Seventy Years Ago This Month at Bletchley Park
Bombing of Britain Intensifies.
The Luftwaffe has suffered 2,433 German aircraft shot down over Britain in three months, with 6,000 airmen killed. On 3rd November there is no air-raid on London, the first night off since 7th September. Yet this turns out to be but a short respite before a renewal of the assault with new tactics and in greater strength. On 14th November German bombers attack Coventry in what is the most devastating raid of the war so far. Using new tactics that must lead to widespread area bombing, a firestorm is generated that destroys over a square mile of the city centre. On 19th November the target is Birmingham, and then on 28th November 200 planes attack Liverpool. The RAF had bombed the Krupp armament factories at Essen on 7th November and then attacked Munich on the 8th. This raid was aimed ostensibly at the railway marshalling yards, but it did happen to be the anniversary of Hitler’s attempt to seize power there in 1923. During a raid on Hamburg two days after the attack on Coventry, the RAF aircraft release their bombs even though cloud cover masks the military targets. The pattern of indiscriminate area bombing warfare has been established.
The Immolation of Coventry.
In full moonlight on the night of 14th November 509 Luftwaffe bombers attack Coventry, 449 aircraft reach the target and only one of them is shot down. Pathfinder aircraft using firebombing lead them. These start a fire-storm which burns out much of the centre of the city, destroying many buildings and killing 568 people. Whatever our propaganda said at the time about “Baedeker raids”, 27 war factories were hit and production is halted for many months. Could better Intelligence have forestalled this disaster?
An Enigma message of 11th November, broken by BP that day, had given Air Intelligence the warning that an unusually heavy raid was coming; they knew it was going to be important and that its code-name was “Moonlight Sonata”. They learn from the decrypt that it would be led by aircraft using the new navigation aid “X-Gerät”, and deduced from the code-name that it would be at the full moon (15th) and that it would be in three waves or ‘movements’. There were four targets listed under code-names in the decrypt, one of them being the as-yet unrecognised code-word “Korn”. A careless German airman Prisoner of War had volunteered that the dates would be between 15th and 20th November, and he did refer to Coventry and Birmingham as the probable targets but Air Intelligence had a captured map which led them to believe that the four code-names were in London or the Home Counties. Then on 12th November a further decrypted Enigma signal, using the Brown key employed by the German pathfinder navigation beams team, gave planned beam bearings showing that three of the targets must be Wolverhampton, Birmingham, and Coventry. But such messages had been received before, during the testing of the X-Gerät system. On the 14th, the morning before the raid, Air Intelligence sent a minute to the Prime Minister about the coming raid, saying that they believed the target was going to be “… probably in the vicinity of London, but if further information indicates Coventry or Birmingham, or elsewhere, we hope to get instructions out in time”. His Scientific Advisor, Prof Lindemann, had already warned Churchill that the pathfinder technique using a precision navigation system and fire-bombing target marking would soon be used. By 1500 hrs on the 14th it was clear, from radio interceptions and the actual beam bearings, that the raid was coming that night and that Coventry was the target, though this was one of the rare days when Hut 6 was unable to break the Brown key. The alarm was rapidly passed on to the defence and emergency services in the area. This was in good time to get the new beam jammers set-up for the X-Gerät system but unfortunately they at first used the wrong modulation frequency; this error was not corrected until December. Churchill had left London during the afternoon for Ditchley Park where he sometimes spent the night “when the moon was high”, but turned round when he read the minute in his box about the expected large raid on London. It was only when he arrived back in the early evening that he learnt that Coventry was the target. Even had he learnt earlier it is not clear that any improvement could have been made in the defence, had he chosen to intervene? The ground anti-aircraft defence of Coventry had been strengthened some days before, not in response to any specific threat but as part of a programme to strengthen the Midland’s industrial centres. The planned RAF counter-measures to the “Moonlight Sonata” threat were mainly air-attacks on the bombers’ bases while they were out on the mission, and this would have been implemented in the same way whatever city proved to be the target.
As the weeks of bombing raids went on the ability of the Intelligence Services to predict the target, the time of the raids, the forces involved and the routes they would take, improved markedly. This was benefiting from ever-increasing use of the Enigma decrypts from BP, especially the Brown key giving the settings of the beams, and on low-grade intelligence that Cheadle obtained from the bombers and their controllers. (By the end of 1940 BP had broken, amongst other Luftwaffe codes & cyphers, some 75 different air to ground codes). While there is no doubt that Churchill was prepared to make hard decisions to protect the Enigma secret, there is no justification for the people of Coventry to feel that he allowed the destruction of their city. But perhaps a better target for blame would have been the Air Ministry because of the long time it took for Air Intelligence to learn to cooperate closely with, and take full advantage of, the ever more well-informed BP Hut 3 Intelligence staff.
Countering the Navigation Beams.
Unlike the RAF who, at this time, considered they could navigate without such aids, the Luftwaffe employed four different radio navigation systems. They first set up 38 medium frequency beacons and 11 broadcasting stations across continental Europe. Initially the British Y service had no difficulty in breaking the simple call-sign system and so warning the defences of the frequencies being used, which changed daily. The counter-measure, first used by the UK in August 1940, was to reradiate the original beacon signals from “meacons” so giving the enemy bombers false bearings. The countermeasures proved effective and so on 8th December 1940 the Germans started to use their Wireless Safety Service higher frequency beacons for operational navigation. Anticipating this, 80 Wing, the recently formed countermeasures wing of the RAF, had installed special meacons the day before! During 1941 the Germans introduced many steps to avoid the meacons, such as changing the frequencies and call-signs up to 15 times a day. But, with the help of BP in unravelling the changes, it remained possible for Cheadle to “meacon” all the enemy beacons, though with a delay of up to 25 minutes to reset the meacons after a German frequency change.
