Marker Beacon

A Marker Beacon is a ground‑based VHF radio transmitter that radiates a narrow, fan‑shaped signal vertically upward. It is used in conjunction with the Instrument Landing System (ILS) to provide distance‑fix information to pilots during the final approach. The system typically consists of three marker beacons located at specific distances from the runway threshold:

• Outer Marker (OM): Located approximately 4–7 nautical miles (7.4–13 km) from the threshold, at or near the glide slope intercept altitude. It identifies the initial approach fix and the final approach segment. The signal is modulated at 400 Hz and produces a continuous series of dashes (— — —) in the cockpit.
• Middle Marker (MM): Located approximately 1.1 km (0.7 miles) from the threshold. It identifies the decision height point for CAT I approaches. The signal is modulated at 1300 Hz and produces alternating dots and dashes (· — · —).
• Inner Marker (IM): Located approximately 300 metres (1,000 ft) from the threshold. It identifies the decision height point for CAT II/III approaches. The signal is modulated at 3000 Hz and produces a continuous series of dots (· · · ·).

Each marker beacon transmits a 75 MHz carrier, which is received by the aircraft’s marker beacon receiver, triggering both an aural tone and a visual light (blue, amber, white) in the cockpit to inform the pilot of the aircraft’s position along the approach.

The marker beacon's purpose is to give the pilot a positive, unambiguous confirmation of specific distances along the final approach, independent of the localizer and glide slope signals that provide continuous lateral and vertical guidance. Because each of the three beacons uses a different modulation tone and produces a distinct aural and visual cue in the cockpit, a pilot flying the approach gets a series of confirmed checkpoints, initial approach fix, then decision height for CAT I, then decision height for CAT II/III, without needing to interpret a continuously varying deviation indicator. This distance-fix role is largely what DME now provides on most modern approaches, but where marker beacons remain installed they still serve the same underlying purpose of marking a fixed point along the approach path with a signal the pilot cannot mistake for anything else.

Marker beacons are installed on precision approach runways that support ILS operations. They are used for:

• CAT I Precision Approaches: The outer and middle markers provide critical distance and decision height references.
• CAT II and CAT III Precision Approaches: The middle and inner markers provide enhanced distance and decision height references for low‑visibility operations.
• Military Airfields: Supporting military ILS approaches and training.
• Non‑ILS Approaches: In some cases, marker beacons are also used for localiser‑only or VOR approaches as a distance‑fix aid.

Each marker sits directly on the extended runway centreline, at the fixed distance from the threshold its role requires — the outer marker roughly 4-7 nm out, the middle marker around 3,500 ft from the threshold, and the inner marker (where fitted) just short of it. The antenna radiates a narrow fan-shaped pattern straight upward, so the site must be clear of overhead obstructions and levelled to keep the beam correctly shaped; a small equipment hut at the base houses the transmitter, monitoring electronics, and standby power supply. Because each marker's exact position defines the distance and decision-height cue it gives the pilot, siting is tied to a precise survey referenced to the runway threshold, with the location fixed on the published approach chart.

The marker beacon system operates on a simple but reliable principle:

• Signal Transmission: Each marker beacon transmitter generates a 75 MHz carrier signal that is modulated with a specific audio frequency (400 Hz, 1300 Hz, or 3000 Hz). The signal is radiated vertically upward using a directional antenna that creates a narrow, fan‑shaped pattern (approximately 20° to 40° wide).
• Aircraft Reception: As the aircraft passes over the beacon, the aircraft’s marker beacon receiver detects the 75 MHz signal, demodulates the audio tone, and triggers both an aural tone and a visual indication in the cockpit.
• Visual Indication: The cockpit marker beacon display typically includes three coloured lights: blue (outer), amber (middle), and white (inner). The appropriate light flashes in sync with the received audio tone.
• Distance Fix: The known distances of the markers from the runway threshold provide the pilot with a precise distance‑fix, which is used to confirm the aircraft’s position along the final approach and to verify the decision height.

The marker beacon is one of the simplest transmitting facilities on the airfield and carries a correspondingly thin control loop. The transmitter runs continuously at fixed power and frequency (75 MHz) with a fixed keying pattern for its identity (outer, middle or inner), so there is no dynamic command input to regulate during normal operation. An internal monitor checks transmitted power and modulation depth against tolerance; if either falls out of tolerance, the unit reports a fault — and on more advanced installations may shut down — so that a controller or maintenance system is alerted rather than an aircraft continuing to receive a degraded or false marker indication.

The transmitter and its monitoring electronics are housed in a weatherproof equipment shelter or rack cabinet near the antenna site, built with dual-redundant, hot-standby transmitter modules so a fault in the active unit triggers an automatic changeover without interrupting the signal. The antenna itself is a fixed, vertically polarised array — typically a set of dipole or Alford-loop elements arranged to produce the directional, fan-shaped radiation pattern that defines the beacon's coverage — mounted on a low, frangible support structure set well clear of the approach or runway centreline so it poses no obstruction hazard. Cabling runs from the shelter to the antenna in a weatherproof feed line, and the whole installation is bonded to a local earthing system for lightning protection. Power comes from the mains supply backed by an uninterruptible supply and standby generator, and a continuous self-monitoring circuit compares transmitted signal parameters against tolerance limits, raising an alarm to the control tower or maintenance system the moment the beacon drifts out of specification.