Most Secret War

by R. V. Jones

  He assured me that he wished to interpose no hindrance between me and the pilots and interpreters; in fact he immediately took me to lecture to the pilots of the Photographic Reconnaissance Unit which had just moved to Benson near Oxford.

  We soon had a chance to link photographic reconnaissance with the interception of wireless signals associated with the control of German day fighters in France. This came about because across the road from my office was the large block of Edwardian, if not Victorian, flats known as Queen Anne’s Mansions. An Army Signals Interception Unit belonging to Home Forces was stationed there, under the command of Lieutenant Colonel B. E. Wallace. Somehow we came to know him, and to learn that his Unit was intercepting radio traffic of German origin. Because the codes involved appeared to be simple, we thought that we would try to break them ourselves. This turned out to be a foolish thing to do, since we were not members of the code-breakers’ trades union, and it was a long time before they forgave us. But first Derek Garrard and then Charles Frank succeeded in deciphering the traffic, which turned out to be plots by the same coastal radar stations that we were photographing, and the information in the messages could therefore provide us with valuable clues about the performance of German radar.

  Once we came to identify the traffic we could see that the vital information was contained in the form of ranges in kilometres and bearings in degrees from the particular radar station involved, and it was often so elementary that only the bearing alone was encoded, and this in such a way that the figures were substituted by letters which did not change for several days on end. It was therefore a matter almost of common sense to find the appropriate de-coding figure-for-letter substitution after the reception of a few messages. We were never clear why the Germans used such an elementary system. Possibly it was because there was little available time—radar plots had to be used almost immediately if fighters were to intercept—and so there may not have been enough time for a more elaborate coding procedure.

  It was perhaps because the messages were so easy to decipher that our code-breakers had tended to think that they could not contain information of much importance, and had therefore put them aside. Alternatively, some code-breakers had misgivings, perhaps subconsciously, at making much use of their decodes: this would be tempting a bountiful Providence which had given the code-breaker the luck to make his break and which could at any time withdraw the luck by leading the enemy to change the code.

  The German plots presented us with much opportunity. We could, for example, fly our own aircraft in sorties towards any particular radar station that we had identified, coming in at various heights and seeing when we were first detected. We could thus chart the extent of the German radar cover, and also investigate the accuracy of their plotting. In general, this was high, because they had built their radar equipment with their usual eye to precision of performance. Moreover, we could ascertain the German reactions to any one of our raids and we could build up a minute-by-minute picture of what the German fighter controller was seeing. We took this to the stage of organizing a plotting table at Fighter Command Headquarters alongside the famous table on which our own radar plots were displayed, and so the controller for our fighter sweeps over north France could see not only what our own radar stations were recording there but also the information available to his German counterpart. We set this up at Fighter Command, much to the approval of the Commander-in-Chief, Sir Sholto Douglas, and arranged for plots to be fed in from Colonel Wallace’s Unit and various other R.A.F. and Army Interception Units.

  Most of the plotting was done by two girls who had now joined us: Mary Francis, a mathematician from the radar establishment at Swanage and ‘Ginger’ Parry, who before she had married was ‘Ginger’ Girdle-stone. She had started from school as cashier in a butcher’s shop in Felixstowe, and had been taken on with two other girls at Bawdsey by Watson-Watt to see whether girls would make good radar-plotters. Not only did these girls succeed but they went on to train as their successors the famous band of W.A.A.F.s who staffed the plotting tables of the Battle of Britain.

  A further benefit from the German plots was that they enabled us to locate stations that we had not previously found. We at first achieved this by chance when a photographic reconnaissance aircraft had flown a sortie over the cruiser Hipper in Brest, making an easily recognizable track which we could plot on to a map from the photographs. It happened that this sortie was also plotted by a German radar station north-west of Brest; we did not know exactly where the station was but we could plot the ranges and bearings from this unknown station on a piece of transparent paper on the same scale as we had used for the photographic track, and we found that these gave a track of similar shape to that which we had plotted from the aerial photographs. We then slid the transparent paper over the photographic map until there was the best fit between the photographic and radar tracks (Figure 6), and the plotting origin on the transparent paper had to lie, within a small limit of error, at the point where the radar station itself was. All we had then to do was to request that the next aircraft flying a sortie over Brest should photograph the small region in which we now knew the station must be located, and it promptly showed up. As a result of this success, Peter Stewart arranged that all photographic aircraft would run their cameras for a few minutes on crossing the French coast, which was the region in which they were most likely to be plotted by German radar anyway, and we would then examine the radar plots at the time shown on the photographs, and correlate the radar and photographic plots, thus locating any station that was plotting the aircraft at the time. This proved to be a very fruitful method of locating the remaining stations whose whereabouts were not exactly known to us.

