An instrument landing system (ILS) uses two separate ground-based radio transmitters to provide both lateral and vertical guidance to pilots during an instrument approach procedure that normally includes the lowest landing minimums. A Category I ILS approach, for example, typically offers a 200-foot decision height above the ground. ILS approach visibility minimums are measured either in statute miles or by using runway visual ranges down to zero-zero for a Category IIIc ILS approach. (Approaches of Cat 2 and greater require special flight crew certification.)
How an Instrument Landing System Works
Key Takeaways:
- An Instrument Landing System (ILS) is a ground-based radio navigation system that provides precise lateral (localizer) and vertical (glideslope) guidance for aircraft during instrument approaches, enabling landings in low visibility conditions.
- The ILS signal sensitivity increases significantly as the aircraft nears the runway, requiring pilots to make increasingly precise and smaller corrections to maintain course and glideslope.
- The system can include optional marker beacons for distance information, and a localizer-only approach is possible if the glideslope fails (though with higher minimums), but there is no glideslope-only option.
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