by Brian Ford
Radar was not always a success. We have already seen, good radar reflections from incoming Japanese warplanes were detected prior to the attack on Pearl Harbor — but radar was relatively new, as were the observers, and so the all-important advance warning was fatally disregarded. Radar detectors were subsequently erected by the Germans on the coast of northern Europe, facing England. In February 1942, one of their Würzburg radar stations was detected by British reconnaissance near Bruneval in Normandy and close-up photographs were taken in a daring daylight raid. The British realized that the obvious answer would be to raid the radar establishment and bring the important components back to England. The idea was quickly approved, and R. V. Jones was the first to volunteer to go along and act as the technical specialist — but the authorities decided against sending anyone with specialist knowledge. If captured, they would be aware of details that the Germans could perhaps extract.
The plan was for this audacious raid to be carried out by ‘C’ Company of the 2nd Battalion, 1st Parachute Brigade commanded by Major John Frost. Their specialist would be radar operator Flight-Sergeant C. W. H. Cox. None of the men was told anything about the raid until the last minute, and training went on with scale models of the enemy terrain and with trials on the beaches of southern England.
On the night of 27 February 1942 the teams were dropped by parachute from Whitley aircraft flying at just 600ft (180m). The beach was secured, and teams rushed to the villa where they found a single German soldier guarding the equipment. German troops from a nearby pillbox were firing into the site as the British troops disassembled the Würzburg radar aerial, removing the important components which were all packed and taken quickly to the beach. There were problems making radio contact with the Navy, who were supposed to be ready to collect the men and their spoils, but had encountered a German warship and had to take evasive action. The German pillbox had been silenced and red flares were shot into the sky — at which point the Navy appeared in rough seas to retrieve the teams. Under enemy fire six landing craft arrived to collect their precious cargo. The ship was escorted back to Portsmouth Harbour by Royal Navy destroyers with RAF Spitfires flying overhead; the patriotic song Rule Britannia was played at high volume from loudspeakers. The raid was entirely successful. The airborne troops suffered only a few casualties, and the pieces of the radar they brought back, along with a German radar technician, allowed British scientists to understand German advances in radar and to create counter-measures to neutralize those advances.
Brave members of the French resistance continued to supply information to Britain and it was soon discovered that all German radar stations operated on a small number of frequencies, and they could easily be jammed. When a huge and devastating Allied bombing raid on Hamburg was planned for July 1943, fragments of metallic strips were dropped — once again, exactly one-quarter as long as the wavelength of the German radar beams, to maximize reflections — and the signals from the radar stations were completely obliterated. This chaff was known as ‘window’ by the Royal Air Force and had been independently discovered in Germany, where it was known as Düppel. It is a sign of the confused strategic planning by the Nazis that, having pioneered the use of reflecting chaff, the Germans did nothing to prepare themselves for its use by the Allies.
American radar
The Germans lacked the ability and knowledge to manufacture radar systems in which the radio beams could be tuned to a desired frequency, and this capability gave the British superiority in this vital area of defensive warfare. The early radar trials in Britain used a frequency of about 10MHz and the Chain Home stations began at 20MHz and later extended their range up to 70MHz — but the tracking radars operated at 200–800MHz. In 1940 the Americans began to use a cavity magnetron transmitter which went from the megahertz range up to gigahertz frequencies. It was small enough to fit into aircraft, and the H2S radar which was used by United States Air Force bombers had a frequency of 3GHz. It could show the details of the ground beneath the aircraft with unprecedented accuracy — but it was decided never to use it over Germany. Allied intelligence had reported that the Germans believed 800MHz was the highest frequency that radar could use, and it was feared that — if an American plane was shot down, and the secret of its radar was discovered by the Germans — the balance of power could radically change.
In the event, this proved to be a groundless fear. In the spring of 1943 an American bomber was brought down near Rotterdam, Netherlands, and the onboard H2S radar and the intact magnetron fell into German hands. They set out at once to reconstruct their own version, code named Rotterdamgerät, but the first examples were manufactured only when the war was nearing its end.
Installations of British radar detectors on Royal Navy ships allowed them easily to detect German submarines on the surface of the sea, and so the Germans worked on methods of preventing their U-boats from generating an echo, so they could thus escape detection. They soon found that a mixture of rubber and carbon painted onto a submarine’s superstructure greatly diminished the strength of its radar signature, and they tested it in a dry dock until they had perfected the ideal recipe. Surprisingly, the British warships continued to detect the submarines as though nothing had happened. The reason was simple — when wet with seawater, the protective layer no longer worked and radar detection could function as normal.
We must not dismiss the German idea out of hand, however: this was the origin of what we now know as ‘stealth technology’. Once again, a development from World War II underpins a crucial aspect of present-day warfare.
The importance of radar
Radar in Britain had a decisive effect on the conduct and duration of the war. Following enemy aircraft and ships that were otherwise invisible was of crucial importance. The Chain Home network was even shown to be able to detect V-2 rockets long before they reached the British coast and, although nothing could be done to prevent their arrival, this is historically significant as the first ballistic missile radar detection system in the world. These systems are now everywhere. It is also important to note that Hungarian scientists used their own radar to bounce signals off the moon in 1944. This was the first time it was ever achieved, and they accurately measured the distance of the moon from the earth for the first time in history.
