Where does the ILS sit on the airfield, and where does it meet the lighting?
The localizer array and the glide slope mast each need clear space around them, and the AGL layout uses holding positions, signs and stop bars to keep vehicles and aircraft out of it.
An ILS has two main antenna sites on the airfield. The localizer array sits beyond the stop end of the runway, on the extended centreline. The glide slope mast stands beside the runway, level with the touchdown zone and set back from the centreline. Each site needs clear space around it, because aircraft, vehicles and structures can reflect or block the signal and bend the course or the path. Other equipment, such as a DME or marker beacons, can be part of the installation and has its own entry.
Two protected zones are used. The ILS critical area is the zone around the antennas that vehicles and aircraft must stay out of while the ILS is in use. The ILS sensitive area is a larger zone where movements are controlled, usually during low visibility operations. The navaid owner and the national authority set the size of both areas, and the airport turns them into holding positions on the ground.
This is where AGL meets the ILS. The AGL team places ILS holding positions with markings, ILS critical area signs and, where provided, stop bars that the tower switches on when the protected area is in use. Lighting fixtures and signs near the antennas are kept low and frangible, and metal structures, fences and cable routes are kept out of the antenna fields unless the ILS owner agrees. The PAPI angle is set with the glide path in mind. Works and outages near an ILS site are agreed in advance, and the ILS owner decides whether a check or flight check is needed before the ILS is used again.
Frequently Asked Questions
Q1Where are the ILS antennas placed on an airfield?
The localizer array is placed beyond the stop end of the runway, on the extended centreline. The glide slope mast stands beside the runway, level with the touchdown zone and set back from the centreline. The exact positions are set in the ILS design and the airport's obstacle and safety rules.
Q2What is the difference between an ILS critical area and a sensitive area?
The critical area is the zone around the antennas where vehicles and aircraft are not allowed while the ILS is in use, because they could disturb the signal. The sensitive area is a larger zone where aircraft and vehicle movements are controlled, usually during low visibility operations. Their sizes are set by the navaid owner and the airport.
Q3How does airfield ground lighting help protect the ILS?
It marks and controls the places where traffic must stop. ILS holding positions are shown by markings and signs and, where provided, by stop bars that are switched on when the protected area is in use. In low visibility procedures the stop bars and the ILS protection are usually worked together by the tower.
Q4Why should lighting cables, signs and fixtures be kept away from the antennas?
Metal objects and long metal cable runs close to the localizer array or in the ground reflection area of the glide slope can reflect the signal and bend the course or the path. Fixtures and signs near the antennas are kept low and frangible, and routes and positions are agreed with the ILS owner.
Q5Who decides the size of the critical and sensitive areas?
The navaid owner and the national authority set them from the ILS type, the category of operation and the airfield layout. AGL designers use those areas to place holding positions, signs and stop bars, and they do not set the areas themselves.
Q6What should be done before working near an ILS antenna?
Tell the ILS owner and the tower before the work, agree the dates and the method, and keep vehicles and equipment out of the protected areas while the ILS is in use. After work that may have changed the signal, the ILS owner decides whether the ILS needs checking or a flight check before it is used again.
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