Fennessy worked at Standard Telephones and Cables
before he joined the radar team at Bawdsey in 1935 where
he played an important role in the development of the
equipments and in planning for their effective use. He was
commissioned into the RAF in 1940 and played a major role
under Air Commodore Gregory setting up No 60 Group.
This was responsible for planning
the installation and maintenance of the whole ground radar
control and reporting system with the associated radar and
radio navigational equipments for the UK.
In 1944, promoted to wing commander, he was responsible for preparing the master plan for the deployment of the navigation, path-finding and bombing systems for the Allied landings in Normandy in 1944. By 1945 he was a group captain in 60 Group, in charge of organising all the RAF radio-navigation systems in the UK and Europe.
He joined the Decca Navigation Co in 1946 and became
of Decca Radar Ltd in 1950. After many senior appointments
he became the first Managing Director of British Telecom in 1969. He was
President of the Royal Institute of Navigation from 1975
17 January 1912 - 21 November 2009