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Instrument landing system
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==Alternatives== * The [[Microwave Landing System]] (MLS) allowed for curved approaches. It was introduced in the 1970s<ref>Microwave Landing System For Jets Is Demonstrated. New York Times. May 20, 1976.</ref> to replace ILS but fell out of favor because of the introduction of satellite based systems. In the 1980s, there was a major US and European effort to establish MLS. But a combination of [[airline]] reluctance to invest and the rise of [[Global Navigation Satellite System]] (GNSS) resulted in its not being adopted in civil aviation. <!-- Two somewhat different points of view have been juxtapositioned here regarding the popularity of MLS. --> At the time ILS and MLS were the only standardized systems in Civil Aviation that meet requirements for Category III automated landings.<ref>{{cite web|url=http://www.caa.govt.nz/ICAO/Annex_10_Vol_1.pdf|title=Annex 10 β Aeronautical Telecommunications, Volume I (Radio Navigation Aids) Amendment 81|url-status=live|archive-url=https://web.archive.org/web/20081015034238/http://www.caa.govt.nz/ICAO/Annex_10_Vol_1.pdf|archive-date=2008-10-15}}</ref> The first Category III MLS for civil aviation was commissioned at Heathrow airport in March 2009 and removed from service in 2017.<ref>{{cite web |url=http://www.atc-network.com/News/29534/Worlds-first-low-visibility-microwave-landing-system-comes-into-operation-at-Heathrow |title=Worlds first low-visibility microwave landing system comes into operation at Heathrow |author=NATS |date=March 26, 2009 |publisher=atc-network.com |url-status=dead |archive-url=https://web.archive.org/web/20110707182801/http://www.atc-network.com/News/29534/Worlds-first-low-visibility-microwave-landing-system-comes-into-operation-at-Heathrow |archive-date=July 7, 2011 }}</ref> * [[Transponder Landing System]] (TLS) can be used where a conventional ILS cannot work or is not cost-effective. * [[Localizer Performance with Vertical guidance]] (LPV) is based on the [[Wide Area Augmentation System]] (WAAS), LPV has similar minima to ILS for appropriately equipped aircraft. {{As of|2008|November}}, the FAA has published more LPV approaches than Category I ILS procedures. * [[GNSS Augmentation#Ground Based Augmentation System|Ground-Based Augmentation System]] (GBAS) ([[Local Area Augmentation System]] in the United States) is a safety-critical system that augments the GNSS Standard Positioning Service (SPS) and provides enhanced levels of service. It supports all phases of approach, landing, departure, and surface operations within the VHF coverage volume. GBAS is expected to play a key role in modernization and in all-weather operations capability at CATI/II and III airports, terminal area navigation, missed approach guidance and surface operations. GBAS provides the capability to service the entire airport with a single frequency (VHF transmission) whereas ILS requires a separate frequency for each runway end. GBAS CAT-I is seen as a necessary step towards the more stringent operations of CAT-II/III precision approach and landing. The technical risk of implementing GBAS delayed widespread acceptance of the technology. The FAA, along with industry, have fielded Provably Safe Prototype GBAS stations that mitigate the impact of satellite signal deformation, ionosphere differential error, ephemeris error, and multipath.
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