instrument-landing system (ILS), ground-based radio system designed to provide an airplane pilot with precise guidance for the final approach in landing. The pilot flies his aircraft along a course delineated by the intersection of two radio beams—the localizer beam for guidance in the horizontal plane and the glide-slope beam for guidance in the vertical plane. These beams activate an indicator in the aircraft that contains a horizontal needle sensitive to deviations from the glide slope and a vertical needle sensitive to deviations from the localizer path. By keeping both needles centered, the pilot can guide his aircraft down to the end of the landing runway aligned with the runway center line. Limitations inherent in the system prevent it from being used safely in locations where the land beyond the approach end of the runway is not level. Also, false guidance can result from distortion of the radio beam by nearby buildings or mountains. Newer systems using microwave beams overcome most of these limitations. Radio marker beacons are also installed at several locations along the approach path to tell the pilot on the landing approach how far he is from the end of the runway. ILS is an approach rather than a landing system. It is called instrument low approach system (ILAS) by the U.S. military air forces. As a supplementary safety measure, especially in bad weather and for emergency landings, the ground-controlled approach (GCA) system is used. Precision radar indicates the location and movement of an aircraft to the ground controller at an airport, enabling him to direct the pilot by voice radio.
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