Radar was firmly established and had become refined by the end of World War II. During the Vietnam War of 1955–75, anti-radiation missiles were developed which would home onto the beam emitted by a radar transmitter and destroy the installation, thus using the radar against itself. As devices became increasingly compact, radar speed detectors entered service with the police force. And so we can look back at the stimulus that World War II gave to the development of radar — and can bear in mind that the same principle had been used to detect shipping before World War I began. Radar has enjoyed a lengthy history. It can now entrap you on a highway, it is an integral part of global transportation, and it promises to have an illustrious future.
RADIO GUIDANCE FOR AIRCRAFT
Harnessing reflections of radio waves bounced back from aircraft and ships was the principle behind radar — but great effort also went into using radio beams themselves as an aid to navigation. This was born in 1932 as an aircraft landing system developed by the German company Lorentz AG, and was the brainchild of Dr E. Kramar. Like all good ideas, it was rooted in simplicity and was a brilliant innovation. It worked by having three radio masts that were transmitting 38MHz signals towards an approaching plane from the end of the runway. The antenna in the middle sent a single, continuous signal; the others (to the left and right) turned on and off alternately. The antenna to the left sent dashes each lasting ⅛th second, whereas the right antenna sent alternate dashes each ⅞th second long. The pilot of an approaching aircraft that was exactly on the correct, central flight path would tune to the radio signal and hear a continuous tone. If the plane was too far to the left, dashes would be heard; if it was over to the right then the short dots would be received. It was the first su
ccessful remote landing system and within two years it had been installed by Lufthansa on their aircraft, and was widely sold around the world. It was ahead of its time, and worked reliably over a distance up to about 30 miles (50km). As the Luftwaffe expanded, they experimented with a range of alternative solutions to the same problem. The British, meanwhile, trained their pilots in celestial navigation to aid flying at night; the Luftwaffe ignored something so old-fashioned, concentrating on wireless systems instead.
JAMMING ENEMY RADAR
The British worked hard to discover the German secrets and to block or jam their radar, as Robert Cockburn, a scientist with the Royal Aircraft Establishment, explains:
My job was to find out how to jam — or if you like, bend — the German beam. These beams were such an obvious device for the Germans to use but it took no account of the possibility of countermeasures. It was a fairly straightforward job. They used a dot dash Lorentz beam and all I had to do was radiate additional dots. Initially, we did it in synchronism — in other words we received the dots down at Worth Matravers and transmitted them by telephone line to Beacon Hill near Salisbury where we had a jammer. But I very soon realised that it didn’t matter a damn whether they were synchronised or not. They just had to be at the same time because when the German pilot got on to the signal beam, he would still hear those extra dots coming through which would make him go off to one side. We were being too meticulous for the rough and tumble of war.
Robert Cockburn, Imperial War Museum Sound 10685
The Lorentz system was widely installed in airfields across Germany and its use soon became a standard procedure. During the early war years, the German Air Ministry introduced a long-range alternative code named Sonne. Some of the receivers and documentation were captured by agents of the British, who adopted it for the RAF. The British renamed the system Consol.
As the transmitters became more powerful with each new generation, the range was extended and the Lorentz transmitters were used to guide bombers out across the North Sea to London. Here they encountered a problem, for the signals could guarantee that a bomber was flying along the correct straight course, but gave no information as to how far the bomber had flown. Headwinds could have the pilot far from the target, even though pointing in the right direction. For this reason, the Germans decided on a modification: they would set up two beams from Lorentz aerials that were widely separated. The beams would intersect over the target. The pilot’s task was near foolproof: he would fly along one beam, monitoring the signal in order to keep on course, until the second signal was suddenly encountered. At this point the crew knew they were on target, and the bombs were dropped. The Germans code named the system Knickebein — the crooked leg.
Knickebein transmitters were first tried in 1939, with one transmitter set up in Stollberg, northern Germany, another at Kleve in the far west of the country near the frontier with the Netherlands, and a third at Lörrach in south-western Germany. Once France had capitulated in June 1940 the Germans constructed more aerials along the French coast with more in the Netherlands and even in Norway. The German code name Knickebein was a very appropriate allusion to the L-shaped beams and they were extremely effective as a world-beating navigational aid. This may explain the British code name for the system: they called it the Headache.
The first evidence that the British had of how the system worked was when a German bomber was shot down and the radio receivers on board were examined. R. V. Jones, the brilliant physicist at the Air Ministry in London, was convinced that the system was far too sophisticated and sensitive to be a mere landing aid. At the same time, code-breakers at Bletchley Park had heard mention of ‘bombing beams’ being used by pilots. Jones’s beliefs were not widely shared, and the government’s Chief Scientific Adviser, Frederick Lindemann, dismissed the idea out of hand. His reasoning was that the radio beams could not be used, due to the earth’s curvature. R. V. Jones persisted, pointing out how high the bombers were, and managed to persuade Churchill to instruct the RAF to send an aircraft with a suitable detector, to search for the beams. The only receiver they could find which could pick up the required frequencies was obtained from an amateur radio shop in London and it was installed in a twin-engined Avro Anson aircraft. Jones’s rivals tried to cancel the flight at the last minute, but he reminded them that it had been ordered by Churchill himself — and so it went ahead.
