Editing Instrument landing system (section)

{{Short description|Ground-based visual aid for landing}}
{{for|the Preston Reed album|Instrument Landing (album)}}
[[File:ILS diagramsimplified.png|thumb|upright=2|Diagram of an instrument landing system (ILS) approach]]

In [[aviation]], the '''instrument landing system''' ('''ILS''') is a precision [[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 {{convert|200|feet}} over the ground, within a {{convert|1/2|mi|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 increases the range of weather conditions in which a safe [[landing]] can be made. Other versions of the system, or "categories", have further reduced the minimum altitudes, runway visual ranges (RVRs), and transmitter and monitoring configurations designed depending on the normal expected weather patterns and airport safety requirements.
[[File:Lumières et ILS de la piste 06 L (3083202657).jpg|300px|thumb|View of the primary component of the ILS, the [[localizer]], which provides lateral guidance. The transmitter and antenna are on the centerline at the opposite end of the runway from the approach threshold. Photo of [[Indra Sistemas|Indra's Normarc]] localizer, taken at the runway 06L of the [[Montréal–Trudeau International Airport]], Canada.]]
ILS uses two directional [[radio signal]]s, the ''localizer'' (108 to 112&nbsp;MHz frequency), which provides horizontal guidance, and the ''glideslope'' (329.15 to 335&nbsp;MHz frequency) for vertical guidance. The relationship between the aircraft's position and these signals is displayed on an [[Flight instruments|aircraft instrument]], often additional pointers in the [[attitude indicator]]. The pilot attempts to manoeuvre the aircraft to keep the indicators centered while they approach the runway to the [[decision height]]. Optional ''[[Marker beacon|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. Markers are largely being phased out and replaced by [[distance measuring equipment]] (DME). 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.<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=1st |publisher=Osprey |date=1973 |page=143 }}</ref>