by R. V. Jones
By this time Norman and his colleagues in Hut 3 at Bletchley were so fascinated by the use that I was making of their information that they asked me if I would give them a lecture explaining what I had found. Bletchley had been organized into a series of Huts; and Hut 3, which had originally been Winterbotham’s headquarters if we had to evacuate Broadway, became the centre for co-ordinating the signals of air interest that came out of Enigma. The head of Hut 3 was Malcolm Saunders, a naval officer with an excellent knowledge of German and a great sense of history, and he had on his staff a number of distinguished academics including, besides Norman, F. L. Lucas and Geoffrey Barraclough. ‘A. J. Alan’ chaired my lecture, which aroused such enthusiasm for divining intentions from code names that one cryptographer, learning that the Germans had in hand an operation called Freischütz, spent a whole night reading the score of Der Freischütz in the hope of finding a clue.
But if there was enthusiasm at Bletchley, there was reviving doubt in some quarters of the Air Staff. In particular, the Principal Deputy Director of Signals, O. G. W. G. Lywood, now dismissed Knickebein as a ‘nine days’ wonder’, since after all the fuss at the end of June it had so far not materialized as a major threat: this was a singularly ungrateful reaction to a timely warning but Lywood may have felt understandably resentful of my youth. In fact, there were no air attacks on London or other major cities in July, and the only night alarm in London had been caused by an aircraft that had been sent up to investigate the beams and had accidentally strayed across London. We knew, indeed, from Enigma that Hitler had specifically forbidden the bombing of London for the time being; and fortunately Lywood’s view did not prevail because I continued to receive pointers to the fact that the Luftwaffe intended to make use of the beams on a large scale.
On 27th July Felkin circulated a document recovered from a German bomber which said that to use Knickebein at long range, the aircraft receivers were to be tuned up by special tuning squads, whose services would be made available to bomber formations on request. And on 27th July an Enigma signal said that one of these squads was requested by Kampf Geschwader No. 54 (a Bomber Group, with a nominal strength of 3 Gruppen each of 27 aircraft) in the week beginning 5th August. Since this Geschwader was primarily to operate in the west of England up to Liverpool and Manchester, which would be beyond the range of the Cleves Knickebein, the information implied that the foreshadowed Knickebein beam near Cherbourg was coming into operation early in August. I was able to give this warning on 4th August: major night bombing appeared imminent.
During the same period we decided to institute bombing surveys to record the pattern of German bombing. I was asked to nominate two towns for particularly careful observation, and I suggested Liverpool and Birmingham, my argument being that Liverpool with the outline of its docks visible even on fairly dark nights would provide a useful contrast with a target well inland such as Birmingham, where we could expect the beams to be especially important. P. M. S. Blackett, who was then with Anti-Aircraft Command, was at the meeting and afterwards he told me that he could not see how the Germans could have made beams as sharp as I was claiming, on an argument that I had had to meet earlier; this was because he had not realized what could be done by over-lapping two broad beams in the way that I have already described.
(For the benefit of students of physics, the problem is how precisely one can define a direction by a beam generated by an aperture which is a given number of wavelengths wide. At first sight this may appear to be the resolving power as calculated by Lord Rayleigh—and indeed Rayleigh thought it was. But the Rayleigh criterion applies properly to the closeness to which two sources can approach before they appear to merge into one when viewed through the aperture, i.e. of establishing that two separate diffraction patterns are involved. By contrast, the precision with which the direction of a single source can be defined is the precision with which the central direction of a single diffraction pattern can be established when no other pattern is present, and this precision may be a million or more times greater than the resolving power.)
The day battle in the air had been going on since about 10th July, the target being almost entirely coastal shipping. On 12th August the German attack switched to our radar stations and aerodromes, and it seemed only a matter of time before the night attacks developed, too. Vera and I went up to Hoar Cross for a short holiday in the second half of August, and on the 23rd we watched a night attack on Birmingham some twenty miles away, reminding me of the scenes of searchlights, bomb flashes and fires that I had witnessed in the first war. As I watched, I wondered whether Knickebein was now coming into large-scale use.
