Marker Beacon

A Marker Beacon is a low-power, ground-based VHF transmitter (75 MHz) that emits a focused vertical beam to identify specific positions along an ILS approach path.

• Outer Marker (OM): Located 4–7 NM from the runway threshold — blue light, continuous dashes at 400 Hz. Indicates the point to begin final descent on the glideslope
• Middle Marker (MM): Located ~3,500 ft from the threshold — amber light, alternating dots and dashes at 1,300 Hz. Approximately at decision height for Category I ILS
• Inner Marker (IM): Located at the runway threshold — white light, continuous dots at 3,000 Hz. Used for Category II approaches

Marker beacons are being phased out and replaced by GPS-based fixes and DME distances.