Fighter, p.12
Fighter, page 12
Berlin and London shared a determination to believe that the other side knew nothing of radar technology, even though there existed plenty of evidence otherwise. And Milch had shown a German radar set to a visiting French air force delegation in August 1938. In the autumn of 1937, during an official visit to England with Udet and other officers, he had boasted about German radar to an astonished audience of high-ranking RAF officers during a formal lunch held in Milch’s honour.
In the ante-room of the Officers’ Mess at Fighter Command HQ Milch suddenly addressed a loud question to the assembled officers. ‘Now, gentlemen, let us all be frank,’ he said. ‘How are you getting on with your experiments in the detection by radio of aircraft approaching your shores?’
There was some embarrassed laughter, and an attempt to change the subject, but Milch persisted. ‘Come, gentlemen, there is no need to be so cagey. We’ve known for some time that you were developing a system of radio-location. So are we, and we think we are a jump ahead of you.’*
Perhaps, in technical terms, Milch was correct but radar could never be a practical warning system for any frontier except a coastline. But the system itself was born out of a unique British ability to compromise.
It has been said that the motto of the team at Bawdsey, Suffolk, where practical radar was developed, was ‘Second best tomorrow.’ This meant that they could not afford to spend a year or two in search of perfection but must get radar working soon, even if its performance was below peak. Although an egoist, Watson-Watt never promised more than he knew he could deliver.
It was this restraint that caused Watson-Watt to name their experiments RDF (Radio Direction Finding). It was a deliberate attempt to deceive the curious, because although they believed they could crack the problem of range-finding, they thought the problem of direction-finding would be difficult or even impossible.
Watson-Watt’s original memorandum of 27 February 1935 is a remarkable document. It not only set out the alternative paths of research but guessed rather accurately what might be achieved. At the end it noted the importance of identifying aircraft, and suggested a radio method by which friendly aircraft could give a coded reinforcement of the reflected radio wave. He added a note about the need that all this would bring for really good radio-telephone communication with the fighter pilots, realising that his radar would be measured by how efficiently it placed fighter planes in a position to attack enemy bombers. (At this time the envisaged equipment was very large and few men dreamed that it would ever be made small enough to fit inside even the largest aircraft.)
But the most remarkable thing about radar is that no one had invented it long before Kühnold’s experiments. There were hundreds, perhaps thousands, of scientists and experts paid by their governments to advise on such scientific matters. The phenomenon of radio waves re-radiating from distant aircraft was repeatedly mentioned in professional journals, and the Post Office got endless complaints about the way aircraft spoiled radio reception. Yet these were simply treated as problems to be solved. None of the experts was able to link these ‘problems’ of interference with the threat of the bomber, which continued to get tremendous publicity.
FIGURE 13. Radar (RDF)
The Germans could see the Dover radar masts from France. There was no way of hiding or disguising the huge girder-work towers upon which radar (then called Radio Direction Finding) depended. In any case, the stations were all emitting radio signals.
At the base of the two sets of towers (receiver towers and transmitter towers) there was a ‘receiver hut’. Here the operators, often women, watched the cathode-ray tubes.
As the signal went from the transmitter, a blip (see A) came on the screen. That short pulse of energy either disappeared, never to be seen again, or hit something, bounced back, and was received by the second set of tall masts. It made another blip (see B) on the screen. By measuring the time between the blips, the operator could estimate how far away the aircraft reflecting it was. However, this was more difficult when there were formations of aircraft.
A screen registering 24 twin-engined aircraft.
The map shows coverage of the ordinary CH (Chain Home) stations for aircraft flying below 15,000 feet. Aircraft higher than this could be detected at a greater distance, although the radar coverage was very poor for aircraft above 20,000 feet.
To prevent aircraft coming in under the radar at sea level, CHL (Chain Home Low) sets were used (see dotted lines). These rather more sophisticated sets, with rotating aerials, transmitted at 1.5 metres (compared with 10 metres for the CH) and had been developed by the navy for detecting ships. Notice the way in which the coverage at this stage of the war gave priority to ports (marked with dot).
Note. The radar coverage only faced seawards. Aircraft that had crossed the British coastline could not be plotted (except by the Observer Corps) while flying over the land.
By measuring the time it took the pulse to bounce back from an aircraft, range could be accurately assessed. But direction-finding was much more difficult. However, the Filter Room staff could plot the ranges provided by two adjacent radar stations (R1 and R2) and intersect them to get the position of the enemy.
This technique was called range-cutting and was an important part of the work done in the Filter Room. It was because direction-finding was so difficult to do with this kind of radar equipment that the system was called Radio Direction Finding. This was intended to deceive unauthorised enquirers about the purpose of the tall towers.
