Instrument Landing System (ILS) – Glossary & In-Depth Guide

What is the Instrument Landing System (ILS)?

The Instrument Landing System (ILS) is an internationally standardized, ground-based radio navigation aid that enables aircraft to approach and land safely in low visibility conditions. ILS provides both lateral and vertical guidance, ensuring that aircraft remain aligned with the runway’s centerline and descent path during final approach, even when pilots cannot see the runway due to fog, rain, or low clouds. Used at airports worldwide, ILS is essential for supporting safe, efficient air traffic operations under instrument flight rules (IFR).

ILS consists of two primary ground-based radio transmitters:

• Localizer (LOC): Provides horizontal guidance, aligning the aircraft with the runway centerline.
• Glideslope (GS): Provides vertical guidance, establishing the optimal descent angle.

These signals are received by aircraft navigation systems and displayed on cockpit instruments, which pilots (or autopilot systems) use to make precise corrections throughout the approach. ILS is further supported by marker beacons or Distance Measuring Equipment (DME) for position verification and approach lighting systems (ALS) to aid the transition from instrument to visual flight near the runway.

ILS approaches are charted in official Aeronautical Information Publications (AIPs) and commercial charts, specifying frequencies, procedures, and weather minimums for each runway. Since its introduction in the 1930s, ILS has become the gold standard for precision approach systems, especially for landings in challenging weather.

How Does ILS Work?

ILS works by transmitting highly directional radio signals from ground antennas located near the runway. These signals create an invisible, three-dimensional approach corridor in the sky. Aircraft receivers interpret these signals and display the aircraft’s deviations from the ideal path on cockpit indicators, allowing pilots to maintain precise approach alignment and descent.

• Localizer (LOC): Located beyond the departure end of the runway, it broadcasts two overlapping beams at 90 Hz (left) and 150 Hz (right) on VHF frequencies (108.10–111.95 MHz). The aircraft detects which frequency is stronger and displays lateral deviation on the cockpit indicator. When the indicator needle is centered, the aircraft is aligned with the runway centerline.

• Glideslope (GS): Located about 300 meters from the runway threshold and offset from the centerline, it broadcasts overlapping UHF signals (329.15–335.00 MHz) at 90 Hz (below) and 150 Hz (above). The aircraft’s receiver determines if it is above or below the ideal descent path, displaying a vertical deviation needle. Centered means on the correct glide path, typically set at a 3° descent angle.

• Marker Beacons or DME: Provide pilots with position fixes at critical points along the approach, such as the Final Approach Fix (FAF) or Decision Height (DH).

• Approach Lighting System (ALS): High-intensity lights extending from the runway threshold assist pilots in visually identifying the runway environment during final approach.

The entire ILS system is continuously monitored for signal integrity. If an anomaly is detected—such as signal distortion from an obstacle or interference—the ILS can automatically deactivate, preventing misleading indications.

ILS Components Explained

Localizer (LOC)

The Localizer is the ILS component responsible for lateral (left/right) guidance. Its antenna array, positioned at the far end of the runway, emits two overlapping beams modulated at 90 Hz (left) and 150 Hz (right). The aircraft’s navigation receiver compares these signals, and the difference is shown as a vertical needle on the cockpit’s Course Deviation Indicator (CDI) or Horizontal Situation Indicator (HSI).

• Frequency Range: 108.10–111.95 MHz (odd tenths only).
• Coverage: Typically 18 nautical miles (NM) within 10° of the centerline; 10 NM within 35°.
• Precision: The signal width is about 5°, with full-scale deviation representing roughly 2.5° either side of centerline—equating to just a few meters at the runway threshold.

The localizer’s Morse code identifier (e.g., “ISXU”) must be verified by the pilot before commencing the approach. Critical and sensitive areas around the antenna are tightly controlled to prevent signal distortion from aircraft or vehicles.

Glideslope (GS)

The Glideslope provides vertical (up/down) guidance, ensuring the aircraft descends at the correct angle—generally 3°, though this may be adjusted for obstacles or terrain. The glideslope antenna is offset from the runway centerline and about 300 meters from the threshold.

