How does the ILS localizer guide aircraft along the runway centreline?
An antenna array beyond the far end of the runway sends two tones that tell the aircraft receiver whether it is left or right of the centreline, and the AGL layout keeps vehicles and aircraft out of the way of the beam.
The ILS localizer is the part of the instrument landing system that gives lateral guidance to the runway centreline. An antenna array placed beyond the stop end of the runway, on the extended centreline, radiates two signals on one VHF frequency, modulated at 90 Hz and 150 Hz. The aircraft receiver compares the two: when they are equal the aircraft is on the centreline, a stronger 90 Hz tone means it is to the left, and a stronger 150 Hz tone means it is to the right. The pilot or autopilot steers to centre the course indicator.
The localizer is not part of the AGL system, but the two work as a pair. The localizer lines the aircraft up with the runway in cloud, and the approach lights, runway centreline lights and touchdown zone lights take over visually when the pilot sees the runway. The glide slope gives the vertical guidance that goes with it.
Aircraft and vehicles near the antenna can reflect or block the signal and bend the course, so the ILS critical area around the antenna is kept clear whenever the ILS is in use, and the larger sensitive area is controlled during low visibility operations. This is where AGL helps: ILS critical area signs and markings, holding positions and, where provided, stop bars show where aircraft and vehicles must stop. Lighting, signs, cables and other metal structures close to the array are reviewed with the ILS owner, and the ILS may need a flight check after changes.
Frequently Asked Questions
Q1What does an ILS localizer do?
It gives the pilot or autopilot lateral guidance to the runway centreline. An antenna array beyond the far end of the runway sends signals that the aircraft receiver turns into a left or right indication, so the aircraft can be lined up with the runway in cloud or poor visibility.
Q2How does the localizer know which side of the centreline the aircraft is on?
The array radiates two tones, 90 Hz and 150 Hz, on one radio frequency. On the centreline the aircraft receiver finds them equal. If the 90 Hz tone is stronger the aircraft is to the left of the centreline, and if the 150 Hz tone is stronger it is to the right. The difference is shown on the course deviation indicator.
Q3Is the ILS localizer part of airfield ground lighting?
No. It is a radio navigation aid owned and maintained by the air navigation service provider or the airport's navaid team. The localizer guides the aircraft by radio, and the approach, centreline and touchdown zone lights take over visually when the pilot sees the runway. The two meet at the airfield layout and at the rules that protect the ILS signal.
Q4Why are there ILS critical areas near the localizer?
Aircraft and vehicles can reflect or block the localizer signal and bend the course. The critical area around the antenna is kept clear whenever the ILS is in use, and a larger sensitive area is controlled during low visibility operations. Holding positions, ILS critical area signs, markings and, where provided, stop bars show where aircraft and vehicles must stop. The exact areas are set by the navaid owner and the national authority, with the airport.
Q5Can airfield lighting installations affect the localizer?
They can. Metal structures, signs, fences and long metal cable runs close to the antenna array or in front of it can reflect the signal. New or moved lighting, signs and civil works near the localizer are reviewed with the ILS owner, and the ILS may need a flight check afterwards.
Q6What happens if the localizer signal goes out of tolerance?
The localizer is monitored all the time. If the signal moves outside its limits, the monitor removes the signal or changes over to the standby transmitter, within 10 seconds for CAT I, 5 seconds for CAT II or 2 seconds for CAT III, so that pilots are not guided by a wrong course. The category and monitor limits are set in the project design documents and the national rules.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.