How do marker beacons tell a pilot where they are on the approach?
Small transmitters under the approach path send a beam straight up, and the aircraft receiver turns each one into a tone and a coloured lamp, near the same centreline where the approach lights stand.
A marker beacon is a low-power VHF transmitter on the extended runway centreline, under the ILS approach path. It sends a narrow fan-shaped beam straight up. When the aircraft flies through the beam, its receiver sounds a tone and lights a lamp, so the pilot gets a position fix along the approach. The outer marker gives a 400 Hz tone in dashes with a blue lamp, the middle marker gives 1300 Hz in alternating dots and dashes with an amber lamp, and the inner marker, where fitted, gives 3000 Hz in dots with a white lamp.
The outer marker is typically several kilometres from the threshold, where the aircraft checks its height against the glide path. The middle marker is about a kilometre from the threshold and warns the pilot that visual guidance is close. Many airports now use a DME or another fix instead of some or all of the markers, so a runway may have all three, some or none.
Marker beacons are not part of the AGL system, but they share the approach centreline with the approach lights. The middle marker sits near the outer end of a full-length approach lighting system, and an inner marker, where fitted, falls inside the approach lighting area. Land, access, cable routes and obstacle rules are therefore shared and agreed with the navaid owner. Marker antennas are low, new lighting structures close to them are reviewed, and works near a marker are agreed in advance.
Frequently Asked Questions
Q1What do marker beacons do?
A marker beacon is a small VHF transmitter on the extended runway centreline, under the approach path, that sends a narrow beam straight up. When the aircraft flies through the beam, its receiver sounds a tone and lights a lamp, so the pilot gets a position fix along the approach.
Q2What is the difference between the outer, middle and inner marker?
The outer marker is the farthest from the runway and uses a 400 Hz tone in dashes with a blue lamp. The middle marker is about a kilometre from the threshold and uses 1300 Hz in alternating dots and dashes with an amber lamp. The inner marker, where fitted, is closest to the threshold and uses 3000 Hz in dots with a white lamp.
Q3Are marker beacons still used?
Fewer than before. Many airports have replaced marker beacons with a DME or another fix on the approach, so a given runway may have all three, some or none. What is installed at an airfield is set in its ILS design and the published procedures.
Q4Are marker beacons part of airfield ground lighting?
No. They are radio navigation aids owned and maintained by the air navigation service provider or the airport's navaid team. They meet the AGL system on the ground, because the markers sit on the same centreline as the approach lights.
Q5Why does the middle marker matter to the approach lights?
It is placed to warn the pilot that visual guidance is close. On many runways it sits near the outer end of a full-length approach lighting system, so the pilot hears the middle marker shortly before the approach lights come into view.
Q6What should be done before working near a marker beacon?
Tell the navaid owner before the work and agree the dates and the method. Keep tall equipment and large metal objects away from the antenna while the ILS is in use, and have the navaid owner decide whether the marker needs checking before it goes back into service.
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