• Frequency Range: 329.15–335.00 MHz.
• Coverage: Typically 10 NM.
• Precision: Full-scale deflection equals about 0.7° above/below the glide path.

To avoid capturing “false glideslopes” (spurious signals at multiples of the true angle), pilots always intercept the glideslope from below. The system’s sensitive area is protected from interference, and the glideslope is monitored for signal integrity.

Marker Beacons

Marker Beacons are transmitters placed along the approach path to provide pilots with fixed position references:

• Outer Marker (OM): 4–7 NM from threshold; blue light and “dah-dah-dah” tone.
• Middle Marker (MM): ~0.5–0.8 NM from threshold; amber light and “dot-dash-dot-dash” tone.
• Inner Marker (IM): Close to threshold (CAT II/III); white light and high-pitched “dot-dot-dot-dot” tone.

When passing over, the corresponding cockpit light and tone activate, confirming the aircraft’s position at key approach points. Many airports now use DME or GPS fixes instead of marker beacons for greater flexibility and accuracy.

Approach Lighting System (ALS)

The Approach Lighting System is a series of high-intensity lights extending from the runway into the approach path, aiding pilots in transitioning from instrument to visual flight. ALS configurations vary:

• ALSF-2: Advanced system with steady-burning bars and sequenced flashers (“rabbit”).
• MALSR: Medium-intensity with alignment indicator lights.
• SSALS: Simplified, shorter systems.

ALS is usually the first visual cue acquired below the cloud base and is critical for approaches in low visibility. The presence and type of ALS are depicted on approach charts and directly impact approach minima.

Distance Measuring Equipment (DME)

DME is often paired with ILS localizer antennas (ILS/DME) and shows the aircraft’s slant-range distance to the runway. This allows pilots to verify position at key fixes, such as the FAF or step-down points, and is now more common than marker beacons at many airports.

• Frequency Range: 960–1215 MHz.
• Display: Distance in nautical miles on cockpit navigation instruments.
• Use: Continuous updates throughout approach; often replaces marker beacons.

Course Deviation Indicator (CDI)

The Course Deviation Indicator is the cockpit display used to track localizer (lateral) and glideslope (vertical) deviations. When both needles are centered, the aircraft is precisely on course and glide path. The CDI is highly sensitive during ILS approaches, highlighting even small deviations and enabling timely corrections.

ILS Categories and Minimums

ILS approaches are divided into categories, each supporting different minimum visibility and decision heights:

• Decision Height (DH): The specified altitude at which a pilot must decide to land or go around.
• Runway Visual Range (RVR): The distance over which a pilot can see runway markings or lights.

Higher categories require more advanced ground and airborne equipment, as well as special crew training and airport procedures.

Importance and Future of ILS

ILS remains the benchmark for precision approaches, enabling safe landings in adverse weather and high-traffic environments. While satellite-based systems like GBAS and GPS-based approaches are increasingly used, ILS continues to be indispensable for the most demanding operations, including CAT III autoland procedures at major airports.

ILS’s reliability, global standardization, and ability to support low-visibility landings ensure its continued role in aviation safety. Airports and airlines invest heavily in maintaining, upgrading, and protecting ILS installations to support safe, efficient, and predictable air traffic operations.

Related Terms

• MLS (Microwave Landing System): An alternative to ILS, now rarely used.
• GBAS (Ground-Based Augmentation System): Satellite-supported precision approach, increasingly found at major airports.
• RNAV (Area Navigation): Navigation based on waypoints, often GPS-derived, with increasing role in modern instrument approaches.
• Autoland: Automatic landing system relying on ILS for guidance during CAT III approaches.

Summary

The Instrument Landing System (ILS) is a cornerstone of modern aviation, enabling safe, precise landings under all weather conditions. Its combination of localizer, glideslope, marker beacons or DME, and approach lighting forms the backbone of precision approach procedures at airports worldwide. As aviation evolves, ILS continues to play a vital role in ensuring safety and efficiency in increasingly complex and demanding airspace.

For more information on implementing or upgrading ILS at your airport, contact us or schedule a demo with our experts.