In 1940 arrangements were made to protect certain Vulnerable Points with an “Anti-Dive-bombing Barrage”. This included Harwich Harbour (other sites were Thames Haven Oil Installations, Tilebury Docks, Chatham Dock Yards, Sheerness Dock Yard, Dover Harbour and Purfleet).
The barrage was designed for a height of 3,000 feet with a dive angle of 60˚. A barrage circle was to be prepared with its centre being the centre of the VP or its most vital part. For a small VP the barrage circle would have a set radius of 577 yards – for larger sites the radius would be set by the Anti-aircraft Defence Command (AADC) in each case.For each gun site the barrage circle would be divided into four quadrants symmetrical along the line gun site-VP. Each site would then arrange a concentration at a point in the centre of the arc of each quadrant (points W,X,Y and Z and the diagram below) at an average height of 3,000 ft. A height zone of 400 ft to 500 ft distributed between the four guns was desirable.
If the VP was only protected by one gun site the barrage would have only a limited effect as the concentration was limited to only a small part of the quadrant. If there was a number of gun sites good coverage would be achieved due to the effect of overlapping barrage circles.
The barrage would be employed at the discretion of the Gun Position Officer (GPO) who would determine which quadrant to fire at. For example if a dive-bombing attack was developing to the NE of the VP he would order a barrage in quadrant A. The barrage may move around the barrage circle as a result of changing lines of attack.
Right: The Anti-Dive-bomber Barrage