ILS Localizer

The ILS Localizer is a ground-based radio navigation system that forms the lateral guidance component of the Instrument Landing System (ILS). It consists of a phased array of antennas located beyond the far end of the runway (typically 300 to 600 metres from the runway end). The localizer transmits a modulated radio signal at a frequency between 108.10 MHz and 111.95 MHz. The signal creates two distinct lobes: one modulated at 90 Hz (on the left side of the runway centreline) and the other at 150 Hz (on the right side). The aircraft's receiver measures the difference in modulation depth between the two signals, providing the pilot with a deviation indication that guides them to the runway centreline.

The localizer's purpose is to keep the aircraft's track aligned with the extended runway centreline throughout the approach, a role that becomes critical precisely when visual cues that would normally confirm alignment, runway lighting, markings, the runway edge itself, are obscured by low visibility until very late in the approach. The two-lobe signal structure means any lateral drift away from centreline immediately produces an imbalance between the 90 Hz and 150 Hz modulation depths, giving the pilot a continuously updated correction cue rather than a single go/no-go indication. Because the localizer signal is usable from much further out than the point where visual contact with the runway is typically established, it is what allows the aircraft to be stabilised on the correct lateral track well before touchdown rather than making a late visual correction close to the ground.

The ILS Localizer is installed on precision approach runways that support ILS operations. It is a critical component for:

• CAT I Precision Approaches: Providing lateral guidance down to a decision height of 200 feet (60 metres) with RVR of 550 metres or greater.
• CAT II Precision Approaches: Providing lateral guidance down to a decision height of 100 feet (30 metres) with RVR of 350 metres or greater.
• CAT III Precision Approaches: Providing lateral guidance down to a decision height as low as 15 feet (5 metres) with RVR as low as 200 metres (CAT IIIA), 50 metres (CAT IIIB), or even zero (CAT IIIC).
• Military Airfields: Supporting military aircraft ILS operations.
• Civil Airports: Supporting commercial and general aviation ILS approaches.

The localizer antenna array is sited beyond the stop end of the runway, on the extended runway centreline, at a distance (typically 300-600 m past the far threshold) set so the course line it radiates is centred precisely on the runway. The array sits within a graded, cleared area free of fencing, vehicles, or vegetation that could distort the radiated pattern, and the site is chosen to keep the array clear of the runway end safety area while remaining within the obstacle-limitation surface for the approach it serves. The transmitter and monitoring equipment are housed in a shelter at the base of the array, fed from dual power supplies with automatic changeover, and linked by cable to a remote control and status panel in the ATC tower or navaid maintenance facility.

The ILS Localizer operates on the principle of amplitude modulation and spatial signal distribution:

• Signal Generation: The localizer transmitter generates an RF carrier signal at the assigned frequency. Two modulating signals (90 Hz and 150 Hz) are applied to the carrier.
• Antenna Array: A multi-element antenna array (typically 8 to 24 radiating elements) creates a directional pattern with two overlapping lobes. The 90 Hz signal is predominant on the left side of the runway centreline, while the 150 Hz signal is predominant on the right side.
• Course Line: Along the runway centreline (the "course line"), the modulation depths of the 90 Hz and 150 Hz signals are equal. This results in a zero deviation indication on the aircraft's Instrument Landing System indicator (CDI).
• Aircraft Reception: The aircraft's ILS receiver measures the difference between the two modulation depths. The difference is displayed as a deviation indication on the Course Deviation Indicator (CDI), showing the pilot whether the aircraft is left or right of the runway centreline.
• Coverage: Per ICAO Annex 10, the localizer signal is usable to 25 nautical miles (46 km) within ±10° of the front course line, 17 NM (31 km) between 10° and 35°, and 10 NM (18.5 km) beyond that out to ±35°–45°. Where terrain or frequency congestion requires it, a reduced-coverage localizer (down to 18 NM / 33 km within ±10°) may be sited instead — this is a siting option, not a distinction between ILS categories.

The localizer facility is not remotely commanded in real time for its RF output — its control logic is a self-monitoring and protection loop. Integral near-field monitors sample the radiated signal directly at the antenna array and check course alignment, displacement sensitivity and RF power against tolerance, with some installations adding a far-field monitor for independent confirmation. Dual-equipment sites run online and standby transmitters, and the monitor logic executes an automatic changeover to standby on an out-of-tolerance reading; if neither channel can meet tolerance, the facility shuts itself down and radiates no signal rather than risk erroneous lateral guidance. Status, alarms and changeover events are reported to the airport's remote control and monitoring system for ATC and maintenance visibility.

The localizer antenna array, a phased arrangement of 8 to 24 radiating elements, is mounted on a frangible support structure beyond the far end of the runway, with each element individually fed and phased from a weatherproof equipment shelter through carefully matched-length coaxial cable runs to maintain course accuracy. The support structure is designed to break away cleanly if struck by an aircraft during an overrun, similar to elevated runway light fixtures, while still holding the array rigidly enough to preserve the precise element spacing and height the system depends on. The equipment shelter houses the dual redundant VHF transmitters, 90 Hz/150 Hz modulation electronics, and continuous course-monitoring equipment in a temperature-controlled, weatherproof cabinet consistent with other ILS ground equipment. Feeder cables between the shelter and antenna array are sealed and often buried or ducted for protection, and automatic monitoring watches course alignment and signal strength, switching to the standby transmitter and raising a remote alarm if performance drifts outside tolerance.