Instrument Landing System (ILS)

Instrument Landing System (ILS) is a ground-based precision approach system that enables aircraft to land safely and accurately under low-visibility conditions. As defined in ICAO Annex 10, the system provides simultaneous horizontal (lateral) and vertical position guidance to the aircraft, allowing it to align precisely with the runway centerline and descend along a specified glide path. ILS is the most widely used precision approach system worldwide and plays a critical role in enhancing landing safety during adverse meteorological conditions such as fog, rain, or snow. In CAT IIIb systems, automatic landings can be performed even when visibility approaches zero.

Historical Development

The foundations of ILS date back to initial experiments conducted in the United States in 1929, which primarily utilized signals from localizer and glideslope antennas. The first operational applications were implemented between 1938 and 1939, and in 1949 the system was standardized internationally through ICAO Annex 10. During the 1960s and 1980s, CAT II and CAT III categories were developed and integrated with automatic landing systems. Today, ILS remains the foundational precision approach system and operates in conjunction with satellite-based systems such as GNSS and Ground Based Augmentation System (GBAS).

System Architecture and Components

ILS is built upon three primary components: the localizer, the glideslope, and marker beacons or Distance Measuring Equipment (DME). The localizer provides lateral guidance to align the aircraft with the runway centerline. The glideslope provides vertical guidance to ensure the aircraft descends along a precise glide angle. Marker beacons or DME determine the aircraft’s distance from the runway threshold. In addition to these core components, approach lighting systems, monitoring units, and regular calibration infrastructure are essential for the system’s reliable operation.

Localizer (LOC) – Lateral Guidance

The localizer consists of an antenna array aligned with the extended centerline of the runway. Typically installed approximately 300 meters behind the runway threshold, the array transmits in the VHF band between 108.10 and 111.95 MHz. The localizer signal contains two main side lobes modulated at 90 Hz and 150 Hz. The 90 Hz modulation dominates on the left side of the runway centerline, while the 150 Hz modulation dominates on the right. The aircraft’s receiver compares the amplitude ratio of these two signals to determine deviation from the centerline. When the signals are equal, the aircraft is considered centered on the runway axis. The coverage extends up to 25 nautical miles (46 km) and provides a sensitivity of ±2.5° at the runway threshold. In CAT I operations, the maximum allowable deviation from the centerline is ±10.5 meters, while in CAT III operations this tolerance is reduced to ±3 meters.

Localizer signals can be affected by various sources of interference. Hangars, terminal buildings, and topographical irregularities near the runway can cause signal reflections (multipath), leading to erroneous indications on the pilot’s instruments. Additionally, signals radiating from the rear lobe of the localizer antennas, known as “back course,” are not used as they may lead to incorrect approaches. Localizer systems are designed with dual transmitters; if the primary system fails, the backup automatically activates. Monitoring mechanisms automatically deactivate the system if a deviation beyond tolerance from the runway centerline or an abnormal signal strength is detected.

Glideslope (GS) – Vertical Guidance

The glideslope provides vertical guidance through an antenna array positioned approximately 300–380 meters behind and 120–210 meters to the side of the runway threshold. The aircraft approaches along a predetermined glide path, typically 3°, though it may vary between 2.5° and 3.5° in special cases. The glideslope transmits in the UHF band between 329.15 and 335 MHz and is paired with the localizer frequency. When the pilot tunes the localizer frequency, the receiver automatically selects the corresponding glideslope frequency.

The glideslope signal also consists of two side lobes modulated at 90 Hz and 150 Hz. The 90 Hz energy is stronger above the glide path, while the 150 Hz energy dominates below it. The point where the two signals are equal defines the ideal glide path for the aircraft. The glideslope coverage is approximately 10 nautical miles (18 km) with a sensitivity of ±0.7°. However, glideslope signals are susceptible to false guidance errors. This phenomenon, known as “false glideslope,” occurs when the glideslope signal reflects off terrain ahead of the runway, creating a second, steeper glide path at angles such as 9° or 12°. Pilots are trained to capture the glideslope from below to minimize the risk of locking onto a false path. Glideslope systems are continuously monitored; if the angular deviation exceeds 0.1° or if a power loss occurs, the system automatically shuts down.

Marker Beacons and DME – Distance Information

Distance information in the ILS system is provided by marker beacons or DME. Marker beacons are low-power transmitters operating at 75 MHz and are installed at specific distances from the runway threshold. The Outer Marker (OM) is located approximately 7.5 kilometers from the threshold and transmits a 400 Hz modulated dash (-) signal. The Middle Marker (MM) is positioned 1,050 meters from the threshold and emits a 1,300 Hz modulated dot-dash (-.) signal, typically indicating proximity to the decision altitude. The Inner Marker (IM) is placed 300–450 meters from the threshold and transmits a 3,000 Hz modulated dot (.) signal, used in CAT II and CAT III operations. In modern ILS installations, DME has largely replaced marker beacons. DME continuously calculates distance by measuring the time delay between signals transmitted by the aircraft and those returned by the ground station, providing more accurate and uninterrupted information than marker beacons.

