by R. V. Jones
The final paragraph discussed the timing of countermeasures.
10.3.7 It is unwise to be squeamish about taking countermeasures against any enemy development because of the danger of reciprocation. The enemy is not altogether lacking in ingenuity, and has probably thought of most of the counters. The true reason against undertaking countermeasures is that ultimately the enemy will learn to overcome them, and that it is only during the period of his education that we shall reap the advantage. This period must be made to occur only when we want it, no matter what action the enemy may take against our own system in the meantime. When the correct time arrives there should be no hesitation. Spoofing may require a very considerable degree of effort to be successful, but its cost must be weighed only against the relevant D.T. effort and against the additional damage which we could do with its aid.
The background to the foregoing paragraph had been a remarkable ‘squeamishness’ on our part regarding technical countermeasures against German radar, as far as the Air side was concerned. This contrasted with the readiness with which tactical countermeasures such as concentration and evasive action had been adopted. It also contrasted with what the Royal Navy had been doing in jamming the German coast-watching radar ever since we had discovered it in October 1940. I was amazed to sit through meetings of the Radio Counter Measures Board right up to the end of 1942 discussing whether it was advisable for us to start an ‘R.C.M. War’ and so invite German retaliation by the jamming of our own radar, when our Navy was already jamming German radar, and when the Germans had already jammed our radar very successfully in the Scharnhorst-Gneisnau episode. Although I repeatedly pointed to these facts, I encountered a nebulous but strong reluctance against taking technical measures against Luftwaffe radar.
The reluctance went right back to the early days of Bawdsey, and at the heart of it was Watson-Watt. No trials of spurious reflectors had been made after I had suggested their use in 1937; and A. P. Rowe, who succeeded Watson-Watt as Superintendent at Bawdsey, told me after the war that he did not hear of the suggestion until 1941. Writing to me in 1962 Rowe said ‘When I took over from W. W. at Bawdsey, I found it was “not done” to suggest that the whole affair would not work’.1 In defence of Watson-Watt it could be argued that anything that might have thrown doubts on the value of radar, such as the early development of a successful jamming system, might have caused the Air Staff to lack confidence, and thus not to support radar enthusiastically. We might then not have had such an effective system ready in time for the Battle of Britain. But it could equally well be argued that, had we been up against a more thorough opponent who had developed a proper system of jamming, our 1940 radar would have been rendered powerless, and we should have been culpably unprepared to deal with such a situation.
Such arguments apart, the fact was that in 1941 and for almost the whole of 1942 we shrank from technical countermeasures, and the most generous interpretation of Watson-Watt’s attitude was that he had developed a ‘bridge on the River Kwai’ attitude towards radar, and it hurt him emotionally to think of radar being neutralized, even German radar. Perhaps, remembering our old enmity, I was in turn committed to the idea of wrecking radar with clouds of spurious reflectors; but of course I would never have wanted to see our own radar wrecked. With some justice events were to lead to an ultimate confrontation between Watson-Watt and me on this very issue, and I will now trace their contorted evolution.
Among the various technical countermeasures to German radar, one possibility was to fit a warning device to every bomber, telling the crew whenever a Würzburg or a Lichtenstein transmission was directed at them, so that they could take evasive action. This came into use as ‘Boozer’ in the spring of 1943. A much more elaborate device was that of sending back amplified and extended echoes, to suggest a large formation of aircraft; this was used on a small scale, following my visit to No. II Group after the Bruneval raid (p. 243) from August 1942 onwards. On the first occasion when it was tried (6th August) eight Defiant aircraft off Portland produced such a response that thirty German fighters were sent up around Cherbourg to meet the imaginary threat. And on 17th August our ‘Moonshine’ Defiants with supporting aircraft managed to divert nearly 150 German fighters from an American Flying Fortress attack on Rouen. But the scheme was elaborate, and it would only work if a very limited number of Freyas had to be dealt with, so it was dropped after a few months, to be later revived for D-Day. We were able to use ‘Moonshine’ earlier than the simpler countermeasures, perhaps because it was too elaborate for the Germans to use on a large scale against us, and because our Moonshine aircraft could stay so far outside the German defences that there was little chance of their being captured. But the main reason probably was that the whole episode had occurred at the lower level initiated by my visit to No. 11 Group, and that formal approval was never sought at the highest level.
With the two major possibilities, jamming and spurious reflectors, the situation was different for their use would be on such a scale that formal approval would have to be obtained. Jamming appealed to me less, because it was cruder and moreover would jeopardize any aircraft carrying a jammer because it could be homed on by fighters carrying suitable receivers. Spurious reflectors would be simpler, and contained an element of hoaxing. The phenomenon on which they depended was that of resonance. If a reflector is made of a simple wire or strip of metal of length equal to half the wavelength used by the radar station, it resonates to the incoming radio waves and re-radiates them to such effect that it is roughly equivalent to a whole sheet of metal whose dimensions are a square which has sides equal in length to half a wavelength. Thus a few hundred such strips or wires would reflect as much energy as a whole Lancaster bomber. Originally I suggested that wires should be suspended from balloons, because the long wavelengths that were usual in 1937 would require lengths of at least 10 feet; but we found that the predominant wavelengths in the German radar that we had to counter were about 50 centimetres, so each wire or strip need only be 25 centimetres long, and could be made light enough to fall through the air at a slow rate, and thus remain active for many minutes.