  Fig. 6. Example of locating an unknown German radar station through its broadcast plots on a British photographic aircraft. The radar track is plotted on transparent paper which is then slid over the track as mapped from the aircraft’s photographs until the best ‘fit’ is obtained, the unknown station must lie at the origin of the radar plots

  Besides de-coding and using the German radar plots of our aircraft, we could of course examine the characteristics of the radar transmissions themselves. This told us such details as the width of the radar beam, and the way the individual stations scanned their field of view. And evidently the Germans found that the Freya equipment was capable of detecting aircraft at greater ranges than those allowed by the original pulse repetition rate of 1000 per second, for they subsequently lowered the pulse repetition rate to 500 per second, which would allow a detection range of 300 kilometres.

  We also noticed changes in German radar philosophy during 1941. It appeared to us that at the beginning of the war the Germans had not thought nearly so much about the use of radar as we had ourselves, in contrast to the navigational beams, where they had obviously thought a great deal further than we had. The difference could be explained by our having thought almost entirely defensively whilst they were giving priority to offensive action. As a result in Britain, the serving officers and the scientists and engineers had been thrown much more together by the bombing threat, and had thus come to appreciate one another’s problems much more than did their German counterparts. What would happen, for example, was that our serving officers would tell us the kind of equipment that they would like to have, and we might well have to tell them that they could not have what they wanted but that if they could relax their requirement in one direction or another we might be able to give them something that would be better than nothing. Similarly we could sometimes ask them whether, when we had thought of some new device, they would have a use for it.

  As a result of this interchange at the working level, Britain had a radar system in 1939 which, although imperfect, was adapted to operational needs and moreover was handled by serving officers who from its beginning had been able to think out how it could best be used. And the essential point of our radar philosophy was that it enabled us to overcome the fundamental problem of intercepting the enemy not by fly
ing continuous patrols, which would have been prohibitively expensive, but by sending up our fighters so as to be at the right place and the right time for interception. In other words, we regarded the main contribution of radar as a means of economizing in fighters, one of our most precious commodities.

  The Germans, by contrast, did not have the same close relationship between their serving officers and their scientists. “When radar became a technical possibility, and this was realized at least as early in Germany as in Britain, the German Services drew up specifications which the scientists and engineers then tried to satisfy. And very well they did so, within the limits imposed by the specifications. German radar was much better engineered than ours, it was much more like a scientific instrument in stability and precision of performance. The philosophy of using it, however, seemed to have been left to the German Services, and the Luftwaffe in particular made a philosophical mistake by focusing on the wrong objective.

  German philosophy ran roughly along the lines that here was an equipment which was marvellous in the sense that it would enable a single station to cover a circle of radius 150 kilometres and detect every aircraft within that range. Thus it could replace a large number of Observer Corps posts on the ground, and so was a magnificent way of economizing in Observer Corps. Moreover, where we had realized that in order to make maximum use of the radar information the stations had to be backed by a communications network which could handle the information with the necessary speed, the Germans seemed simply to have grafted their radar stations on to their existing Observer Corps network which had neither the speed nor the handling capacity that the radar information merited. It was only after nearly two years of war that I saw the real value of radar recognized by a senior German officer. This was in North Africa, where Flieger Führer Afrika (probably Lieutenent General Hoffman von Waldau) said he needed more radar so as to economize in fighters. From that point onwards we could expect the German use of radar to improve.

  As for the scale of German radar deployment in 1941, we could make a guess from the few serial numbers that were mentioned in the Enigma messages referring to Freyas: these numbers were 22, 59, 82 and 132. It was an interesting problem for a gambler faced with these numbers and assuming that they started from unity: how many Freyas were there altogether? My guess was that there was likely to be about 150; and by the end of 1941 we had actually located more than 50. Two of them were brought to our notice in a spectacular way which deserves to be recorded as an outstanding example of courage and individual initiative.

  I was telephoned one morning in the summer of 1941 by an officer on Denys Felkin’s staff of prisoner interrogators, who asked me to accompany him to the Royal Patriotic Schools in Wandsworth because M.I.5 had detained there a man who had just arrrived in this country and said he was a Dane. His story was so improbable that M.I.5 were very suspicious and they wished to have it checked by experts in every detail. So Charles Frank and I accompanied the officer, Flight Lieutenant Gregory, to the Schools, where we found the man under close guard. He said that his name was Thomas Sneum and that he owned an estate on Fanø, the island just offshore from Esbjerg. He had been a lieutenant in the Royal Danish Naval Air Service, but had returned to his estate when the Germans occupied Denmark. He and a friend had found that another of his friends had a Hornet Moth lying dismantled in an old barn on the island of Odense in central Denmark and so they had formed the idea of making their escape to England. They had therefore reassembled the aircraft, sometimes having to use odd bits of wire instead of proper fasteners, and somehow they had gathered enough fuel to take them to England, although this was well beyond the range of a Hornet Moth. Their plan was to carry the extra petrol in cans and to refuel the tanks by a hose from the cockpit while in flight.