I had the impression that Churchill was as sceptical as I was about the imminence of invasion in July 1940, despite military appreciations that it was likely to happen soon. For some weeks there was no suggestion of it in Enigma, and then we began to get strong indications of an intended operation to be mounted by the Germans under the code name ‘See-loewe’ (‘Sealion’). This information was passed up to Churchill by Winterbotham, but it seems that the former was still so doubtful that Winterbotham and Stewart Menzies had to have a special meeting with him to convince him of its authenticity. Winterbotham asked me if I could prepare a note establishing the reliability of our information; this I duly wrote, pointing out that the source was exactly the same as that which had given us the crucial message about the Cleves Knickebein which had been so dramatically verified.
Churchill was now convinced, and he said that he would like to see all future information but that it might compromise our source if we continued to use the German code name. So he told us that in future all teleprints on the subject should be headed ‘Operation Smith’. His instructions were carried out, with the surprising result that the War Office appeared to lose all interest in information coming from Bletchley regarding the invasion.
After some time, the reason was found. It turned out that the War Office had its own Operation Smith, which was indeed concerned with the invasion. It was the code name for the movement from one of its minor administrative branches from its current headquarters in somewhere like Stroud or Tetbury to some place further north if the Germans should have invaded and posed a threat to south Gloucestershire. The result was that when the Bletchley teleprints were received in the War Office, duly headed according to the Prime Minister’s instruction, they were immediately sent to the Colonel in Gloucestershire, who no doubt impressed by the service that the War Office was providing but realizing that the material was too secret for general circulation, locked them in his safe and told nobody.
We returned to London before the end of the month, where I chased a miscellany of details besides, of course, my main targets of Knickebein, Wotan, and Freya. I found some satisfaction in learning that the Germans, too, could form their own wild theories when under stress. The pilots of one bomber formation, Kampf Gruppe 100, had been asked to investigate a theory that whenever one of our Observer Corps posts heard a German bomber overhead at night it switched on a red light, so that our patrolling fighters could thus get a rough clue to the whereabouts of the bomber. Actually, we had no such procedure, but after three weeks the pilots of this crack formation reported that from their own observations the theory was undoubtedly correct. Probably we had so many red lights showing accidentally that at least one was always within visual range of an aircraft flying anywhere over England, but this was a pleasant lesson in how a false theory could be built up.
As an example of the many minor distractions at this time, there was an Enigma message which seemed to say that a particular region in France would be suitable for the use of ‘FLAK GAS’. Since there had been earlier stories of anti-aircraft shells filled with a gas to burst in front of an aircraft which would paralyse its engines as it flew through the cloud of gas, there was an immediate alarm, improbable though the story seemed. The scare arose from nothing more than a missing letter in the message, for there should have been a ‘T’ at the end, the letters GAST standing for �
��Geräte Ausbau Stelle’, which merely meant an antiaircraft equipment depot. I was reminded of this alarm later in the war, when the Germans in turn had a scare when they found that the American Air Force in North Africa was stocking up with enormous quantities of GAS, and so they began to brace themselves for the onset of chemical warfare, which in turn alarmed us. Fortunately a German Intelligence officer realized in time that the Americans were merely referring to petrol.
CHAPTER SIXTEEN
Knickebein Jammed—And Photographed
TOWARDS THE end of August, the Germans began to supplement their day-time efforts with fairly heavy night attacks and on 24th August a few stray bombs fell on central London. This prompted Churchill to order a retaliation on Berlin on the following night. Hitler’s expected response was not immediate, but instead there were successive heavy attacks on Liverpool on the last four nights of August. My recommendation of both Birmingham and Liverpool for our bombing surveys was thus already justified, and it was now time to bring countermeasures to Knickebein into play; the first of these was fortunately ready just in time.