The crew were briefed to search for navigational signals and to note the frequency and the bearing. They managed to find the beam from the transmitters at Kleve, and later encountered the transmissions from the direction of Stollberg. Flying along until both were received, the pilots found they were over the Midlands city of Derby. The place where the beams intersected was directly above the Rolls-Royce factory, where the Merlin engine was manufactured for the fighters of the RAF.
The British effort during World War II was always aimed at finding a simple and effective answer to the German initiatives. As a result, they installed aerials to pick up the navigational signals and then ingeniously rebroadcast them in a different direction. The effect was initially to confuse the German bomber pilots, but as the British became more experienced they were able to fine-tune their transmissions so that the enemy bombers could be induced to drop their payload anywhere the British wanted. Through this simple but effective means, they had found an answer to Headache … the British code named the new system Aspirin, of course.
Engineers in Germany moved as quickly as they could to devise an answer to the British jamming. They code named it X-Gerät — or X-device — and it used a complex series of beams operating at a higher frequency. The signals picked up by the pilot were used to time the distance the aircraft was to fly before dropping its bombs. Reception of the first signal was the cue for the pilot to set a specially designed clock ticking in his cockpit. The moment the second signal was received, the hand of the clock would stop and a separate hand would start to move … when both were aligned the target was directly beneath and the bombs were dropped. The British knew that the Germans were using radio beams to guide their bombers, and Jones had worked out where the beams were probably operating, but they could not detect the X-Gerät signals which were at a much higher frequency. Attempts to jam the transmissions failed. Successful raids against Birmingham, Wolverhampton and Coventry were all conducted by the Germans using this guidance system and without any advance warning being available to the British.
Matters changed on 6 November 1940, when a Heinkel He-111 was shot down off the Dorset coast and sank in shallow water. It was equipped with the new X-Gerät equipment and — once the receivers had been dried out and tested — it was clear that the navigation beams were at 2MHz, much higher than the 1,500Hz which the British had used. Work proceeded at a furious rate, and new jamming transmitters were hurriedly assembled. They were not ready in time to prevent the devastating raid on Coventry on 14 November 1940, but they were in place just five days later, disrupting a massive bombing raid on the city of Birmingham.
Later raids were diverted as the British realized they could send their second beam to trigger the pilot’s clock at the wrong time. This caused the bombs to be dropped early. The Germans soon responded by switching on their second beam for a much shorter period of time, making interference increasingly difficult for the British.
As the cat-and-mouse competition continued, the British managed to maintain equilibrium for most of the time, and R. V. Jones was not surprised when messages were picked up about the next generation of navigational aids. This was the Y-Gerät system — code named Wotan. Jones had recognized how perfectly descriptive the code name Knickebein had been, and knew at once that there would be a hidden meaning within the new code name. The Germans were making a fundamental mistake — in choosing witty allusions in their code names, they were giving away the nature of the weapon. When Jones was at Bletchley Park, looking at the incoming messages as they were translated, he took the opportunity to discuss the hidden meaning of Wotan with a German
specialist. He was told that Wotan was one of the ancient gods — a god with one eye. At once Jones knew what it meant: the new system would involve a single navigational beam. It could be modulated in some way, and not used in conjunction with a second signal. It would be much harder to fake, and the British knew they would face a new problem in diverting the bombing raids away from English cities. Jones also recalled that something similar had been mentioned in the Oslo Report.
The new way in which the Y-Gerät worked was to transmit a single beam directed over the target. The planes would be fitted with transponders that would transmit the beam back towards the originating transmitter station. The returned signal was measured automatically and compared with the timing of the original signal; so this gave the exact position of the aircraft along the narrow beam. If any correction was needed, the radio operators could send coded instructions to the pilot, making outside interference difficult. At least, that is how the Germans saw it. It was viewed very differently by the British. The new Wotan signals were soon detected in England, and it was discovered that they were transmitted on a frequency band of 45MHz. This was a standard radio frequency, and was exactly the same as the BBC television transmitter at Alexandra Palace in North London. Alexandra Palace (known as Ally-Pally) had broadcast a regular television service from 1936 but had been closed down by order of the government when war broke out. Jones simply ordered that the signal be turned back on, but operated at very low power. This was calculated to interfere with the timing of the Wotan navigational transmissions — but too weak for the Germans to detect. Jones was a warm and amusing character, and an inveterate practical joker and as time went by he instructed the crew at Ally-Pally gradually to increase the strength of the signal. Communications were picked up, in which the German bomber pilots accused their control room of incompetence; later the Germans believed that the Wotan equipment was at fault. The British counter-measures remained undetected, and the new guidance system could not be made to work.