Night fighter
When Bomber Command began attacks by night in May 1940, the Germans had no adequate means of intercepting incoming formations of RAF bombers. Pre-war trials aimed at creating a night fighter defence had used a warning service based on sound detectors and searchlights. Night fighters orbited the beacons at altitude outside illuminated area, and when a bomber was caught in the light, the fighter engaged the aircraft. Any focusing of searchlights at altitude signaled the night fighter to enter the illuminated zone and attack. AAA units were ordered to fire at every given opportunity, other than when the fighters were in the combat zone. These experiments ceased in August 1939 and in 1940 were still reliant on searchlight-aided AAA with fighters in a subordinate role.