What is an ILS marker beacon?

An ILS marker beacon is a ground-based transmitter that provides pilots with precise range information at specific points along the ILS approach path.

Where are the outer, middle, and inner marker beacons located?

The outer marker is typically about 4 NM from the threshold, the middle marker about 0.6 NM, and the inner marker at or near the runway threshold.

What are the audio and visual indications for each ILS marker beacon?

The outer marker gives a low-pitched tone and blue light, the middle marker a medium-pitched tone and amber light, and the inner marker a high-pitched tone and white light.

Are marker beacons always present on ILS approaches?

No, many modern ILS installations use DME paired with the localiser frequency instead of marker beacons for range information.

What is the modulation and Morse code pattern for ILS marker beacons?

Each marker beacon uses a specific audio frequency and Morse code: outer marker (2 dashes/sec), middle marker (alternating dots and dashes), inner marker (6 dots/sec).

ILS Marker Beacons

Medium4 min readRadio Navigation

Moderately Examined

Why this matters

Knowing how ILS marker beacons work allows pilots to verify their position and altitude during critical phases of approach, enhancing situational awareness and safety, especially in poor visibility. This understanding is fundamental for executing precision approaches and avoiding controlled flight into terrain.

ILS marker beacons are ground-based transmitters used in aviation to provide precise range information to pilots during an Instrument Landing System (ILS) approach. Located along the ILS glide path, these beacons—outer, middle, and inner—emit signals that help pilots verify their position relative to the runway threshold, especially in low-visibility conditions.

Quick Check

At what frequency do all ILS marker beacons transmit?

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In depth

Explanation

What Are ILS Marker Beacons?

ILS marker beacons are short-range radio transmitters placed at specific points along the ILS approach path to indicate predetermined distances from the runway threshold. Their primary function is to give pilots a positive indication of their position during the final approach phase.

Types and Locations

Outer Marker (OM): Usually located about 4 NM from the runway threshold, marking the point where the aircraft should intercept the glide path.
Middle Marker (MM): Found approximately 0.6 NM from the threshold, indicating the point where the aircraft is typically at decision height.
Inner Marker (IM): Positioned at or near the runway threshold, used mainly for Category II/III approaches.

Frequencies and Identification

All ILS marker beacons operate on a standard 75 MHz VHF frequency. Each beacon uses a unique audio modulation for identification:

Outer Marker: 400 Hz tone, continuous dashes (2 per second), blue cockpit light.
Middle Marker: 1300 Hz tone, alternating dots and dashes, amber cockpit light.
Inner Marker: 3000 Hz tone, rapid dots (6 per second), white cockpit light.

Visual and Audio Indications

When flying over a marker, pilots receive both an audio Morse code and a visual light in the cockpit, each color-coded to the marker type. This dual indication ensures unmistakable recognition of the aircraft's position.

Modern Alternatives

Many modern ILS installations now use DME (Distance Measuring Equipment) paired with the localiser frequency instead of physical marker beacons. DME provides continuous distance readout, improving situational awareness.

Operational Significance

Marker beacons or DME fixes are required for cross-checking the aircraft's altitude against range, ensuring the aircraft is on the correct glide path during approach.

The essentials

Key Points

ILS marker beacons transmit on a standard 75 MHz VHF frequency.

Outer, middle, and inner markers are located at specific distances from the runway threshold.

Each beacon uses a distinct audio tone and Morse code pattern for identification.

Cockpit indicator lights are color-coded: blue (outer), amber (middle), white (inner).

Marker beacons provide fixed position references for cross-checking altitude on approach.

Many modern ILS systems use DME instead of physical marker beacons.

Marker beacons enhance safety by confirming aircraft position during approach.

Watch out

Exam Traps & Typical Mistakes

Confusing the 75 MHz carrier frequency with the audio modulation frequencies (400, 1300, 3000 Hz).

Mixing up the color coding of cockpit indicator lights for each marker.

Assuming all ILS approaches still use physical marker beacons instead of DME.

Believing marker beacons provide continuous range information rather than discrete position fixes.

Misidentifying the location of each marker relative to the runway threshold.

Test yourself

Example Exam Questions

Question 2Medium

What is the cockpit light color for the middle marker beacon during an ILS approach?

Question 3Medium

What is the primary purpose of the outer marker in the ILS system?

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