Airborne Early Warning radar for the Royal Navy and Royal Air Force, from WWII to the present day

The use of radar in aircraft to achieve long-range early warning of airborne threats was developed during WWII in Britain and the USA and is now a key component of air defence.  The first operational dedicated AEW systems in the UK were for the Royal Navy Fleet Air Arm (FAA), with the Douglas Skyraider AEW.1 aircraft entering service in 1951.   These aircraft were manufactured in the USA and fitted with the AN/APS-20A radar (operating in S-band) and other components, which had all been developed for the US Cadillac I system during WWII.  

The Skyraider aircraft were replaced in 1959 by the Fairey Gannet AEW.3, fitted with the AN/APS-20F/I, an updated version of the AN/APS-20 radar family.  In 1978, the Gannet AEW.3 left service.  This was due to the introduction of the Illustrious Class carriers, which could only carry helicopters and VTOL aircraft (the Harrier).  The resulting capability gap was not fully filled until the arrival in 1985 of the Mk.2 AEW Sea King, fitted with a version of the Searchwater radar (operating in X-band).  These were replaced in 2002 by the ASaC Mk. 7 Sea Kings, fitted with the Searchwater 2000AEW radar.  These in turn left service in 2018 and are being replaced by Crowsnest, fitted in Merlin Mk. 2 helicopters, with an upgraded Searchwater radar.

The Royal Air Force (RAF) acquired an AEW capability in 1971 when the AN/APS-20F/I, as used in the Gannet AEW.3, was fitted in Shackleton AEW.2 aircraft.  Following the cancellation of the Nimrod AEW.3 programme, these Shackleton systems had to remain in service until 1991 when the RAF acquired the E-3D AWACS.  The AWACS aircraft remained until 2021 and the RAF is currently awaiting delivery of the E-3A Wedgetail, fitted with a MESA L-band Active Electronic Scanned Antenna (AESA) radar.