The S-band AN/APS-20 was the world first AEW radar, initially developed by the MIT Radiation Laboratory during WWII. Entering service in
1945, it saw service for 45 years, phasing out in only in 1991 when the Royal Air Force retired its Shackleton AEW2. In US Navy service it
was succeeded by the Hazeltine AN/APS-82.
The US Navy, in 1944, under the threat of Kamikaze attack, ordered the development of a radar system that could be carried aloft in an
aircraft. Which would expand the radar horizon under which the Fleet was to operate during a series of campaigns through the Philippines and
northwards to Japan. Massachusetts Institute of Technology (MIT) was tasked with developing a workable system in February 1944 under Project
Cadillac and a prototype system was built and flown in August on a modified Grumman TBM Avenger torpedo bomber. With tests proving
successful, the system being able to detect low flying formations at a range in excess of 100 miles, the US Navy ordered production of the
TBM-3W Avenger, the first AEW aircraft to enter service. TBM-3Ws fitted with the AN/APS-20 radar entered service in March 1945, with some
36-40 eventually being constructed. The TBM-3W Avenger was purely an AEW radar aircraft, as the aircraft had a crew of only a single pilot
and one radar operator. All control functions were conducted on surface ships, with radar data transmitted via a data link which gave the
video image and radar antenna angle to enable a tactical picture to be developed in the Combat Information Center (CIC). Following the
success of Project Cadillac, Project Cadillac II was commenced in 1944, with the aim of producing a flying command center. This lead to the
development of the PB-1W, a modified Boeing B-17 Flying Fortress, using the same AN/APS-20 radar as the TBM-3W Avenger, but with several
operators on board who could steer defending fighters towards targets via radio. The PB-1W was specifically designed to counter the Kamikaze
threat, operating from land bases in support of the Fleet at sea. The AN/APS-20 radar as fitted to the TBM-3W and PB-1W became the mainstay
of AEW aircraft developments following World War Two.
Post WWII development
After WWII, the US Navy and Royal Navy adopted AEW very promptly and deployed AN/APS-20 radar equipped AD-3W, AD-4W Skyraider and AEW.1
Skyraiders on several carriers, while the US Navy and US Air Force fitted the radar to long range land based WV-1/2 and EC/RC-121C/D
derivatives of the Lockheed Constellation (with dorsal mounted 'nodding' AN/APS-45 height finding radar) . While not designed specifically
as an AEW aircraft, the Grumman AF-2W Guardian, when fitted with the AN/APS-20, also had a secondary AEW capability.
These AN/APS-20 based systems represent the first generation of AEW systems and saw extensive use well into the seventies, when the US Air
Force deployed its E-3A Sentry and the US Navy phased out the last of its land based AEW in favour of carrier based E-2B/C Hawkeyes. Carrier
based AEW has had a colourful history with the US Navy deploying the AD-3W, AD-4W, E-IB Tracer and finally the very successful E-2B/C
Hawkeyes family, while the Royal Navy transplanted its AN/APS-20s into the Gannet AEW.3 and subsequently passed these radar sets to the
Royal Air Force where they equipped the long serving Shackleton AEW upon arrival of its E-3D Sentry fleet.
1 MW peak power version, released in 1946.
Fitted to EA-1E Skyraider, P-2 Neptune.
1 MW peak power version. Essentially electrically and physically identical to the AN/APS-20A, differing mainly in a newer
flux-gate compass system. Fitted to P-2 Neptune.
2 MW peak power version, resulting in rediation health hazard concerns.
Unlike its early-model predecessors, the aircrafts movement was compensated for by using links with the internal Doppler
navigation kit and by calculating ground speed and drift. Included IFF interogator, data link (to the ground ship) was via AN/ART-28
Bellhop. Fitted to EA-1E Skyraider and Royal Air Force, Royal Navy Gannet AEW.3 and Shackleton AEW2.