Background In 1963, the USAF asked for proposals for an Airborne Warning and Control System (AWACS) to replace its
EC-121 Warning Stars, which had served in the
airborne early warning role for over a decade. 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", i.e. to detect the movement of low-flying aircraft, and discriminate, even over land, target aircraft's movements; this was previously impossible due to the inability to discriminate an aircraft's track from
ground clutter. Contracts were issued to
Boeing,
Douglas, and
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 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. 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. , a 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 engines. It would carry its radar in a rotating dome mounted at the top of a forward-swept tail, above the fuselage. Boeing was selected ahead of
McDonnell Douglas's
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. 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. Hughes' radar was initially thought to be a certain winner due to its related development of the
APG-63 radar for the new
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.
Full-scale development Approval was given on 26 January 1973 for the full-scale development of the AWACS system. To allow further development of the aircraft's systems, orders were placed for three preproduction aircraft, the first of which performed its maiden flight in February 1975.
IBM and
Hazeltine were selected to develop the mission computer and display system. The IBM computer was designated 4PI, and the software was written in
JOVIAL. A
Semi-Automatic Ground Environment (SAGE) or
back-up interceptor control (BUIC) operator would immediately be at home with the track displays and tabular displays, but differences in symbology would create compatibility problems in tactical ground radar systems in
Iceland, mainland Europe, and South Korea over
Link-11 (TADIL-A). In 1977, Iran placed an order for ten E-3s, however this order was cancelled following the
Iranian Revolution. in 1977|alt=Black-and-white photograph with angled front view of four-engine jet aircraft on ramp with front fuselage door opened: A contingent of people are there to welcome the jet, which has a disc-shaped radar perching on top of struts on the dorsal fuselage. Engineering, test and evaluation began on the first E-3 Sentry in October 1975. Between 1977 and 1992, a total of 68 E-3s were built.
Future status Because the Boeing 707 is no longer in production, the E-3 mission package has been fitted into the
Boeing E-767 for the
Japan Air Self Defense Forces. The
E-10 MC2A was intended to replace USAF E-3s—along with the
RC-135 and the
E-8 Joint STARS, but the program was canceled by the
Department of Defense. NATO intends to extend the operational status of its AWACS until 2035, when it is due to be replaced by the
Alliance Future Surveillance and Control (AFSC) program. The
Royal Air Force chose to limit investment in its E-3D fleet in the early 2000s, diverting Sentry upgrade funds to a replacement program. On 22 March 2019, the UK Defence Secretary announced a $1.98 billion contract to purchase five
Boeing E-7 Wedgetails. The order was later reduced to three aircraft. The U.S. Air Force intends to retire 15 of its 31 E-3s and acquire the E-7. On 31 March 2023, the USAF retired an E-3 from service for the first time. NATO will replace the E-3 with
Saab GlobalEye on
Bombardier Global airframes. ==Design==