Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 134,039 pages of information and 213,154 images on early companies, their products and the people who designed and built them.
of Cowes, Isle of Wight, and of Addlestone
1965 Plessey Co took over the non-marine radar part of Decca Radar  under Edward Fennessy, which formed the basis of Plessey Radar. The company had to move out of Decca's premises at Chessington, and relocated to Addlestone.
1968 Item about apprentices 
1974 Addlestone, Weybridge, Surrey, KT15 2PW: Divisional managing director P. E. G. Bates; general manager E. R. B. Wills. National air defence radar system; mobile air defence radar systems; three-dimensional air defence radar; terminal area and en route air traffic control systems; primary and secondary radars for defence and civil aviation; naval and coastal radars; naval electronic equipment including displays, data handling, IFF, weapon control and action information systems; meteorological systems including weather and windfinding radar for civil and military operations; upper air systems; hydrological systems; oceanographic systems; message-switching and software systems; defence communications networks; information handling systems. 
Plessey Isle of Wight: Production of radars for military and commercial Air Traffic control (ATC), naval and land based applications.
Products included ACR-430 motion detection radar used to control traffic on airfields, AR-1, AR-5, AR-5D, AR-15, AR-15AB, AR-15D, AR-15M, AR-15/2, DASR-1, DASR-3 then, in cooperation with ITT the AR-320 3D radar system for various ATC and air defence (AD) applications. Plessey also cooperated with Siemens in the production of the Astral and AWS-2, AWS-3, AWS-4, AWS-5, AWS-5A/B/C/D, AWS-6, AWS-6/100/200/300 series, AWS-6D/E, AWS-7, AWS-8, AWS-9(2D) and (3D) all for maritime applications whilst the Guardsman-C, Guardsman-2 and Guardsman-S were widely deployed in both civil and military airfields. The Company also produced the CDR-431 radar for coastal surveillance and the Commander S, a mobile system employed by the Army in 1996. Plessey was variously involved in the production of IFF (Indicate Friend / Foe) equipment including the Condor system and IFF Mk 10 that became a World standard for NATO and commercial IFF. The Company also produced the HPR and MPR signature measurement systems that were used to evaluated radar cross sectional reflection areas of ships and aircraft, which in turn lead to the development of stealth aircraft and ship designs. Over time, with improving experience and skill Plessey designed and manufactured a range of additional specialist radar systems such as MSAM, MTCALS, P-SCAN 2000, Pike, PTR-826, PVS 2000, Sentinel, Sigma, Spectar, the Rapier fire control system for the British Army; the Type 93 and Type 101 for the Royal Air Force while the Types 993, 994 and 996 series were produced for the Royal Navy.
Other businesses that existed alongside Plessey Radar, or were spun off from Radar, included Plessey Airports.