The new system divided searchlights into belts known as:
Gun Defended Areas
The system was similar to that used by the Germans where searchlights were used to indicate the course of Allied bombers. One War Diary reported that Searchlight troops had put a lot of work into accomodation at Cluster sites and were not pleased to have to move again. They were to be told that the new method was as a result of a request by the RAF following lessons learnt from raids over Germany although the details of the tactical purposes were to be kept secret from the troops. - see Lone Sentry.Com for details of the German system for searchlights
The proposed layout was as follows:
The Indicator belt was not to be less than 12 miles deep and was to be on the enemy side of the Killer belt (in some cases on both sides of the Killer belt). Single searchlights were to be spaced at 10,400 yards at existing Cluster sites. The function of the lights was to indicate to the fighter the entry of an enemy aircraft and to provide a continuous mark on which to fly to intercept the enemy in the Killer belt.
The Killer belt was not to be less than 16 miles deep. Single searchlights were to be spaced at 6,000 yards (one third at existing Cluster sites) and the function of the lights was to illuminate the enemy aircraft so the interceptor fighter could engage it. It was finally decided that the Killer belt should be 20 miles in depth between the first and last row of lights. If topography prevented this it could be reduced to a minimum depth of 16 miles.
Gun Defended Areas were to be provided with S.L.C single lights at 6,000 yard spacing. The spacing could be thickened to 3,500 yards if required with the temporary deployment of mobile units. Fighter aircraft were to avoid the Gun Defended Area if possible. When orders finally confirmed the new layout in September the deployment of mobile units had to be abandoned due to the fact that AA Command had to give up 50,000 men of which 30,000 were to be found from Searchlights.
This reorganization and shortages of manpower at certainly caused problems initially with the new layout. The CO of 69th Search Light Regt noted a “dismal and discouraging failure” of what the Regt tried to achieve in Nov 1941 and further went on to note an “epidemic of training, operational and administrative paralysis”. Perhaps rather unkindly he also noted that manpower shortages resulted in the Regt having “to persevere with the ‘Category C’ man who has no brains at all”.
The tactics to be deployed required interceptor fighters to orbit a beacon and not fly patrol lines as previously under the old systems. When lights in the Indicator belt detected a raid the fighter was to direct himself onto searchlights in the Indicator belt. The depth of the two belts, calculated on the difference of speed between fighters and bombers, would ensure that eventually the fighter closed with the rear of raiders in the Killer belt. Orbit points would be approximately 14 miles apart on a line eight miles in the rear of the front edge of the Killer belt.
Above: In theory the fighter sighted the exposure of searchlights at A, the forward edge of the Indicator belt. He continually directs himself on the successive exposure of searchlights until he closes with the raider and engages it at B. No vectoring by ground controllers was required.
Priority for the distribution of equipment was as follows:
Searchlights – 150 cm for Killer belts, 90 cm for Indicator belts and 90 cm / 120 cm for GDA’s.
Sound Locators – the most advanced models in the Killer belt as first priority then the Indicator belt then the GDA’s
SLC – first priority to the Killer belt, then to the GDA’s (Lowestoft/Yarmouth and Harwich), then to the Indicator belt. As SLC became more available the higher marks of Sound Locators would be displaced form the Killer belt to the Indicator belt etc.
During the process of ‘declustering’ labour was to be found to erect hutting etc at the new sites by only operating one light at the existing clusters. Priority of work was to be (i) roads (ii) hutting (iii) essential ground defence works (iv) essential protection of the projector. Once roads and hutting was complete the new site was to become operational. Sites in the Killer belt were to be tested for the suitability of SLC before any huts were erected.
Above: The layout of the Belt system, East Anglia
It was also decided to deploy single barrel U.P equipments at certain searchlight sites. One projector was to be provided for each terminal line searchlight site in the Killer belt and at sites in the Indicator belt. The aim was to allow searchlight sites to engage low flying aircraft (below 3,000 ft) and to act as a further indicator to interceptor fighters. Only seen targets were to be engaged using pre set fuses – no ancillary equipment (e.g. range finders or wind deflection calculators) were to be deployed to keep the operation as simple as possible.
32 Searchlight Regt papers, TNA
2 AA Div papers, TNA