In the absence of inland radar coverage, 1st AA Division's Chief Signals Officer, Lt-Col G.C. Wickens, devised a system of 14 fixed base-lines of sound locators to detect night raids approaching the IAZ. These were linked by automatic telephone equipment to the Brompton operations room, where the angular plots were resolved to indicate grid squares where the HAA guns in range could fire an unseen barrage. Detachments of 75th Searchlight Regiment from 38th AA Bde were trained to operate the base lines. This 'Fixed Azimuth' system came into action in June 1940, in time for the opening of the night Blitz on London. It was later replaced as searchlight control (SLC) and gunlaying (GL) radar systems were introduced. However, the performance of the AA defences in the early weeks of the Blitz was poor. AA Command moved 108 HAA guns to the IAZ from other divisions, and arranged 'fighter nights' when the guns remained silent and Royal Air Force (RAF) night fighters were allowed to operate over London with the searchlights. Gun-laying (GL) radar, modern sound-locators and larger (150 cm) searchlights were introduced as rapidly as possible, and by February 1941 Searchlight Control (SLC or 'Elsie') radar began to be issued, replacing the fixed azimuth system. The number of raiders shot down steadily increased until mid-May 1941, when the Luftwaffe scaled down its attacks. The composition of 38 LAA Bde during this period was as follows:   The Blitz ended in May 1941, but there were occasional raids thereafter and AA Command continued to strengthen its defences. Newly formed units joining AA Command were increasingly 'mixed' ones into which women of the Auxiliary Territorial Service were integrated. AA Command had a critical shortage of Light AA gun (LAA) units, and began a process of converting some S/L units to the role. At the same time, experienced units were posted away for service overseas. This led to a continual turnover of units, which accelerated in 1942 with the preparations for Operation Torch. The AA defences of Southern England were severely tested in the summer of 1942 by the "Luftwaffe's" 'hit-and-run' attacks along the South Coast, and there was much reorganisation, accounting for some of the turnover of units listed earlier. LAA units waiting to go overseas were sometimes lent back to AA Command to deal with the hit-and-run raiders.