Watson-Watt’s first guesses set British radar off to a good beginning. His decision to treat an aircraft as though its wings were a horizontal antenna started him off using a wavelength of 50 metres, calculating this to be twice the wingspan of the average bomber. Almost immediately Watson-Watt changed to half that wavelength to avoid commercial radio signals. His decision to treat the bomber as if it were a horizontal wire led him to horizontal polarisation and to stacked aerials. His experience with cathode-ray tubes (which had been improving rapidly at this time) contributed to the presentation. It was essential that only existing parts could be used: there was no time to start inventing new components. It was upon these basic decisions that rapid British progress depended. Britain was an ideal country for a radar defence chain, for the sea provided no obstructions to the signals transmitted.
In fact, progress was faster than any had hoped: a team assembled in May 1935 had 70-foot-high masts erected, and tests started within the following month. By the end of the year, results were far beyond anything Watson-Watt had promised. He had interpreted the fluctuations of a signal to guess that a formation of three Hawker Harts had strayed across the test area. He had measured aircraft heights to within 1,000 feet and had got some way to solving the direction-finding problem. The team mounted directional aerials to face north, south, east and west and then measured the relative strength of signal. The results were enough to distinguish the British work from anything done elsewhere, and more than enough to provide Watson-Watt with government money for higher masts and to get research started on alternative wavelengths (in case of enemy jamming) and detection of low-flying aircraft. The money was approved by Dowding but the gruff old man never did endear himself to the scientists, and many of them thought – wrongly – that Dowding did not understand the scientific principles of the new device.
There was a unique atmosphere at Bawdsey, to which the researchers moved in 1936. The old manor house was by the sea. It had extensive grounds that included a cricket pitch, peach trees, and the biggest bougainvillaea in the country. The high-grade academic physicists lived and worked in the manor house. There was no red tape and they stopped work for a swim or a bit of gardening as they felt like it. On the other hand, it was not unusual for the laboratory to be in full operation long after midnight. Visitors came from the famous Cavendish Laboratory at Cambridge to sit round the fire and talk shop. These sessions grew into what the Bawdsey men called ‘soviets’ in which visiting civil servants, Air Marshals – and eventually air crew straight from operations – could say anything they liked to anyone they chose. An Assistant III was often actively abetted in arguing with an Air Marshal, said an unrepentant Watson-Watt, who was often fomenting such excitement.
It was in this atmosphere of middle-class comfort that senior officers met scientists, with no clearly defined division of authority. No visitor to Bawdsey could fail to see its value as a way of applying scientific method to war. From Bawdsey, in 1937, teams went to study the discrepancies between radar tracks and navigators’ logs. Another team went to Fighter Command. The name ‘Operational Research’ was coined by Watson-Watt. He defined this as ‘investigation by scientific method on actual operations – current, recent or impending – and explicitly directed to the better, more effective and more economical conduct of similar operations in the future’. Although it never got the public attention that radar attracted, Operational Research eventually became just as important to the progress of the war.
IFF
By 1938 many airmen were worried that the radar could not identify friendly aircraft. It was typical of the negative attitude of most brass hats when the Commander in Chief Bomber Command said he would do everything in his power to oppose the radar work unless this problem was solved. Eventually it was solved, up to a point. It was called Identification Friend or Foe – IFF – a device for every aircraft. This re-radiated a much more powerful pulse than the one it received (but on the same frequency) so that its blip on the radar screen could be identified as that of a friendly aircraft.
The Reporting Network
The sort of radar defence that Britain had built by 1939 could only have grown out of the informal interaction of scientist, airman and civil servant. Part of Watson-Watt’s genius was knowing what was possible, so that as the government put money into research they found his promises fulfilled. But the great achievement of British radar was not to be found in the rather crude Chain Home RDF stations – or the more sophisticated CHL (Chain Home Low) sets – but in the way its information was interpreted and used. In this respect it was most fortunate that Dowding, who gave the first go-ahead for radar, then became Chief of RAF Fighter Command.
Bawdsey became the first radar station, as well as the scientific laboratory. Here, too, there were RAF officers planning the training of the personnel needed to man the other stations. Additionally they set up a full-scale experimental Group Operations Room, and experts were already planning the immense network of telephone cables that would be needed to feed all the information back to other such control centres.
By January 1938, Fighter Command aircraft at Biggin Hill airfield were working under radar control from the station at Bawdsey. From this time onwards, civil airliners passing within range of the Bawdsey apparatus had fighters sent to intercept them for practice. When war began, the operators were able to track German bombers mine-laying in the Thames Estuary. Sometimes the radar was so accurate that RN minesweepers could find the mines immediately.
The canny Scotsman Watson-Watt had proved to be the perfect man for the job, in spite of many bitter disputes in which he was involved. His knowledge of pure science gave him the basis upon which to work and his experience with electrical storms stood him in good stead at a time when electrical disturbance was one of the worst problems of practical radar. His career as a scientist for government departments equipped him for the internal politics he now encountered, and above all he was driven by a sense of urgency that made him set time limits to research, after which equipment went into production whatever its state of development.