Approach Light System

Approach lighting systems are used during ILS approaches, particularly under low-visibility conditions, to assist pilots in visually identifying the runway centerline. On precision approach runways, these systems must be at least 720 meters long according to ICAO standards. Although not a direct component of ILS, these lights are integrated with the system to enhance landing safety.

Operating Principle

The fundamental principle of ILS is based on the electromagnetic signals transmitted by the localizer and glideslope antennas to determine the aircraft’s position. Upon entering the ILS approach sector, the aircraft first captures the localizer signal to align with the runway centerline. Once lateral alignment is achieved, the glideslope signal is acquired, and the aircraft begins descending along the designated glide path toward the runway. Distance information provided by marker beacons or DME indicates the aircraft’s proximity to the runway threshold, enabling a precise approach along both the lateral and vertical profiles.

The system operates by comparing two modulated frequencies. The localizer transmits two signals modulated at 90 Hz and 150 Hz on either side of the runway centerline; the aircraft’s receiver measures the amplitude difference between them to determine whether it is left or right of the centerline. The glideslope uses the same principle to indicate whether the aircraft is above or below the ideal glide path. This allows the pilot or the automatic flight control system to make necessary corrections during the approach.

ILS Categories

The International Civil Aviation Organization (ICAO) classifies ILS systems into three main categories based on their precision.

CAT III systems operate in conjunction with automatic landing and automatic braking systems. For these systems to be utilized, both the aircraft and the airport must be certified for CAT III ILS operations.

Pilot Procedures and Instrument Monitoring

During an ILS approach, the pilot must follow published altitudes and must always capture the glideslope from below to reduce the risk of locking onto a false glideslope signal. During the final approach, the aircraft must be flown to maintain the localizer (lateral) and glideslope (vertical) indicators centered. The position of the instrument needles represents the aircraft’s deviation from the desired path.

• If the localizer indicator is to the right, the aircraft is to the left of centerline; the pilot must turn right to recenter.
• If the localizer indicator is to the left, the aircraft is to the right of centerline; the pilot must turn left to recenter.
• If the glideslope indicator is above center, the aircraft is below the glide path; the pilot must raise the nose slightly.
• If the glideslope indicator is below center, the aircraft is above the glide path; the pilot must increase the descent rate.

At the Decision Height (DH) on the glideslope, if the runway is in sight, the pilot continues the landing; if not, a go-around maneuver is initiated. This point is the critical decision point in an ILS approach.

Monitoring and Calibration

The precision of ILS systems is maintained through continuous monitoring and regular calibration. Signals from the localizer and glideslope antennas are constantly monitored. Any deviation beyond tolerance from the runway centerline, significant signal strength reduction, or angular drift triggers automatic deactivation of the affected component or activation of the backup system.

Calibration flights are critical to verify system accuracy. Aircraft equipped with specialized measurement devices regularly test signal deviations, sensitivity, and compliance with category standards. Calibration standards for CAT II and CAT III systems are non-negotiable due to their extremely low tolerance for error.

Advantages and Limitations

ILS provides operational and economic benefits to airlines by enhancing flight safety under low-visibility conditions and reducing landing cancellations and delays. CAT III systems enable landings even when visibility approaches zero, increasing the capacity of high-traffic airports.

However, the installation and maintenance costs of ILS are high. Buildings and terrain features near the runway can cause signal reflections, and electromagnetic interference may compromise system accuracy. Not every runway length, slope, or surrounding environment is suitable for ILS installation; therefore, its deployment is limited at smaller airfields.

GBAS and GNSS-Supported Systems

The Ground Based Augmentation System (GBAS) is an alternative to ILS that provides precision approach capability to multiple runways from a single ground station. Based on GNSS, GBAS enhances satellite signals with correction data transmitted from the ground station. This eliminates the antenna placement constraints of ILS and offers lower costs and greater operational flexibility. However, GBAS currently has limited CAT III capabilities, so ILS remains the most widely used precision approach system worldwide. Satellite-supported systems such as GNSS-based LPV (Localizer Performance with Vertical Guidance) are also becoming increasingly common at medium and small airports.

ILS Applications in Türkiye

In Türkiye, Istanbul Airport, Ankara Esenboğa, Erzurum, İzmir Adnan Menderes, and Antalya airports are equipped with CAT II and CAT III ILS systems. These systems undergo regular calibration flights under the supervision of the Directorate General of Civil Aviation (SHGM) and the Directorate of State Airports Authority (DHMİ), in accordance with ICAO standards.

ILS Applications Worldwide

ILS is actively used at hundreds of major airports worldwide. National civil aviation authorities conduct regular inspections and calibration flights in compliance with ICAO standards to ensure system reliability. In Europe, major airports such as London Heathrow, Paris Charles de Gaulle, Frankfurt, and Amsterdam Schiphol are equipped with CAT IIIb ILS systems. In the United States, airports including Chicago O’Hare, Atlanta Hartsfield-Jackson, and Los Angeles International operate under CAT II and CAT III categories. In Asia, Tokyo Narita, Hong Kong, and Singapore Changi; in Africa, Johannesburg O.R. Tambo; and in the Middle East, Dubai International are equipped with the most advanced ILS categories. The system is essential for ensuring safe and uninterrupted flight operations in regions with high air traffic density.