Trials and initial use The first aircraft equipped with the AN/APS-20 was a converted TBM-3 Avenger, designated XTBM-3W, which first flew on 5 August 1944. The radar was mounted in a radome under the forward fuselage. A series of aircraft were converted from existing TBM-3 airframes by the
Naval Aircraft Modification Unit and designated TBM-3W, with initial training aboard the
aircraft carrier starting in May 1945. The crew of the Avenger consisted of a single Radar Operator (RO) and the
pilot. The radar picture was transmitted back to the aircraft carrier for processing. At the same time, 31 large
Boeing PB-1W aircraft were converted from B-17G Flying Fortresses to become the first land-based aircraft equipped with the radar. 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. The latter team was able to direct aircraft to the target, adding the ability of the aircraft to control
fighter aircraft and enabling operation independent of shipboard or ground-based control. This capability was later developed into the
Airborne Warning and Control System (AWACS). The war ended before either aircraft became operational. Post-war, the USN quickly accelerated a program to get the radar into service. Between 24 February and 23 March 1953, PB-1W were used as an airborne command information centre, operating in a zone between from a naval task force. However, newer airframes were quickly introduced. AN/APS-20 radars were installed in the
Douglas AD-3W Skyraider, which replaced the Avenger, and the
Lockheed PO-2W Warning Star, later designated the EC-121 in 1962, a specialized platform developed from the
Lockheed L-1049 Super Constellation airliner. Both also saw service outside the USN. 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. The radar was used to detect aircraft as they approached over water, although it struggled to identify low flying aircraft against a background of trees, topographical features or moving land vehicles. At the same time, the
Royal Navy procured fifty examples of the AD-3W as the Skyraider AEW.1. This was the first use of the radar by a non-US operator. Meanwhile, in an attempt to extend endurance, in 1954 the USN ordered the installation of the radar in a
blimp, the
ZP2N-1W, later redesignated ZPG-2W and then, in 1962, EZ-1B. The last example retired in October 1962, not only ending the use of the radar in airships but also all lighter than air operations by the service.
Expanding capabilities Alongside these developments, new uses of the radar were explored. One arena where the radar broke new ground was in
weather research, particularly with the
hurricane hunters that flew into
tropical cyclones. The first trials of the radar took place in 1946 with a PB-1W and on 15 September 1953 the newly created weather squadron VJ2 flew the first aircraft equipped with an APS/AN-20 into a hurricane,
Hurricane Dolly. 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. At the same time, VW-1 was providing a similar service tracking
typhoons in the Pacific. Other users of the radar included the
Environmental Science Services Administration (ESSA) and its successor the
National Oceanic and Atmospheric Administration (NOAA). 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. 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. 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. Nonetheless, the
Royal Canadian Air Force (RCAF) used the radar in their
maritime surveillance Canadair Argus Mk.1. Of greater impact, however, was the subsequent use of the improved AN/APS-20E in the role aboard the
Lockheed P2V Neptune. The radar first flew in the third airframe, designated P2V-2S, which was the first one developed for the anti-submarine role. Despite it being ineffectual when tracking periscopes, it proved effective at identifying large surface ships up to away. 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. The AN/APS-20E was also fitted to
US Marine Sikorsky HR2S transport helicopters. Tests proved that fitting the radar to the airframe did not jeopardise flight characteristics but the radar components were damaged by the helicopter's high level of vibration. 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. It was particularly used to help find returning space capsules after
splashdown. For example, the radar was installed aboard aircraft of
VP-5 that were involved in the recovery of
Commander Alan Shepard from
Mercury-Redstone 3,
Captain Gus Grissom from
Mercury-Redstone 4 and Commander
Wally Schirra from
Mercury-Atlas 8. Replacement of the radar took many decades. 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. In the meantime, the USN introduced the AN/APS-82 on board carriers in the airborne early warning role in 1959 as the design of the Grumman WF-1 equipped with the AN/APS-20 was superseded at the production stage by the
Grumman WF-2 Tracer. 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. The Canadians finally replaced their AN/APS-20 with
AN/APS-115 in 1981. The last operator of the radar was the
Royal Air Force (RAF), which had previously used it between 1952 and 1957. When the Royal Navy retired their last AN/APS-20, they were refurbished and fitted to retiring
Avro Shackleton MR.2 maritime patrol aircraft. 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. ==Variants==