What is a Marker Beacon?
A marker beacon is a specialized VHF radio beacon operating at 75 MHz—a key component of the Instrument Landing System (ILS). These beacons deliver critical position indicators along approach paths through both visual and auditory cockpit cues. They are essential during low-visibility conditions.
The International Telecommunication Union (ITU) Radio Regulations, specifically Article 1.107, define a marker beacon as “a transmitter in the aeronautical radio navigation service which radiates vertically a distinctive pattern for providing position information to aircraft.” Their purpose is straightforward: transmitting distinctive vertical signals to aircraft overhead.
Types of Marker Beacons
Three distinct marker beacon types are used in traditional ILS approaches, strategically positioned at precise distances from the runway threshold:
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Outer Marker (OM): Located 4–7 nautical miles out, it marks the final approach fix where descent begins.
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Middle Marker (MM): Positioned 0.5–0.8 nautical miles out, it indicates a critical transition point near the decision height.
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Inner Marker (IM): Situated at the runway threshold, it is used for Category II and III precision approaches.
The Locator Outer Marker (LOM) is a variation, combining an outer marker with a non-directional beacon (NDB) to improve navigation. Pilots identify it through a distinctive two-letter Morse code signal—modulated at 1020 Hz—derived from the parent ILS’s identification code.
Outer Marker – Function and Signals
Positioned at the final approach fix, the outer marker (OM) signals a critical milestone: the aircraft has reached the precise point where controlled descent toward the runway commences.
During instrument meteorological conditions (IMC), this confirmation is essential. This allows pilots to time their descent accurately and mentally prepare for the approach segments ahead.
Middle Marker – Key Features
Strategically positioned to coincide with decision height on standard ILS approaches, the middle marker (MM) announces that the aircraft is entering the final landing phase.
The MM represents the transition—from instrument to visual flight. Here, pilots must establish visual contact with the runway to continue safely. Without it? A missed approach becomes mandatory.
Inner Marker – Usage in Approaches
Reserved for high-precision Category II and III approaches—those demanding decision heights below 200 feet—the inner marker (IM) sits directly at the runway threshold.
The IM confirms that the aircraft is precisely positioned over the runway threshold. This confirmation is important during the near-zero visibility conditions characteristic of these specialized approaches.
Marker Beacon Signals and Identification
| Marker Type | Typical Distance from Runway | Cockpit Light | Audio Signal |
|—|—|—|—|
| Outer Marker (OM) | 4–7 NM | Blue, flashing | 400 Hz tone, continuous dashes (–) |
| Middle Marker (MM) | 0.5–0.8 NM | Amber, flashing | 1300 Hz tone, alternating dots and dashes (.-.-) |
| Inner Marker (IM) | At runway threshold | White, flashing | 3000 Hz tone, continuous dots (….) |
The Future of Marker Beacons in Aviation
Marker beacons continue serving as dependable position verification tools at numerous airfields alongside Instrument Landing Systems (ILS). Yet their role in modern aviation is changing. Previously central to instrument approaches, these transmitters now face gradual supplementation—or outright replacement—by advanced navigation technologies.
GPS-based systems—RNA (Area Navigation) and RNP (Required Navigation Performance)—are rapidly becoming the new standard. These systems offer several advantages:
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Continuous position information throughout the approach, rather than discrete position fixes.
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Reduced ground infrastructure requirements.
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Greater flexibility in approach path design.
Distance Measuring Equipment (DME) provides another option, providing continuous distance information rather than discrete position fixes. Many airports have already decommissioned their marker beacon infrastructure, favoring DME or GPS-based solutions instead. This shift reflects aviation’s broader migration toward satellite-based navigation systems.
Yet marker beacons remain important in specific contexts. Regions with limited advanced navigation infrastructure still rely on them heavily. They also serve as crucial backup systems during GPS outages. These simple, reliable systems continue to provide essential position awareness during low-visibility approaches worldwide—particularly in developing regions where cutting-edge technologies remain partially implemented.
Aviation authorities worldwide are adopting varied strategies regarding marker beacon infrastructure. Some maintain these systems as integral components of layered, redundant navigation frameworks. Others are systematically phasing them out as newer technologies demonstrate proven reliability. This shift reflects aviation’s ongoing challenge: balancing technological advancement with time-tested safety systems.
Pilots must now be proficient with both systems—mastering both traditional marker beacon systems and modern GPS-based approaches. The distinctive audio tones and visual indicators of marker beacons will likely persist in aviation training curricula for years ahead, even as physical infrastructure becomes increasingly scarce. This dual knowledge ensures safe navigation regardless of available ground-based or satellite navigation aids at any destination.