Instrument landing system

Lore & Background

The ILS was preceded by several radio-based landing systems developed between the 1920s and 1940s, notably the Lorenz beam, a blind-landing system that initially provided only horizontal guidance until a patent for vertical guidance was added. During World War II, the US-developed SCS-51, a mobile ILS system contained in a truck and trailer, offered better accuracy for both axes and was installed at many airbases in the United Kingdom. After the International Civil Aviation Organization (ICAO) was formed in 1947, ILS was selected as the first international standard precision approach system and published in ICAO Annex 10 in 1950. Further development enabled ILS to provide up to Category III approaches.

The system operates using a localizer transmitter on the runway centerline at the opposite end from the approach threshold, providing lateral guidance, and a glideslope transmitter for vertical guidance. Optional marker beacons, such as the middle marker placed near the Category I decision height, provide distance information, though these are largely being replaced by distance measuring equipment (DME). Approach lighting systems extend high-intensity lights from the runway toward the decision point to aid the transition from instrument to visual landing.

Other precision approach systems include the radar-based ground-controlled approach (PAR), the microwave landing system (MLS), and the ground-based augmentation system (GBAS). MLS was planned to replace PAR but saw limited deployment due to the promise of free GPS service and later differential GPS (DGPS). GBAS, standardized by ICAO as the third precision landing system capable of up to Category III, has been deployed at a limited number of airports, such as Frankfurt am Main, serving single or parallel runways. Despite these alternatives, ILS remains the only precision approach system supported by all IFR-equipped civil aircraft.

Reader's Guide

The instrument landing system has been the foundational precision approach system for civil aviation since its adoption by ICAO in 1950. By providing both lateral and vertical guidance down to a decision height of 200 feet and within half a mile of the runway, it dramatically increases the range of weather conditions in which a safe landing can be made. Its categories further reduce minimum altitudes and runway visual ranges, tailored to airport safety requirements and expected weather patterns.

While alternative systems such as MLS and GBAS have been developed and standardized, their deployment has been limited compared to the worldwide proliferation of ILS installations. The promise of free GPS signals and later augmentation systems like GBAS led many states to delay MLS implementation, and even today only a limited number of GBAS systems are in use. By 2015, the number of US airports supporting ILS-like LPV approaches exceeded the number of ILS installations, suggesting a potential eventual removal of ILS at most airports. However, ILS remains the only precision approach system universally supported by all IFR-equipped civil aircraft, ensuring its continued relevance in aviation infrastructure.