GCI radar was used for guiding night fighters onto attacking bombers. It had three stages of development: ‘mobile’, ‘intermediate transportable’ and ‘final’.
Early stations (from 1940) had equipment on wheeled caravans and temporary wooden hutting; these were replaced by intermediate stations which had the aerial arrays mounted above and below a wooden gantry, with operations carried out from wooden huts.
Final stations, built from 1942 onwards, had brick operations blocks, known as ‘Happidromes’. These stations had a single rotating aerial array with the transmitter and receiver housed in a well underneath.
GCI stations were fitted with a Type 7 radar initially operating on a frequency of 209 MHz, though later equipment operated on 193 and 200 MHz. This was a parallel development of the Chain Home Low (CHL) equipment by the addition of a height-finding capability and a Plan Position Indicator (PPI) display.
In a PPI display the cathode ray tube is scanned radially from the centre of the rim of the tube face. The angle of the scan is synchronised to the rotating aerial and the intensity of the beam is modulated by the signals received from the aerial.
The effective range of a GCI station was 90 miles with a range of 30 miles at 1000 feet.
The Happidrome at Cricklade (Code 31G) has been demolished with only the concrete base of theoperations building remaining; no trace of the Type 7 radar well could be found in the adjoining fields. The earlier ‘intermediate transportable’ GCI station was located on the opposite side of the road and this too has gone without trace. The domestic camp was probably shared with Down Ampney airfield which is a mile to the north east.