The air situation was quiet until 21 October, when the Germans began a bombardment of Antwerp and Brussels with V-1 flying bombs. With experience drawn from defending against the V-1 attacks on Southern England (Operation Diver), GHQ AA Troops with 21st Army Group had a contingency plan for this, known as 'X' defences. This involved three lines of warning stations in the path of the V-1 attacks, then heavy AA guns at least 10 miles from the target cities. Searchlights were deployed to aid the guns at night, and to mark the boundaries of the X defence areas to warn off friendly aircraft. British units took responsibility for Brussels, but the two batteries of 42nd S/L Rgt deployed at Antwerp were under the command of the US 50th AA Artillery Brigade.