The scientists realised that the quality of radar would depend upon generating very high power for very short wavelengths. Already the original experiment’s 50-metre wavelength had been reduced to 10 metres for the chain of stations that was being erected round the British coast. For a supplementary chain (CHL) the wavelength was only 1.5 metres. The shorter wavelengths provided a narrow beam that was far more directional. So the CHL masts had rotating aerials that swept the horizon to find the maximum intensity of response. These CHL sets were largely due to the work of an Australian named W. A. S. Butement, a War Office scientist who had started such beamed radar experiments as early as 1931 but had been discouraged from continuing with them.
But, even by 1940, it was still very difficult to read the blips on the cathode-ray tubes and height estimation was done by comparing the signals of different aerials. Judging the number of aircraft in any formation just from the wobble of the cathode’s glow was even more illusive. And when the operators were reading many blips at once, and trying to distinguish single aircraft from large formations, the results were confused and contradictory.
The Filter Room
From each of the Chain Home stations they phoned the details seen on the cathode-ray screens to a Filter Room at Bentley Priory. (This room also received reports from the Chain Home Low stations that searched for low-flying raids.) The Filter Room exemplified the way in which the whole radar system reconciled man with machine. Here the reports from the radar stations were weighed against the accuracy of their previous reports and against known faults in the apparatus. Only after the reports were compared, judged and interpreted were they passed on to the Operations Rooms. A good example of the value of the Filter Room was ‘range-cutting’. The Chain Home stations were far more accurate at measuring range than finding direction, so the range reports from two neighbouring stations could be intersected to provide an accurate position.
Another important task was comparing the reports of enemy raids with the estimated position of any RAF aircraft that might be seaward of the radar chain. The IFF system that enabled friendly aircraft to characterise the blip they made on the screen was far from perfect. It remained the weakest link in the system for a long time and eventually was radically changed. Meanwhile the Filter Room was responsible for preventing RAF squadrons from attacking friendly aircraft.
Operations Rooms
The filtered reports were plotted on the Filter Room map table. The counters – red for enemy and black for friendly – had numerals to show estimated height and strength, with an arrow to indicate direction, and a reference number for that particular formation. From the Filter Room balcony the whole map table was watched, and a teller passed details of these filtered plots back to Operations Rooms at Sector, Group and Fighter Command HQ. At each place, the map table was identical. On the wall in every Operations Room there was a special clock, marked to give each five-minute segment a different colour. Each raid’s coloured direction arrow was changed (to match the clock), and moved as each new report was received. Providing that the reports kept coming, all the plots would be the same colour. But a lost, or neglected, raid would be noticed because its colour remained unchanged.
Also available to the controller and his staff on their balcony was ‘the Tote’. This was a board fitted with coloured lights. It showed at a glance which squadrons were available in 30 minutes, which were at readiness (5 minutes) or at cockpit readiness (2 minutes), and which were in the air.
The Observer Corps
Virtually all of Britain’s radar stations were near the coast and facing seawards. But as soon as the raiders passed over the coast, the Operations Rooms were forced to rely upon an army of volunteers equipped with only enthusiasm, binoculars, an aircraft recognition booklet, and a simple sighting device.
As Churchill said, it was like going from the middle of the twentieth century to the early stone age. To cater to this transition there were ‘Lost Property Offices’ which recorded aircraft reported by the Observer Corps but not by radar. (The Observer Corps were only allowed to track aircraft that had been detected by the radar stations.)
The Observer Corps volunteers devoted much of their spare time to aircraft recognition. The system was wholly dependent upon these men for reports of enemy aircraft that had crossed the coast of Britain. On cloudy days this meant that enemy formations over land were reported only on the basis of the sound of their engines.
High-Frequency Direction-Finding (HF/DF) – ‘Pip-squeak’/‘Huff-duff’
For the radar system to work, it was essential that the Sector Controllers have an accurate plot of the position of their own fighters, in order to guide them to the raiders.
The direction-finding stations – there were three of them in each sector – took bearings on transmissions from the fighter pilots’ radio-telephones. To save the pilots effort, these were automatically switched to transmit for fourteen seconds in every minute of flight. The cross-bearings were translated into a map position in a D/F room, which was usually next door to the Sector Operations Room. There the Sector Controller could watch the movements of the enemy formation and of his own fighters.
Calculating quickly the compass course on which to send the fighter squadrons for accurate interception proved a vexing problem. Not only were pages of trigonometry consulted but a number of small computers were built to assist the calculation. Until one day, watching an exercise, an exasperated Wing Commander said he could judge the interception course by eye alone. He was immediately challenged to do so by the irritated boffins. He picked up the microphone that connected the Operations Room with the fighter pilots and gave them courses, until the two RAF formations taking part in the exercise met in a perfect interception.
The Wing Commander’s judgment was greeted by amazed disbelief. Asked to explain how he did it, he said it was a process of imagining an isosceles triangle, with the fighters and bombers at each base corner – interception would take place at the summit. He gave the course accordingly. It was a rough calculation but quite good enough to become standard procedure. The most common discrepancy, due to the superior speed of the fighters, was no real problem. The fighters were simply ordered to orbit until the bombers arrived.*
