The introduction of precision approaches using global navigation satellite systems (GNSSs) instead of requiring expensive airport infrastructure is leading to the replacement of ILS. ILS remains a widespread standard to this day. Several competing landing systems have been developed, including the radar-based ground-controlled approach (GCA) and the more recent microwave landing system (MLS), but few of these systems have been deployed. Many sets were installed at airbases in the United Kingdom during World War II, which led to it being selected as the international standard after the formation of the International Civil Aviation Organization (ICAO) in 1947. The US-developed SCS-51 system was more accurate while also adding vertical guidance. The ILS usually includes high-intensity lighting at the end of the runways to help the pilot locate the runway and transition from the approach to a visual landing.Ī number of radio-based landing systems were developed between the 1920s and 1940s, notably the Lorenz beam which saw relatively wide use in Europe prior to World War II. Markers are largely being phased out and replaced by distance measuring equipment (DME). Optional marker beacon(s) provide distance information as the approach proceeds, including the middle marker (MM), placed close to the position of the (CAT 1) decision height. The pilot attempts to manoeuvre the aircraft to keep the indicators centered while they approach the runway to the decision height. The relationship between the aircraft's position and these signals is displayed on an aircraft instrument, often additional pointers in the attitude indicator. ILS uses two directional radio signals, the localizer (108 to 112 MHz frequency), which provides horizontal guidance, and the glideslope (329.15 to 335 MHz frequency) for vertical guidance. Photo of Indra's Normarc localizer, taken at the runway 06L of the Montréal–Trudeau International Airport, Canada. The transmitter and antenna are on the centerline at the opposite end of the runway from the approach threshold. View of the primary component of the ILS, the localizer, which provides lateral guidance.
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