  The whole escapade was full of danger because the take-off had to be made within earshot of German guards, and the aircraft engine had not been run for some time, at least not since Denmark had been occupied more than a year before. They intended to swing the propeller, fling open the barn doors as soon as the engine started, taxi straight out and take off. They timed their exit to occur as a train was passing nearby, so as to mask the noise of their engine. There were electricity cables across their path and they did not know whether they would have to go over or under them. In the event, with the extra fuel load they had to go under, and they then set course for England. Over the North Sea Sneum had to climb out onto the wing, and insert the end of the hose into the fuel tank, while his companion poured petrol from the reserve cans into the hose at the cockpit end. They were duly intercepted, much to the astonishment of the Royal Air Force, since no one could believe that a Hornet Moth could have flown so far. The story was a tall one, and it was not unreasonable to suspect that the whole thing had been rigged by the Germans so as to infiltrate two agents who would gain our confidence.

  Why I had been drawn into the episode was that Sneum had brought some undeveloped cine film with him which he said that he had taken of the radar station on Fanø, showing the aerials turning. Unfortunately M.I.5 had taken the film and had it processed by, I believe, the Post Office, and between them they had ruined nearly all of it; Sneum was justifiably indignant. There were just one or two frames left from which I could see that he very definitely had filmed two Freyas in operation. Figure (7) shows the sketches that I made by projecting the film on to a sheet of paper and tracing the outlines of the Freyas; these are the sole relics of a most gallant exploit. Gregory, Charles and I were all convinced that Sneum was genuine, and we could entirely sympathize with his indignation. Not only had he and his friend risked their lives several times over, but also they had brought with them very valuable information only to have it ruined by the hamhandedness of our Security Authorities; moreover they were treated as spies because their story was so improbable. At the same time, there was an almost inevitable irony about such episodes, because the more gallant and therefore improbable that they were, the harder it was to believe that they had really happened.

  Fig. 7. The only relic of Thomas Sneum’s film of the German radar station on the Island of Fanø: sketches traced from a ‘still’ of the film

  So we extracted Sneum from M.I.5 custody, and did our best to make up for his wretched treatment. He volunteered to go back to Scandinavia. There was some difficulty in Stockholm because, I believe, he wanted to go into Denmark but S.O.E., which by agreement with M.I.6 was in sole charge of direct activity in Denmark, thought that it would be too dangerous for him, with the result that he fell out with them. He was even incarcerated in Brixton Prison, but was ultimately released. He joined the Royal Air Force, finishing the war in a Mosquito Squadron. I tried at the end of the war to make some slight amends for the way in which he had been treated, when I persuaded the Royal Air Force to let him lead his squadron into the airport at Copenhagen as the first of the Allied Forces to take it back from the Germans.

  It might be thought that after all this Sneum would have become the national hero that he deserved to be. But he found himself cold-shouldered by those in control of Denmark at the end of the war, perhaps for this very reason. Some of them had been equivocal so long as Germany was in the ascendant, and their patriotic record would bear no comparison with that of Sneum, who had committed himself to resistance as soon as the Germans invaded Denmark. It would have endangered their positions if Sneum came back, and they were able to make play of his imprisonment in Brixton; he ultimately left Denmark to live in Switzerland. If they survive, the men who go first are rarely popular with those who wait for the wind to blow.

  CHAPTER TWENTY-FOUR

  Beams On The Wane

  ONE REASON why we had been able to pursue Freya to the extent described in the previous chapter was that our countermeasures had gained us a respite from the beams. Even so, we dared not relax our watch for new stratagems and devices. It looked as though the Germans hoped to bludgeon their way through our jamming by building so many beam stations that we could not jam them all. They erected
seven more Knickebeins, making a total of eleven: the position of each is indicated by a ‘K’ on the map in Figure 8, which also shows the positions of the X- and Y-beam transmitters. (The Cleves and Husum Knickebeins were much larger than those erected subsequently, their aerials being about 300 feet wide by 100 feet high; there was a third larger Knickebein at Lörrach near the Swiss border. These three huge constructions were clearly set up deliberately before the outbreak of World War II; they were so sited on the western border of Germany as to give the best possible crosses on targets in England and France.) To counter any one beam effectively we needed to deploy three jammers, because these were fixed whereas the beam could be swung from one target to another over the whole of southern Britain. Addison at 80 Wing therefore had a heavy task; although the Luftwaffe was never to return in force, his preparations had to be made.

  Fig. 8. Map showing the increased deployment in Summer 1941 of German beam stations (K, Knickebein; X, X-Beam; Y, Y-Beam), which the Luftwaffe hoped to use on its return from Russia

  At first we expected to see the Luftwaffe back in the autumn, but our hopes began to rise. There was always the promise offered by the Enigma decodes, although for one reason or another this promise was not always fulfilled. As Wavell’s troops pushed right forward in Africa, Enigma messages showed that German rations and fuel supplies in some quantity were being sent to Tripoli; yet Wavell himself told me after the war that the presence of the Germans was a complete surprise to him when his forces encountered them, with the result that Rommel was able to roll us back easily. Wavell said that he would have changed his plans had the information been available to him.