The left side of the approach area is filled with a VHF carrier wave modulated with a 90 Hz signal. The field tapers to runway width near the landing threshold. They create an expanding field that is 21⁄2° wide (about 1,500 feet) 5 miles from the runway. Two modulated signals are produced from a horizontally polarized antenna complex beyond the far end of the approach runway. The localizer broadcast is a VHF broadcast in the lower range of the VOR frequencies (108 MHz–111.95 MHz) on odd frequencies only. Components of an instrument landing system (ILS).
Together, all of these radio signals make an ILS a very accurate and reliable means for landing aircraft.
Marker beacons provide distancefrom- the-runway information. Compass locator transmissions for outer and middle approach marker beacons aid the pilot in intercepting the approach navigational aid system. A separate glideslope broadcast provides vertical guidance of the aircraft down the proper slope to the touch down point. It is used to provide horizontal guidance to the center line of the runway. A localizer is one of the radio transmissions. Multiple radio transmissions are used that enable an exact approach to landing with an ILS. This radio navigation system guides the aircraft down a slope to the touch down area on the runway. An ILS is used to land an aircraft when visibility is poor.
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