As our knowledge of German radar built up during 1941 I pressed the idea again, especially with Lindemann, and it finally resulted in trials being done late in 1941 and early 1942. They were undertaken under Robert Cockburn’s direction at Swanage by Mrs. Joan Curran, now Lady Curran. Her results were all that we expected, and she tried various forms of reflector ranging from wires to leaflets, each roughly the size of a page in a notebook, on which, as a refinement, propaganda could be printed. The form that we finally favoured was a strip about 25 centimetres long and between 1 and 2 centimetres wide. The material was produced and made up into packets each weighing about a pound, and the idea was that the leading aircraft in a bomber stream would throw them out at the rate of one every minute or so, to produce the radar equivalent of a smoke-screen through which succeeding aircraft could fly. So much progress was made, after the years of delay, that by April 1942 enough material had been produced for it to be used by Bomber Command. It was given the code name ‘Window’ by A. P. Rowe, the Superintendent of T.R.E., and the scheme went up to the Chiefs of Staff on 27th April 1942 for their sanction.
The Official History The Strategic Air Offensive Against Germany 1939–45 suggests (Volume 1, pp. 400–401) that Window was thought of at this stage as primarily a weapon against radar-controlled A-A guns, but this was certainly not so in my mind. If the Chiefs of Staff took this view, it must have been because of the Bomber Command obsession with anti-aircraft guns rather than nightfighters. For the first three years of the war the Command seemed to be far more concerned about guns, and it needed much convincing that the fighters were the greater threat, my own conclusion being that 70% of our losses were due to fighters at a time when the Command would not agree to a higher figure than 50%. So wary were they of the guns that the Command Intelligence Officer told me they never routed a raid over the island of Overflakkee because of the possibl
e effect on the morale of the crews.
With the Chiefs of Staff’s approval, the Window packets were, I believe, actually loaded on to bombers for a raid in May 1942, and were then off-loaded again because there was a last-minute prohibition on their use. What had happened in the meantime was that my old Oxford colleague Derek Jackson had heard of them for the first time: he had given up research for Active Service at the beginning of the war and had become one of the most skilled nightfighter radar operators in Fighter Command, and he was now the Command’s Airborne Radar Officer. Naturally, he was concerned about the reduction in our own nightfighting efficiency if the Germans were to use Window against us. He was always able to put his arguments with remarkable force, and on this occasion he called on Lindemann and swung the latter round from supporting Window to opposing its use. Lindemann went to Sinclair, convinced that Window should be withheld, and succeeded in getting its introduction deferred until the effect on our own night-fighters had been evaluated. Lindemann now had Jackson bombarding him on one side and me on the other. For the time being Jackson prevailed; and he was, of course, strongly supported by Watson-Watt.
All through the summer of 1942 the battle went on. I was quite sure that the arguments against using Window were ill-founded, because there was no prospect of the Germans attacking us on anything like the scale that we were now mounting against them. What was left of the German bomber force was mainly tied up in Russia. The only good argument for withholding Window was the one that had been given in my main report on German radar, which was that we should pick our time when its introduction would have greatest effect; we could not expect this effect to last indefinitely, as the Germans gradually found ways of getting round the difficulties that it presented for them, and it could be debated whether or not the optimum time had yet come. But I was sure that we were delaying too long.
I thought that my chance had finally come when in October 1942 I received a report which came from a Danish agent who had heard two Luftwaffe women personnel talking in a railway train. The report ran:
A. A considerable number of women have lately been taken on for work in aircraft control stations. Formerly this work was done by men, but since this summer women are being used and their number is constantly increasing.
B. Operations of night fighters are controlled entirely from such stations, the pilot being almost a passenger. The direction of flight, altitude and even the moment to open fire are given (in latter case by a man) from the ground stations which are equipped with special detectors (or predictors) for this purpose. The principle on which such detectors (or predictors) work has something to do with the metal in an aircraft.
C. Informant has heard of an instance when a British machine in the Rhineland deceived a German control station by throwing out aluminium dust and then changing its altitude. As the dust cloud was nearer the control station than the plane, the detectors (or predictors) guided the German night fighters in this direction and they are said even to have opened fire on the dust cloud.
It was an extraordinary Intelligence situation, and unique in my experience, because the value of the report was independent of whether it was genuine or ‘a plant’. The whole of the opposition to our use of Window had been based on the argument that although the idea was obvious, it had not occurred to the Germans. And yet, now we had the report, either it was genuine, in which case Luftwaffe personnel were discussing the effects of Window or it was ‘a plant’, in which case the German Security Service had shown that they were well aware of the possibility of Window. In either case, it proved that someone in Germany knew of the Window principle, and therefore the argument based on the premiss that the Germans did not know of it was now substantially demolished.
