway.
Example Sentence 2
Passing the middle marker beacon confirmed the aircraft was at decision height and aligned with the runway.