5.marker-beacons. Marker Beacons
Marker beacons are low-power transmitters that radiate a fan-shaped signal vertically upward, providing pilots with a positive indication of passage over a specific point along an instrument approach course or airway. Operating on a single, standardized frequency of 75 MHz, marker beacons are received by a dedicated receiver in the aircraft and provide both an aural tone and a visual light indication when the aircraft passes through the beam.
Because every marker beacon transmits on the same 75 MHz frequency, the pilot does not tune the receiver — identification of which marker is being crossed is accomplished by the distinct color of the indicator light, the modulation frequency of the audio tone, and the keyed Morse code identifier.
Types of Marker Beacons
Three types of marker beacons are associated with the standard Instrument Landing System (ILS):
- Outer Marker (OM) — Located on the localizer course, typically 4 to 7 NM from the runway threshold. The OM marks the point at which an aircraft on glideslope at the published intercept altitude will intercept the glidepath. It transmits a 400 Hz continuous series of dashes (two dashes per second) and illuminates a blue light on the marker beacon panel.
- Middle Marker (MM) — Located approximately 3,500 feet from the runway threshold, where the glideslope crosses the runway threshold elevation by approximately 200 feet AGL — historically corresponding to Category I decision height. The MM transmits a 1,300 Hz alternating dot-dash pattern and illuminates an amber light. Note that with the proliferation of GPS and DME-based fixes, many MMs have been decommissioned and are no longer required for Category I ILS approaches.
- Inner Marker (IM) — Used on Category II and III ILS approaches, located between the MM and the runway threshold (approximately 1,000 feet from the threshold) at the point where the aircraft on glidepath reaches the Category II decision height (about 100 feet AGL). The IM transmits a 3,000 Hz continuous series of dots (six dots per second) and illuminates a white light.
Quick Reference Summary
- OM: 400 Hz, dashes (— — —), blue light
- MM: 1,300 Hz, alternating dot-dash (· — · —), amber light
- IM: 3,000 Hz, dots (· · · · · ·), white light
Airway and Other Markers
In addition to the ILS markers, marker beacons were historically used along Federal airways. The fan marker and Z-marker (or station-location marker, located directly over a low-frequency range station) were employed to identify position fixes along Victor airways. With the advent of VOR, DME, GPS, and RNAV technology, the airway use of marker beacons has been almost entirely phased out, though the equipment specification remains in use for ILS applications.
Receiver and Cockpit Indications
The airborne marker beacon receiver is fixed-tuned to 75 MHz and typically integrated into the audio panel. Most receivers offer a HI/LO sensitivity switch:
- LO sensitivity is used during approach to obtain a sharp, well-defined indication of station passage.
- HI sensitivity provides earlier warning of approaching the station and is useful for general en route awareness.
A TEST function verifies that all three lamps are operational. When passing over a marker, the appropriate light illuminates and the audio tone is heard through the cockpit speaker or headset, provided the marker audio is selected on the audio panel.
Operational Use on the ILS
During an ILS approach, the marker beacons provide an excellent cross-check of altitude versus position:
- At the OM, verify that the altimeter indicates the published glideslope intercept altitude. This is a critical check that the glideslope signal is valid; if altitude and glideslope do not agree at OM passage, the glideslope may be unreliable.
- At the MM (if installed), the aircraft should be approximately at the Category I decision height (typically 200 feet AGL), and the runway environment should be in sight if the approach is to be continued.
- At the IM, the aircraft is at Category II decision height.
Example: On the ILS RWY 36 approach with a glideslope intercept altitude of 2,000 feet MSL and field elevation of 600 feet, the pilot should see a steady blue OM light and hear two dashes per second at 2,000 feet MSL approximately 5 NM from the runway. Crossing the MM, the amber light flashes and a dot-dash tone is heard while the aircraft is approximately 200 feet AGL.
Substitutes for Marker Beacons
Under current FAA policy, DME, GPS, surveillance radar, NDB compass locators, or a published cross-radial may be substituted for the OM where authorized on the approach chart. Compass locators (low-power NDBs colocated with the OM, sometimes called LOM, transmitting the first two letters of the localizer identifier) are common substitutes. Always consult the approach plate to determine what fixes are required and what substitutes are authorized.