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Defending Coventry
n 1940 RADAR was still in its infancy, and night fighters were not very effective at engaging or shooting down enemy planes. Therefore the main form of defence for night attacks on cities comprised anti-aircraft guns and barrage balloons. By 14th November Coventry had 56 barrage balloons, which were typically deployed at various heights up to around 5,000 feet. (Nearly a mile.) They were intended to keep the German bombers at a height at which bombing accuracy was poor, and also to discourage dive-bombing, but some visual accounts by Coventry citizens suggest that this was not wholly effective.
Ford's Hospital was one of Coventry's early casualties of the war, being hit by an HE (high-explosive) bomb exactly a month before the big November raid.
The existence of these guns provided a great morale boost for the people, as they constantly thudded away, giving the impression that we were "giving it" to the enemy. However, despite the positive perception, the guns in reality were unfortunately quite ineffective, and this had indeed been predicted before the outbreak of war when air defences were being discussed, and previous 'ack-ack' excercises had yielded poor results.
Radio interference
Another way that the British could attempt to foil attackers was to interfere with their Radio guidance system. The first wave of bombers sent by the Germans was Kampfgruppe 100, (known as pathfinders) whose job was to accurately locate the target and drop incendiaries, which would then mark the area for succeeding waves of bombers to easily follow.
To enable the pathfinders to find a target in the dark, the Germans had tried several methods; the most common system in the period leading up to the November Blitz being a pair of intersecting beams known as Knickebein. In November, however, the Germans employed a new system known as X-Gerat (simply meaning 'X Apparatus'), consisting of four radio beams. (See map below showing how Coventry was targeted.) Each beam was named after a German river, and the planes would initially follow roughly along the path of 'Weser', transmitted from Cherbourg.
Map showing how the German X-Gerat radio beams were deployed.
Our idea to counter this method was to send out our own radio beams in a slightly different direction, in order to try and send the German pilots off course. The X-Gerat radio signals were highly directional VHF 'carrier waves' modulated with an audio signal, which could be heard in the pilot's earphones to let him know that he was crossing a particular 'beam'. However, these radio signals were usually switched on by the Germans a few hours before an air-raid, (although not in the eventually intended direction) and this enabled British planes fitted with suitable radio equipment to fly across the beams and attempt to measure which frequencies were being transmitted by the German radio equipment.
The measuring equipment used back then had very limited accuracy, but on the afternoon before the big raid of November 14th, Doctor (later Professor) R. V. Jones, the technical wizard working for British Intelligence, used immense skill, and, as he later admitted, some good fortune, to work out what carrier frequency was in use. Another radio engineer, possibly at Farnborough, worked out that the modulated 'audio' signal was at 1.5kHz. At that point, nobody here knew what the intended target was, but at least we knew on which frequencies to set our jamming equipment....
The old Market Tower and two spires - a view never before seen from this vantage point between West Orchard and Smithford Street.
Breaking the codes
It was three days earlier when the British first knew that a big raid was planned. We put pieces of information together on the 11th November from two sources; - a German prisoner of war talking to a room mate (either someone 'planted' or he could have been bugged) about a raid on a larger scale than ever attempted, - and from German signals decoded by the captured Enigma machine. (Intelligence gathered in this way was known as ULTRA.)
An unusual view across Broadgate from the top floor of what is now the NatWest Bank, showing Martin's Bank near the corner of High Street and Broadgate.
The code-breaking staff at Bletchley Park, Buckinghamshire, had deciphered material that suggested a German operation by the code-name 'Moonlight Sonata', and this would most likely take place during the period of full moon, around the 14th/15th November. Also, teasingly, the Germans had sent codes for three possible places as the bombing target - but only two of them had been deciphered by the time the raid came. There were Wolverhampton and Birmingham.
Target 51: Wolverhampton had been worked out from the decoded message "Einheits Preis", which translated as "single price". The slogan for the Wolverhampton based Woolworths store was "Nothing over sixpence", and our scientists soon saw the association.
Target 52: Birmingham was the home of former Prime Minister Chamberlain, and the decoded signal of "Regenschirm", meaning "umbrella", an item famously carried by Chamberlain, gave away the second possible target.
However, the clues for the third target eluded our intelligence staff, and although the word "Korn" kept cropping up in various decrypted signals, its meaning was not obvious, and unfortunately, it was not until the afternoon of the 15th that scientists finally realised that the Germans had, in this case, not used any clever cryptic clues, but simple alliteration.... Korn was Coventry - Target 53. By then, of course, it was too late!