instrument landing system (ILS)

An instrument landing system (ILS) is a combination of three radio systems used in conditions of poor visibility to guide aircraft towards runways. A glide slope beam sent from the runway indicates the proper angle of descent, and the localizer beam indicates its direction. The outer marker beam, set about 8 kilometers (5 miles) from the runway, and the middle marker, at about 0.8 kilometer (0.5 mile), show distance.