Overview
In aviation, the instrument landing system (ILS) is a precision radio navigation system that provides short-range guidance to aircraft, allowing them to approach a runway safely at night or in poor weather conditions. In its original form, ILS lets a pilot follow lateral and vertical guidance indications on a cockpit instrument until the aircraft descends to 200 feet above the ground and within half a mile of the runway; at that point the runway should be visible, and if it isn't, the pilot performs a missed approach and tries again or diverts. Bringing an aircraft this close to the runway using only instrument guidance dramatically expands the range of weather conditions in which a safe landing remains possible, compared to relying on visual approach alone. ILS has been a cornerstone of instrument approach procedures at major airports for decades, though satellite-based approach systems have increasingly supplemented and, at some airports, begun replacing it.
History
Instrument landing system technology was first developed and tested in the 1920s and 1930s, with early systems seeing limited experimental use before World War II demonstrated the clear military and civilian value of being able to land safely in poor visibility. ILS technology matured significantly through the 1940s and 1950s, and the system was formally standardized internationally by ICAO, becoming the dominant precision approach technology at major airports worldwide for the following several decades. ILS categories, distinguishing different levels of precision and the minimum visibility conditions each category supports, were established as the technology matured, letting airports and airlines clearly define what landing conditions a given runway's ILS installation could support. Satellite-based approach systems have increasingly supplemented traditional ILS installations since the 2000s, though ILS remains widely used and, at many airports, still represents the primary precision approach system available.
Design & Specifications
An ILS installation uses ground-based radio transmitters generating lateral and vertical guidance signals that aircraft receivers translate into cockpit display indications.
Operations
Pilots follow ILS guidance during approach, descending along the indicated path until reaching a decision altitude where the runway must be visible to continue landing.
References
| Category | Navigation, Meteorology & Rules |
| Type | Precision approach navigation |
| Origin | 1920s-30s, matured post-WWII |
| Standard Body | ICAO |
| Modern Complement | Satellite-based approaches |