As a result of our finding Knickebein in June, a special R.A.F. organization was set up to deal with the beams. This was No. 80 Wing under Wing Commander E. B. Addison, who had his headquarters at Radlett, just north of London; Addison, a signals specialist who had recently returned from the Middle East, had been present and had supported me at the meeting when Lywood proposed cancelling the vital Knickebein flight. The technical design of countermeasures had been entrusted to a Section under Dr. Robert Cockburn in the Telecommunications Research Establishment near Swanage. Both the organization and the technical development of countermeasures were accorded the highest priority: they made great demands on all concerned. The first jammers were simply diathermy sets which could be made to transmit a ‘mush’ of noise on the Knickebein frequencies, but these were quickly superseded by higher powered equipment called ‘Aspirins’ (to deal with the Knickebein beams which were code-named ‘Headaches’). These transmitted a dash sounding very like the genuine dash transmitted in the Knickebein beam so that if a German bomber was flying correctly along the equi-signal he would in fact hear this signal plus a dash superimposed on it, which would make him think that he was too much into the dash zone of the overlapping beams, and thus steer further into the dot zone to try to make the dots as strong as the boosted dashes. If our dashes had been synchronized with the genuine ones, the effect would have been to ‘bend’ the beam, which would seem to have been displaced to the dot side. In practice, there was not enough time to develop a synchronized system, so that all that we could hope for on most occasions was to confuse the German pilots and thus deprive them of the inherent accuracy of the beams; but as the battle went on the legend grew up on both sides that we were genuinely bending the beams.
Apart from its accidental night bombing on 24th August London was still immune from attack until the afternoon of Saturday 7th September. I was in my office when the sirens sounded, and soon both bombs and machine guns were audible. Since they did not seem to be very near I joined Winterbotham and others on the roof. Against the clear blue sky we could see, away to the east, bombs bursting and smoke billowing from fires in the London docks. High in the sky were formations of German aircraft with our fighters attacking individually. Still higher a dozen or more aircraft, presumably German fighters, were flying in a circular daisy-chain, each aircraft protecting the tail of the one in front of it. There was an occasional parachute.
I went back to Richmond wondering whether this was the beginning of the end. The fires in the docks were enormous: they could never be put out before nightfall. Even if we jammed the beams completely, the night bombers would have perfect markers, for the flames in the docks could be seen from the coast. All the Luftwaffe would then have to do was to keep the fires stoked up with successive raids, while its main force aimed a few miles to the west and so pulverized central London. As we watched from Richmond, it was clear that the fires were still raging, and the night attacks on London began. But they were put out within a few days, despite all the odds, by the gallantry of the Regular and Auxiliary Fire Services, who continued to work throughout the raids. There must be few tasks so frightening as trying to put out a fire knowing that you are at the target for bombers overhead, and have no power to hit back.
One offsetting advantage that might have been expected from the dock fires was that the sky was so well lit up we could hope the German bombers would be visible to our nightfighters; but despite this, and despite their previous successes during the short June nights, our fighters inflicted almost no losses on the night bombers. Clearly we badly needed a good airborne radar, and this was not yet available. Anti-aircraft guns, too, were relatively ineffective, and so our blunting of Knickebein was almost our only hope.
In parenthesis here, I may mention that at least one A-A gun crew started their war well, for about this time I read one of the most elated accounts that I was to see during the war. It came from a Territorial gun crew near Farningham in Kent, whose first prospect of action occurred when three Dornier 17 bombers flew over in formation during daylight on 8th September to renew the attack on the London docks: according to the gunners they had one shot at the formation as a ‘sighter’ and then carefully aimed their second shot at the leading aircraft. All three aircraft promptly vanished, the gunners claiming that they had hit the bombs in the leader which had exploded and blown up the other two. Certainly survivors of two aircraft were picked up from this remarkable shot (A. 1. 1(k) Report 485/1940).
The climax of the daylight attacks came on Sunday 15th September with Fighter Command stretched to the limit; but so was the Luftwaffe, and it decided henceforward to concentrate on attack by night. For this it had a unique weapon in Knickebein—or would have had if we had not been able to jam it. From 7th September until 13th November London was bombed every night except one, the average number of bombers being 160. Even without any beams, it was, of course, not difficult to hit such a large target as London, with the Thames as marker, but a substantial proportion of bombs went astray, all the more so when raids were made on cities of smaller size. At least some of the credit for this must be given to our counter-measures, because in principle any German bomber flying on Knickebein ought to have been able to hit a target of about one mile square.
