He later worked for Plessey before becoming a managing director of British Telecommunications Research and then becoming deputy chairman of the General Post Office.
[1] Fennessy was born in West Ham, Essex and educated at St Bonaventure's Grammar School in Forest Gate.
[2] His early academic career was not spectacular, and he graduated with a second-class degree from East London College in 1934.
One particularly ironic development concerns the Sonne long-range radio navigation system set up by the Luftwaffe.
[4] In late 1943 Fennessy prepared a plan for navigation and pathfinding systems to support an amphibious landing on the continent.
[2] Decca held a commanding lead in the UK civil maritime radar world for many years, and was a major player worldwide.
In 1952 he led efforts that eventually won the contract for the RAF's "Type 80 radar",[2] and opened a large factory at Cowes on the Isle of Wight to produce them.
[1] He moved into telecommunications research when Plessey and GEC formed a joint effort to build satellite ground stations.
[2] After retiring in 1977, he took on a number of consultancy jobs; including IMA Microwave Products and British Medical Data Systems, among others.