Back in the early days of Airborne Early Warning and Control aircraft (AEW&C) they would have massive radar domes and sensors fitted above or below the fuselage such as the Lockheed EC-121 Warning Star (1952-1984 in US service), Grumman E-1 Tracer (1958-1977 US Navy) and Fairey Gannet AEW.3 (1959-1978 Royal Navy). The latter two types operated off aircraft carriers.
AEW&C technology progressed into more modern designs in the 1960’s and 1970’s where AEW&C aircraft had a massive spinning radar dome fitted above the fuselage of the aircraft such as the Grumman E-2 Hawkeye (1964 – present and in service with a number of nations including the US), Boeing E-3 Sentry AWACS (Airborne Warning And Control System – based off a Boeing 707 airframe and operating since 1977 to the present day with a number of nations and NATO) and Soviet era equivalents the Tupolev Tu-126 Moss (1965-1984 in USSR service) and Beriev A-50 Mainstay (based off the Ilyushin Il-76 transport airframe – entered USSR service in 1984 and is still in use with the Russian, Indian and Iranian Air Forces. The Chinese have an equivalent version known as the KJ-2000 Mainring).
The next generation of AEW/C aircraft now entering service use a very different design. They feature a long thin radar system along the spine of the aircraft that offers broad radar coverage and low drag, enabling the system to be fitted to generally smaller aircraft including both turbo-prop and jet engined airframes (this helps make the systems more affordable). Aircraft companies in countries such as the US, China and Sweden have developed these systems and one that has entered is the Boeing 737 AEW&C which was originally developed for the Royal Australian Air Force (RAAF). It features a Northrop Grumman “MESA” electronically scanned array radar system that can track both air and maritime targets. In RAAF service it is known as the Wedgetail, which I have seen on static display a couple of times at the Avalon International Air Show in Australia.
The Boeing 737 AEW&C is also now operated by the Turkish Air Force as the Peace Eagle and by the South Korean Air Force as the Peace Eye. A few months ago I spotted a Turkish Peace Eagle doing a practice landing at Paine Field in Everett, Washington in the USA. It’s a pretty unique looking aircraft!