Editing Boeing E-3 Sentry (section)

===Background===
In 1963, the USAF asked for proposals for an Airborne Warning and Control System (AWACS) to replace its [[Lockheed EC-121 Warning Star|EC-121 Warning Stars]], which had served in the [[airborne early warning]] role for over a decade.<ref>Wilson 1998, p.&nbsp;72.</ref> The new aircraft would take advantage of improvements in radar technology and computer-aided radar data analysis and data reduction. These developments allowed airborne radars to "[[Look-down/shoot-down|look down]]", i.e. to detect the movement of low-flying aircraft, and discriminate, even over land, target aircraft's movements; previously this had been impossible due to the inability to discriminate an aircraft's track from [[Radar#Clutter|ground clutter]].<ref name=Eden_p92>Eden et al. 2004, p.&nbsp;92.</ref> Contracts were issued to Boeing, [[Douglas Aircraft Company|Douglas]], and [[Lockheed Corporation|Lockheed]], the latter being eliminated in July 1966. In 1967, a parallel program was put into place to develop the radar, with Westinghouse Electric Corporation and Hughes Aircraft being asked to compete in producing the radar system. In 1968, it was referred to as Overland Radar Technology (ORT) during development tests on the modified EC-121Q.<ref name="AUR1">{{cite web |url=http://www.airpower.maxwell.af.mil/airchronicles/aureview/1972/may-jun/pearce.html |title=AWACS to Bridge the Technological Gap |access-date=14 February 2009 |publisher=Air University |archive-url=https://web.archive.org/web/20040627142853/http://www.airpower.maxwell.af.mil/airchronicles/aureview/1972/may-jun/pearce.html |archive-date=27 June 2004 |url-status=dead}}</ref><ref name="Davies p2">Davies 2005, p.&nbsp;2.</ref> The Westinghouse radar antenna was going to be used by whichever company won the radar competition since Westinghouse had pioneered the design of high-power [[radio frequency]] (RF) phase-shifters, which are used to both focus the RF into a pencil beam and scan electronically for altitude determination.

[[File:Lockheed RC-121C 1955.jpg|thumb|The piston-engined [[Lockheed EC-121 Warning Star|EC-121 Warning Star]], military version of the [[Lockheed Constellation]], saw service in the mid-1950s.|alt=Black-and-white photograph of piston-engined aircraft with a large hump on midfuselage]]
Boeing initially proposed a purpose-built aircraft, but tests indicated it would not outperform the already-operational 707, so the latter was chosen instead. To increase endurance, this design was to be powered by eight [[General Electric TF34]]s. It would carry its radar in a rotating dome mounted at the top of a forward-swept tail, above the fuselage.<ref name=Eden_p92/><ref>{{cite web|first=Erik|last=Simonsen|url=http://www.boeing.com/news/frontiers/archive/2007/march/i_history.pdf|title=Still keeping watch|date=March 2007|publisher=Boeing|access-date=21 August 2011|archive-url=https://web.archive.org/web/20110629074657/http://www.boeing.com/news/frontiers/archive/2007/march/i_history.pdf|archive-date=29 June 2011|url-status=live}}</ref> Boeing was selected ahead of [[McDonnell Douglas]]'s [[Douglas DC-8|DC-8]]-based proposal in July 1970. Initial orders were placed for two aircraft, designated EC-137D, as test beds to evaluate the two competing radars. As the test beds did not need the same 14-hour endurance demanded of the production aircraft, the EC-137s retained the [[Pratt & Whitney JT3D]] commercial engines, and a later reduction in the endurance requirement led to retention of the JT3D engines in production.<ref name="Davies p2"/><ref name="Janes 76 p246">Taylor et al. 1976, p.246</ref>

The first EC-137 made its maiden flight on 9 February 1972, with the fly-off between the two radars taking place from March to July of that year.<ref name=AUR1/> Favorable test results led to the selection of Westinghouse's radar for the production aircraft.<ref name="Davies 5-6">Davies 2005, pp.&nbsp;5–6.</ref> Hughes' radar was initially thought to be a certain winner due to its related development of the [[AN/APG-63 radar family|APG-63]] radar for the new [[McDonnell Douglas F-15 Eagle|F-15 Eagle]]. The Westinghouse radar used a pipelined [[fast Fourier transform]] (FFT) to digitally resolve 128 Doppler frequencies, while Hughes's radars used analog filters based on the design for the F-15. Westinghouse's engineering team won this competition by using a programmable 18-[[bit]] computer whose software could be modified before each mission. This computer was the AN/AYK-8 design from the B-57G program, and designated AYK-8-EP1 for its much expanded memory. This radar also multiplexed a beyond-the-horizon (BTH) pulse mode that could complement the pulse-[[Doppler radar]] mode. This proved to be beneficial especially when the BTH mode is used to detect ships at sea when the radar beam is directed below the horizon.<ref name="Northrop1">{{cite web |url=http://www.es.northropgrumman.com/solutions/awacs/assets/AWACS.pdf |title=AWACS Surveillance Radar |access-date=10 February 2009 |publisher=Northrop Grumman |url-status=dead |archive-url=https://web.archive.org/web/20090227021024/http://www.es.northropgrumman.com/solutions/awacs/assets/AWACS.pdf |archive-date=27 February 2009}}</ref>