I at once wrote a report about it, and circulated it to those concerned on 24th October 1942. I telephoned Bomber Command and told B. G. Dickins, the Head of the Bomber Command Operational Research Section, that if the Command would now press for using Window I had ample evidence to demolish the opposition. Portal called a meeting for 4th November, and I made a point of seeing Lindemann to try to talk him round before the meeting and to give him the chance of changing his ground. This he refused to do, dismissing my report as merely being something two W.A.A.F.’s had said to one another in a train. When I told him that I would have to oppose him at the meeting he said ‘If you do that, you will find Tizard and me united against you!’ I could not help replying ‘If I’ve achieved that, by God, I’ve achieved something!’ and we parted in laughter. Actually, he must have been quite misinformed about Tizard’s attitude, for the latter had already minuted that my report had completed the argument for the immediate introduction of Window.
The day of the meeting came, and I was looking forward to the fight. I knew that I could expect opposition from Lindemann and also from Sholto Douglas, the C.-in-C. of Fighter Command and, above all, from Watson-Watt. But with Bomber Command and the evidence, I thought that we must win. And had Tizard been there it would have been a very interesting battle. But for some reason he was absent, and Bomber Command, represented by Sir Robert Saundby, my train-playing host of two months before, collapsed on me. Saundby said that his C.-in-C. did not wish to press for Window at the present time since he considered that other countermeasures, the jamming of the Freyas with a device known as ‘Mandrel’ and the interference with the German radio telephonic controls, would be enough for the time being.
The battle was therefore left to me alone, and I heard Lindemann witheringly talk of the irresponsibility of basing major policy on what someone had happened to overhear two W.A.A.F.’s saying in a train. I therefore lost hands down, and in a last defiance I asked Portal whether he would now advise our Security Services to call off the national effort that was being expended in propaganda to prevent careless talk in our own country. For this was the one example in the whole of my experience where careless talk had really been important, and all that Lord Cherwell could do was to dismiss it as talk between two women in a train. Portal saw that I was indignant, and so he added a recommendation that we should now work out in detail just what the effect of Window both on the Germans and on our own defences might be. We should work out the amount of Window that we should need to drop on a typical raid, and estimate the effect that it would have in reducing our casualties. We should also work out how much Window would be needed by the Germans to produce something like the same effect on our defences. To get all sides of the case presented, he asked that Fighter Command, Bomber Command, and I should make independent estimates, after which we should have a further meeting.
In the meantime, jamming of the Freyas was started in December 1942, using the Mandrel jammers. Some of these were carried in Defiant aircraft in the form of an airborne ‘screen’ intended to blind the Freyas to our approach. This was at first partly successful, in that, with the Freyas blinded it was much more difficult for the Giant Würzburgs to be directed on to individual bombers; but the Germans overcame the problem in various ways, one of which was to change the wavelengths of the Freyas to others which the Mandrels could not jam. The other countermeasures brought into use at the same time was the jamming of the German ground to nightfighter communications by transmitters in our bombers, each using a microphone to broadcast the noise of its engines so as to superimpose these on the German conversations. I was rather worried when I heard that the code name given to this device was ‘Tinsel’ because it might suggest to any German Intelligence officer the metal foil strips that we were still withholding.
As for computing of the amount of Window required, we were to assume a raid of a particular magnitude on a typical target, and work out how much Window material would be needed to render the German radar system ineffective. Derek Jackson at Fighter Command was the first to come up with an answer: his figure was 84 tons. He also calculated that one ton of Window used by the Germans would wreck our own radar system. Charles Frank and I made our own calculation, pointing out that Jackson had calculated the worst case, and that if o
ne adopted assumptions that were reasonable in the light of our special knowledge of the German radar system, the figure was much more probably 12 tons instead of Jackson’s 84. Dickins at Bomber Command cautiously waited until Jackson and I had both made our bids, and then produced a calculation showing that 48 tons would be needed.
Our estimate had been completed by 4th January 1943, but Portal did not reconvene the Window meeting until 2nd April 1943. I knew from Lindemann’s secretary that he was still adamant, and that he was going into the meeting to repeat his opposition to the use of Window. However, he was not called on to speak until all three calculations had been presented. On behalf of Fighter Command, now headed by Trafford Leigh Mallory, Derek Jackson explained how he came to a figure of 84 tons, whilst one ton would suffice for the Germans if they wanted to knock our radar out. Portal then called on me, and I showed where I thought the Fighter Command estimates were too stringent. He then called on Harris, who deflected the question to Dickins, who gave a longish account of how Bomber Command had arrived at its figure. When he had finished, Portal simply asked ‘What has Bomber Command done beyond adding the Fighter Command and A.D.I. (Science) figures together and dividing by two?’
At this point, Watson-Watt intervened, saying that he would like to give a warning based on his experience, and that he considered Jackson’s figure of 84 tons much too low, although he agreed that one ton would be sufficient to knock our own system out. His argument was that no radio or radar device in his experience had been more than 20-30% efficient in its initial stages of use and therefore Jackson’s figure of 84 tons should be multiplied by at least three if we were to be sure that the German radar would be knocked out, because of the inevitable mistakes that we would make in using Window properly.
