Localizer (LOC) – Comprehensive Aviation Glossary

Definition of Localizer (LOC)

A Localizer (LOC) is a specialized ground-based radio navigation system providing precise lateral guidance to aircraft during the final approach phase of landing, especially under instrument meteorological conditions (IMC). Operating in the VHF band, the localizer is a core part of the Instrument Landing System (ILS), ensuring the aircraft is perfectly aligned with the runway centerline even when visibility is poor. The localizer can also function independently in non-precision LOC-only approaches, delivering critical left-right alignment for safe landings.

The localizer antenna array is typically installed beyond the departure end of the runway, aligned to match the runway centerline. This ground equipment broadcasts a highly directional VHF signal, which is received and decoded by the aircraft’s navigation systems. Any deviation from the centerline is immediately displayed to the pilot through cockpit instruments, allowing for precise corrections. Localizer accuracy far surpasses that of other enroute navigation aids, offering a course width at the runway threshold between 3° and 6°, or about 700 feet at one nautical mile from the antenna.

The localizer is essential for all-weather operations at airports equipped with ILS, enabling precision or non-precision approaches where visual references are insufficient. Its use is mandated in published instrument approach procedures and regulated by international standards from the ICAO and national authorities like the FAA and EASA.

How a Localizer Works

Signal Structure and Frequencies

The localizer system transmits two overlapping amplitude-modulated signals in the VHF high band, specifically from 108.10 MHz to 111.95 MHz (odd tenths only). This allocation prevents interference with VOR facilities, which use the even tenths and the remainder of the band.

The transmitted signal consists of:

• 90 Hz tone modulated on one side of the centerline (usually the left)
• 150 Hz tone modulated on the other side (usually the right)

A phased antenna array generates a sharply-defined course by overlapping these sectors. The aircraft’s receiver detects the difference in modulation depth. Equal strength means the aircraft is on the centerline; dominance of one indicates deviation left or right. The high sensitivity of the LOC signal allows pilots to make precise corrections; a full-scale CDI deflection typically represents only 2.5° off centerline.

Technical Signal Table

Coverage and Service Volume

The service volume of a localizer is strictly defined. ICAO specifies:

• Full-scale accuracy within 10° of the runway centerline, up to 18 NM from the antenna
• Usable navigation within 35° of the centerline, up to 10 NM
• Vertical coverage from the surface up to 4,500 feet above the antenna

Terrain, obstacles, and airport layout can affect practical coverage. Published procedures account for these factors when establishing initial approach fixes and minimum altitudes. Signal integrity and protection from multipath or interference are regulated by ICAO Annex 10, ensuring high reliability within the certified sector.

Localizer Service Volume Table

Morse Code Identification

Each LOC station transmits a unique Morse code identifier, usually beginning with an “I” (for “ILS”) and followed by a three-letter designator (e.g., “I-RDU” for Raleigh-Durham). This identifier is modulated onto the carrier at least six times per minute. Pilots confirm the correct LOC facility by listening to this code, a step required by procedure to prevent navigation errors.

Most modern avionics display the identifier digitally, but pilots are trained to check the audio as a backup, particularly at airports with multiple ILS/LOC installations.

Components of Localizer-Based Approaches

Course Deviation Indicator (CDI)

The Course Deviation Indicator (CDI) is the cockpit instrument that shows the aircraft’s position relative to the selected navigation course. For localizer approaches, the CDI is extremely sensitive—a full-scale deflection indicates only about 2.5° off centerline. This sensitivity is crucial for flying precise, stable approaches.

Modern aircraft may use a Horizontal Situation Indicator (HSI) or integrated EFIS displays, which combine CDI functionality with compass and other navigational cues.

Glideslope Integration (ILS)

The Glideslope (GS) provides vertical guidance and is paired with the localizer in full ILS. The glideslope transmitter is located about 750–1,250 feet down the runway, 400–600 feet to one side, transmitting a UHF signal (329.3–335 MHz). The GS uses similar 90 Hz and 150 Hz modulation, but in the vertical plane. With both LOC and GS, pilots fly a 3D approach path, typically with a 3° descent.

When only the LOC is available (LOC-only approach), descent is managed via step-down fixes and published minimum descent altitudes.

Marker Beacons and DME

Marker Beacons provide position cues along the approach:

• Outer Marker (OM): 4–7 NM from threshold, intercept glideslope
• Middle Marker (MM): ~3,500 ft from threshold, near decision height
• Inner Marker (IM): Close to threshold, used on CAT II/III approaches

Each emits a 75 MHz signal, with unique cockpit lights and tones. DME is increasingly used instead of markers, providing continuous slant-range distance to a fix or runway, improving situational awareness and accuracy.

Types of Approaches Utilizing a Localizer

ILS Approach

An ILS Approach uses both LOC and GS for a precision approach, enabling landings in low visibility. Minimums may be as low as 200 ft AGL and 1,800 ft RVR for CAT I, and even lower for CAT II/III with suitable aircraft and crew certification.

Localizer Approach (LOC)

A LOC Approach uses only the localizer for lateral guidance (no vertical GS). Step-down fixes and MDAs are used for descent. LOC approaches are common at airports or runways without full ILS installations.

Localizer Back Course (LOC BC) Approach

A LOC BC Approach uses the opposite side of the localizer for an approach to the reciprocal runway. No glideslope is available, and pilots must beware of reverse sensing on some instruments if not set correctly.

Operational Use and Procedures

Tuning and Identifying the Localizer

Pilots must:

1. Obtain the correct LOC frequency from charts or ATIS
2. Tune the NAV receiver and select the correct navigation source
3. Listen to and verify the Morse identifier

Modern systems may display the ID digitally, but pilots are trained to cross-check the audio as a safety measure.

Setting and Intercepting the Approach Course

• Set the published approach course using the OBS or HSI
• Receive ATC vectors or fly a procedure turn to intercept the LOC
• Monitor the CDI/HSI for accurate tracking

Flying the Localizer Approaches

For LOC-only front course:

1. Intercept and track the localizer
2. Descend using step-down fixes and minimum altitudes
3. At MDA, land if the runway is visible; otherwise, execute a missed approach

For LOC BC approaches:

1. Tune and identify the LOC
2. Set the published back course
3. Monitor for reverse sensing as needed
4. Descend using step-down fixes
5. At MDA, land if visual, or go missed

Technical Parameters and Limitations

Localizer Signal Specifications

• Frequency: 108.10–111.95 MHz (odd tenths)
• Course Width: 3–6° (typically 5° at threshold)
• Service Volume: ±10° to 18 NM; ±35° to 10 NM; up to 4,500 ft AGL
• Modulation: 90 Hz (left), 150 Hz (right)
• Identification: Morse code (e.g., “I-RDU”)

Comparison Table: Localizer vs VOR

ILS Categories and Decision Heights

Common Errors and Safety Considerations

Reverse Sensing

Reverse sensing occurs on back course approaches when the instrument indications are inverted, causing pilots to steer away from the correct course. This is most likely on HSIs if the front course is not set. Proper setup and pilot training are essential to prevent navigation errors.

Frequency Selection and Identification

It’s critical to tune the correct frequency and verify the identifier to avoid tracking the wrong localizer, especially at airports with multiple ILS/LOC systems.

Multipath and Interference

Nearby terrain or structures can cause multipath effects, distorting the LOC signal. Pilots should be aware of NOTAMs affecting signal quality and always adhere to published approach procedures.

A Localizer (LOC) is a foundational navigation aid that has enabled safe, all-weather landings for decades. Its precision, reliability, and integration with the ILS make it a critical component of modern aviation safety and efficiency.