The AN/APS-20 was an airborne early warning, anti-submarine, maritime surveillance and weather radar developed in the United States in the 1940s.

Although developed for carrier-borne operation, first being installed in the single-engined General Motors TBM-3W Avenger, it was also used in larger four-engined airframes, the last being a fleet of Avro Shackleton AEW.2 which were converted from maritime patrol aircraft.

[3] The solution to the problem of detecting objects beyond the horizon lay in developing a dedicated airborne early warning radar.

Cadillac I, the initial platform, was to be carrier-based and the first radar was to be fitted to converted General Motors TBM-3 Avenger torpedo bombers.

[7] The larger size of the PB-1W allowed for two ROs, an Electronics Technician and, critically, a Combat Information Center (CIC) Officer supported by two radio operators.

Between 24 February and 23 March 1953, PB-1W were used as an airborne command information centre, operating in a zone between 30 and 40 miles (48 and 64 km) from a naval task force.

The latter, which could carry up to 30 servicemen for long trips, so impressed the newly formed United States Air Force (USAF) that it ordered it into service as the RC-121 Warning Star.

One arena where the radar broke new ground was in weather research, particularly with the hurricane hunters that flew into tropical cyclones.

[27] Equipped with the dedicated Lockheed WV-3 Warning Star from 1956, the squadron frequently flew up to three times a week on tropical cyclone observation.

[30] Aircraft operated by these agencies were also made available to universities, and therefore the radar was also used as a tool for academic research where its attributes proved invaluable in the study of precipitation.

[31] It was used in a wide range of airframes, including converted Douglas DC-6 airliners and the dedicated Lockheed WP-3A Orion which served into the 1970s.

[32][33] Meanwhile, in 1948, the UK evaluated a PB-1W equipped with AN/APS-20 against a Vickers Warwick V fitted with ASV.13 to see if the radar could also be used to identify surface ships, but the peak power was deemed too high and the pulse length too long for the application.

[35] Despite was being ineffectual when tracking periscopes, it proved effective at identifying large surface ships up to 200 nautical miles (370 km; 230 mi) away.

[36] Subsequently, the airframe was adopted by a range of operators in nine different countries, from the Argentine Navy to the Republic of China Air Force.

[38][39] The AN/APS-20 was also briefly used as part of the Space Race, supporting Project Mercury, where the radar's long range enabled it to be used in tracking and other tasks.

The USAF trialled the more advanced AN/APS-82, which provided target height data, in 1956 but waited until after 1962 before replacing the radar with the AN/APS-95 aboard their Warning Stars.

[42][43] The AN/APS-80, which offered similar capabilities to the AN/APS-20 but added continuous 360° area search coverage, replaced the radar in the anti-submarine role from 1961.

Re-entering service in 1972 with the RAF with the designation AEW.2, the aircraft continued to operate until July 1991 in the airborne early warning role.