Marker beacon

Lore & Background

From the 1930s until the 1950s, marker beacons were used extensively along airways to indicate an aircraft's specific position along the route. From the 1960s onward, they became increasingly limited to ILS approach installations. They are now being gradually phased out, especially in more developed parts of the world, as GPS and other technologies have made them increasingly redundant.

Three types of marker beacons may be installed as part of an ILS. The outer marker, normally identifying the final approach fix, is situated on the same course as the localizer and runway centerline, four to seven nautical miles before the runway threshold.

It transmits a 400 Hz tone on a low-powered 75 MHz carrier signal. The middle marker works on the same principle and is positioned 0.5 to 0.8 nautical miles before the runway threshold, alerting pilots that they are descending through the Category I decision altitude. The inner marker, located at the runway threshold on some Category II and III systems, triggers a flashing white light and a series of audio tone dots at 3,000 Hz.

In the United States, the outer marker has often been combined with a non-directional beacon (NDB) to form a locator outer marker (LOM). LOMs are identified by two-letter Morse code modulated at 1020 Hz, using the first two letters of the parent ILS's identification. Some countries, such as Canada, have abandoned marker beacons completely, replacing the outer marker with an NDB and, more recently, with GPS fixes. Older marker beacon receivers sometimes used indicators labeled "A" (for airway or fan marker), "O" (outer), and "M" (middle), with the airway marker used for reporting points along now-obsolete "Red" airways.

Reader's Guide

Marker beacons have played a significant role in aviation navigation, particularly as part of instrument landing systems, providing pilots with precise position information along the approach path. Their use has declined as GPS navigation has become well established, and some countries have abandoned them entirely. The outer marker, often combined with an NDB as a locator outer marker, helped identify the final approach fix, while the middle marker indicated the decision altitude point. The inner marker served Category II and III approaches with lower decision heights.

Older fan markers and back course markers were used for en-route navigation and non-precision approaches, though recent FAA publications no longer mention fan markers. As of August 2024, nineteen fan markers remain in the FAA database, with seven listed as decommissioned. The gradual phase-out reflects the broader shift toward satellite-based navigation, though marker beacons remain in use at many ILS installations worldwide.