Ground Controlled Intercept (GCI)
The CH and CHL stations were designed primarily as early warning and raid reporting stations, however by the out break of the Second World War sector controllers were using CH data to position their fighters with an accuracy of about five miles. This was usually sufficient on a clear day or in bright moon light but useless in poor weather conditions or dark nights. It had been hoped that the development of air interception radar (AI) would have enabled fighters to complete the interception on their own, however the engineering problems encountered with the AI development were immense and it was clear that for the time being a method of ground controlled interception would have to be found.
In early 1940 interception experiments with the CHL at Foreness showed that direct control of the interceptor was possible. Because of the lack of a PPI or mechanical rotation it was found that the best method was to point the aerial at the target and direct the interceptor along the beam. Whilst the experiments showed that interceptions could be achieved by this method, there was reluctance by sector controllers to relinquish control of the radar, mostly because they ceased to provide other plots whilst they were controlling (Watching The Skies), however the experiments served to demonstrate the need for dedicated ground controlled interception (GCI) radars.
Before continuing with the GCI story a digression into radar displays may be useful. The early radar displays (used in CH and CHL for example) comprised of a cathode ray tube (CRT) that displayed slant range, usually across the horizontal (X) axis, and a deflection proportional to the strength of the received signal on the vertical (Y) axis. The target bearing was read off of a vernier scale representing the direction the aerial was pointing (or goniometer in the case of CH). This type of display is commonly call a Type A display. A typical Type A display can be seen in the Radar Theory section here. In the June of 1940 the Telecommunications Research Establishment (TRE) built the first Plan Position Indicator (PPI). A typical PPI display can be seen in the Radar Theory section here. The PPI presented two dimensions, bearing and slant range, by having a time base as a radius on the tube pointing in the same direction as the aerial. Returned signals, instead of producing a deflection in the time base, were used to brighten up the spot at that part of the time base; if the general brightness was turned down, only the bright spot was seen. As the aerial turned, the bright spot appeared as the aerial's beam swept over the target as a small area of light on the tube. If a transparent gridded map was placed over the tube, the geographical position of all targets could be seen at each revolution of the aerial (Watching The Skies). The use of the PPI simplified interception techniques considerably; previously the tracking of target and interceptor relied on accurate range and bearing readings on two separate returns which were then plotted on the tracking table. Errors in calibration, measurement and the delays inherent in the filtering system could place the interceptor in the wrong position in time and space, ruining any chance of an interception. With the PPI however the plotting could take place directly on the tube face, thus eliminating filtering delays. The intercept controller is chiefly interested in the movements of the target and interceptor in relation to each other, so even if the PPI had a five degree rotational error, the error would be applied equally to both aircraft. Other types of display are shown here.
The pressure to develop GCI was intense; the German Luftwaffe had failed to destroy the R.A.F. during the Battle of Britain in the summer of 1940, thanks in part to the early warning provided by CH, and an intensive night bombing campaign was anticipated for the winter. An experimental GCI was constructed from a CHL modified to provide height finding information and utilized one of the first PPI displays. During trials nine out of the ten practice interceptions were successful, a remarkable feat by any standards. Six copies of the trials equipment were constructed by hand at TRE and the Royal Aircraft Establishment (RAE) during November and December of 1940. The first GCI became operational at Sopley on New Years Day 1941, the remaining five becoming operational by the end of January 1941 (see map, click on the map for a larger image). The results of the GCI interceptions improved rapidly as tactics were developed by controllers and pilots from 0.5 percent enemy losses in December 1940 to seven percent in May 1941 despite the always present manpower problems.
GCI stations varied in size and facilities from highly mobile (Type 15) through transportable (Type 8) to large fixed installations (Happidromes - Type 7) (Pioneers of Radar). The final design of the GCI radar was ready by June of 1941 and the Type 7 was developed to that design in six months. The Type 7 provided multiple controlling facilities, an improved aerial, improved height finding and longer range. Ground Controlled Interception had arrived.
Later in the war the fear of jamming in the now heavily used 200 MHz band led to developments in the 500-600 MHz band. In January 1942 six 50 cm Type 11 CHL/GCI equipments were ordered and all had been delivered by the end of the year. The performance of this radar was somewhat disappointing though, due mainly to a large gap in the vertical cover that could not be removed by tilting the aerial.
Constructed by Dick Barrett