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N.B. this is part of Plessey Co
of Ilford, Essex.
1969 Radio Systems Division at Braxted Park, West Leigh (Havant) and Ilford worked on MALLARD, an abortive multi-national project to develop a battlefield communications systems Plessey : in 1969 was part of Radio Systems Division
1972 Awarded contract by Ministry of Defence for shipborne multi-channel communication systems
1973 Plessey Avionics and Communications was at Upminster, Essex
1974 Managing director F. K. Chorley; sales manager avionics M. I. Dodd.
Airborne UHF/VHF transmitter/receivers: PTR175, PTR377, PTR1721 and PTR170 (UHF only); airborne integrated data acquisition and recording systems including advanced large-capacity PV740 system for the Boeing 747, PV1573 system for ARINC 573 for Concorde and wide-bodied aircraft and comprehensive flight-test systems; aircraft weapon control and stores management systems; missile guidance systems; radar altimeters; target aircraft telecommand systems. 
1986 Plessey Avionics, of Havant, announced the Plessey Automatic Launch Test Facility would be installed at all RAF stations to check every aircraft's friend/foe identification system was working at take-off
1989 Plessey Avionics announced a computer-based inflight entertainment and telephone system which it was offering to major airlines with no capital expenses by the airlines and profits shared; the idea has originated with a company called Candlestar and had evolved from military hardware
Later see GEC-Plessey Avionics