The knowledge that Knickebein was jammed spread through the Luftwaffe and there was a story current at the time that although the pilots were well aware of it, no one wanted to take the responsibility of telling Goering, with the result that Knickebein was persisted in for the next two months although it was substantially useless. Dr. Plendl, who was responsible for much of German beam development, told me after the war that ultimately special listening sorties were sent out to check the crews’ reports, and that the German scientists came to the conclusion that the beams were not merely jammed but bent.
But although Knickebein was now effectively countered by the ‘Aspirins’, we had still not seen what a Knickebein beam station looked like. Aerial photography had been neglected between the wars by the R.A.F., and it was left to individual and unorthodox initiative to overcome the deficiency; and since photographic reconnaissance was to play a vital part in my later work, it is worth recounting how this work benefited from my association with Fred Winterbotham.1 About the time of Munich he had approached a buccaneering Queenslander, Sidney Cotton, who had been a pilot in the Royal Naval Air Service. Cotton was already well known for the development of the ‘Sidcot’ flying suit and was currently involved in marketing the Dufay process of colour photography. Winterbotham obtained the money for Cotton to purchase an aircraft and to make unofficial flights over Germany to photograph various items of interest.
In the few days before the actual outbreak of war, Cotton succeeded in photographing Wilhelmshaven, and it soon became clear that he was better at operating both cameras and aircraft than the regular Air Force. The latter’s cameras suffered, for example, from condensation on th
e lenses and other components in the low temperatures prevailing at altitude: Cotton’s mechanical and physical insight enabled him to remedy these faults by ducting warm air to flow around the cameras, and thus to operate R.A.F. cameras when the R.A.F. itself could not. In addition, Cotton recognized the importance of speed, and he succeeded in getting an extra 20 or 30 m.p.h. out of aircraft such as the Blenheim by removing excrescences, and giving the surface a smooth gloss finish.
Before long the Air Force was brought to admit the effectiveness of Cotton’s treatment, and he persuaded Dowding to let him have two Spitfires from Fighter Command. By ‘Cottonizing’ he improved their speed from 360 to 396 m.p.h., and his next problem was to get adequate cameras installed. The original R.A.F. cameras had been F24’s with lenses of 8-inch focal length; but these gave much too little detail to see objects of less than about 30-feet diameter on photographs taken from the operational height of thirty thousand feet.
He therefore wanted to install larger cameras, and he told me how he had succeeded in getting Farnborough to install two 20-inch focal length cameras into a Spitfire. The authorities at Farnborough had said that this could not be done because the cameras had to be installed behind the pilot, and that the weight of two cameras, about 120 lbs., would pull back the centre of gravity of the aircraft making it ‘tail heavy’ and dangerous to fly. Since he was dependent on Farnborough for the actual installation, Cotton could do nothing but profess to accept their ruling. He therefore sent one of the Spitfires to Farnborough for fitting, and he was duly told that the aircraft was ready.
He went to Farnborough himself, and met the men who had done it. He anxiously asked them whether they had tested it in flight and whether the centre of gravity was all right. They told him that it was, but that of course it would not have been had they acceded to his original request to fit two cameras instead of one. ‘You are absolutely satisfied that the aircraft is okay?’ he asked, and when they assured him that it was he then said, ‘Well, now take off that tail panel!’ When they asked him why, he merely insisted that they should do as he said. The removal of the panel revealed a 20 lb. lead weight attached to the tail which he had placed there before he delivered the aircraft to Farnborough. He told them that since they had now agreed that the aircraft was safe with one camera and 20 lbs. of lead in the tail, this would have pulled the centre of gravity as far back as the 60 lbs. weight of a second camera situated alongside the first, and he therefore demanded that the second camera be installed.