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.