Instrument landing system
In aviation, the instrument landing system (ILS) is a radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a 1⁄2 mile (800 m) of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. Bringing the aircraft this close to the runway dramatically improves the weather conditions in which a safe landing can be made. Later versions of the system, or "categories", have further reduced the minimum altitudes.
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. The relationship between the aircraft's position and these signals is displayed on an aircraft instrument, often additional pointers in the attitude indicator. The pilot attempts to maneuver the aircraft to keep these indicators centered while they approach the runway to the decision height. Optional markers provide distance information as the approach proceeds, including the middle marker placed close to the position of the decision height. ILS may also include high-intensity lighting at the end of the runways.
A 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 the war. The US-developed SCS-51 system was more accurate while also adding vertical guidance. 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. 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. ILS remains a widespread standard to this day.
The introduction of precision approaches using low-cost GPS systems is leading to the replacement of ILS. Providing the required accuracy with GPS normally requires only a low-power omnidirectional augmentation signal to be broadcast from the airport, which is dramatically less expensive than the multiple, large and powerful transmitters required for a full ILS implementation. By 2015, the number of US airports supporting ILS-like LPV approaches exceeded the number of ILS systems, and this is expected to lead to the eventual removal of ILS at most airports.