The first German intersecting beams navigation system, Knickebein, had been detected after an Enigma decrypt had used the word in June 1940. The young Air Ministry scientist, Dr R. V. Jones, convinced Churchill on 21st June, against the opposition of the Air Staff, to let him have an aircraft for experimental purposes, and that night an aircraft of the Wireless Intelligence and Development Unit found the two narrow beams in the 30 Mc/s (MHertz) band. Once the jammers were frequency stabilised, the “beam-bending” and “Bromide” jamming countermeasures proved effective, leading to an ever-growing distrust of the system by Luftwaffe pilots. Dr Jones and 80 Wing, based at Radlett, established a close relationship with the Air Intelligence staff in Hut 3 at BP, where Prof. Frederick “Bimbo” Norman, a great character and pre-war Professor of German at Kings College, London, built up a team who specialised in the enemy technical terms and phrases found in the decrypts, so providing much advance warning of the nature and details of the German secret weapons.
X-Gerät was an even more sophisticated system, employing narrow beams that gave a theoretical accuracy for bomb release calculated by Dr Jones to be 35 – 40 feet. BP had first read the Brown key Enigma signals used by the German Pathfinder squadrons in Sept. 1940 and these provided a wealth of information on the system. It operated in the 60 MHertz band that made for difficulties in setting up countermeasures, as transmitters were not readily available. The jammers were first used on the night of the Coventry raid on 14th November, but unfortunately they were set to an audio modulation of 1,500 c/s, when after some weeks it was realised that this should be 2,000 c/s. By mid-January 1941 the countermeasures were proving effective and though the Germans tried various modifications the system was abandoned after 1942. Dr Jones points out in his “Most Secret War” that the problem that BP faced each day was not just to decrypt the Brown key but also to do so in the two or three hours that were available. The signal giving the information on where and when the Pathfinders were going to bomb went out during the afternoon and that message had to be decrypted and made available to our fighters and jammers with little delay if they were to be ready in time: “For such a feat they strained every resource of human intelligence and endurance; and it was a great day, late in October, when they achieved this fantastic feat. Thereafter they were able to repeat it on about one night in three. I was then able … to tell Fighter Command the exact place of the attack, the time of the first bomb to within ten minutes or so, the expected ground speed of the bombers, their line of approach to within 100 yards, and their height to within two or three hundred metres. Could any air defence system ask for more?” However he continues “Despite this detailed information – and much to our disappointment – our night fighters repeatedly failed to locate the Pathfinder aircraft, and I almost began to wonder whether the only use the Duty Air Commodore made of my phone calls was to take a bet with the rest of the Command as to where the target would be”.
The fourth German system, Y-Gerät or Wotan, used a single narrow beam with distance measurement along it by a radio method involving ground and aircraft transmitters. (Dr R. V. Jones had been forewarned about this system in the famous “Oslo Report” which was given anonymously to the British Naval Attaché in Oslo. It contained such remarkable information on a whole series of German technical innovations that many, though not Dr Jones, believed it to be a plant). Enigma traffic gave some information on the system, and the capture of equipment from a crashed enemy aircraft in May 1941 finally revealed the rest, showing that it was highly susceptible to a particular type of jamming, though the Germans were already losing confidence in the system. Overall, the work of the RAF beam detecting teams, taken with the Enigma decrypts, rendered all the remarkably sophisticated German navigation systems open to effective countermeasures.
The Bombing of BP.
On the night of 21st November 1940 a single German aircraft releases a string of bombs that fall across the Park. It is probably returning from bombing Coventry, where its bombs had “hung-up”; having got them untangled perhaps it releases them on the first target it comes to, the BP campus or possibly the adjacent railway sidings. Elmers School, where the BP Diplomatic section is based, suffers a direct hit, which damages the typists’ room and the telephone exchange but there are no casualties. (Hugh Foss and Oliver Strachey, who in 1935 first broke the Japanese diplomatic cypher machine, are working at BP in Hut 8, but some work continues in the School on the Japanese codes. By now the team is struggling with the Japanese Navy General Operational Code, JN25; but US code-breakers have recently broken the new diplomatic machine known to them as Purple). Another bomb damages the nearby vicarage, and the third in the stick falls near Hut 4, alongside the Mansion. This one is said to have lifted that hut off its foundations, and a naval riggers team has to reinstall it the next day – but of course the work on naval Intelligence, going on there, never ceases! The next bomb falls in the stable yard but fails to go off, which is just as well since Dilly Knox and his “harem” of girls are at work there in Cottage 3, breaking the few Italian naval Enigma messages, and struggling to get to grips with the Abwehr Enigma. Two others fall across the site but also fail to go off, which might give today’s site property developers cause to pause! This was the only time that BP was bombed, and it would seem that the Germans never detected that it was anything other than a normal military base. Had they attended some of the high-quality musical entertainments in the Assembly Hall just outside the main gate they might have suspected that this was not quite your normal army camp!
The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed.