Boeing E-3D Sentry (AWACS)

Development and the British version

The E-3 family grew from a strengthened Boeing 707 airframe designed to carry a radar and identification system above the fuselage. Britain selected Sentry in December 1986 after the Nimrod AEW3 programme encountered major mission-system difficulties. RAF E-3Ds began arriving in 1991.

National E-3 versions differ in engines and mission equipment. The E-3D has CFM56 engines and was delivered with electronic-support equipment, Link 16 and colour displays. These distinctions matter when comparing it with American, French or NATO-owned Sentries; a feature of one version cannot be assumed for all of them.

Radar and identification

The radar antenna and the identification-friend-or-foe antenna sit back to back inside the rotodome. At six revolutions per minute, each sensor makes a complete sweep in about ten seconds. That interval helps explain why a radar picture is a sequence of updates, rather than an unbroken view of an aircraft’s exact movement.

The AN/APY-2 scans horizontally by rotating the antenna and changes the beam’s elevation electronically. Its pulse-Doppler and pulse modes serve different air and surface-search tasks. Some modes measure target height; others trade that information for different search performance. Detection depends on the target, height, clutter and selected mode, rather than one universal range.

Secondary surveillance supplements radar echoes with transponder replies. In the 1999 E-3D configuration, AN/APX-103 equipment decoded Modes 1, 2, 3 and C, with Mode 4 interrogation requested for selected tracks or directions. Primary and secondary radar explain the distinction between detecting an echo and receiving a cooperative reply.

The people behind the picture

The flight-deck crew flies and navigates the aircraft. Technicians maintain its mission equipment, while surveillance personnel compile the picture, controllers work with aircraft and a tactical director coordinates the mission team. Data-link and communications specialists help move information between the aircraft and other units.

The RAF’s ©08/03 crew arrangement comprised four flight-deck crew, three technicians and eleven mission crew. Earlier descriptions used different controller allocations. A crew total is therefore an operating arrangement, not a measure of radar coverage or a fixed number of aircraft that can be controlled.

Electronic support and sharing information

British E-3Ds were delivered with Yellow Gate electronic-support equipment. It passively detected emissions from other equipment, adding a different source of information to radar and transponder returns. Passive sensing does not require the target to reply to an interrogation.

Radios and tactical data links connected the Sentry to aircraft, ships and ground organisations. Link 11 and Link 16 helped exchange the surveillance picture. The result was both an airborne sensor and a place from which trained personnel could coordinate activity; possessing a radar alone does not provide all those functions.

Height, coverage and time on task

Operating at altitude extends the view towards low-flying aircraft compared with a sensor near the ground. The Earth’s curvature still matters: the radar horizon depends on the heights of both sensor and target, as well as propagation conditions. An airborne radar does not see through the Earth or remove every blind area.

Long sorties combine transit, work on station and the return flight. Both flying-boom and probe-and-drogue refuelling were available to the E-3D. The November 2015 figures were approximately 5,000 nautical miles range and eleven hours mission endurance, extendable by refuelling. Time over a patrol point is shorter than total mission endurance because transit and the return flight use part of that time.