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Battle of the Beams
Radio electric warfare was a novel and key battleground in World War II. British and German advances in radar technologies spurred both sides to innovate. One of these battlefields was the Battle of the Beams, the German effort to develop an effective night bombing aid, and the British counter efforts to render it useless.
The research into a means of detecting approaching enemy bombers started in the 1930s. The accidental invention of radar (inventor Robert Watson-Watt was, in fact, trying to create a death ray) drove the first nail into the coffin of the axiom "the bombers will always get through."
By 1939 the British had 30 stations in the Chain Home System to give warning of German air activity. This and the valiant efforts of the Royal Air Force (RAF) soon made daytime bombing missions too costly for continuation. The Luftwaffe therefore switched to night bombing missions but needed a guidance system to lead the planes to their blacked-out targets.
Efforts to convert the Lorenz blind landing system into a target beacon started in 1939. The Lorenz system worked with modulated radio signals transmitted in two overlapping beams, one transmitting dots, the other dashes. Where the two signals overlapped, the pilot heard a continuous tone, the equi-signal. This equi-signal led him to the runway, whereas if he heard dots or dashes only, he knew he was to the left or the right of the correct approach route.
The Knickebein night guiding system used the same basic idea only with narrower, therefore more accurate beams operating with a far longer range. The Germans installed two transmitting stations, one in Kleve and one in Stollberg, close to the Dutch and Danish borders, respectively.
The Kleve beam designated the approach route for bombers. Their targets were marked by the crossing of the Kleve and Stollberg beams. The British were notified of the existence of a German night guiding system by the "Oslo report," in which German scientist H. F. Mayer described several German inventions, and intercepted and decoded Enigma traffic that gave away the location of the transmitting stations. These facts led R V. Jones, head of the scientific intelligence of the Air Ministry, to commence a hunt for Knickebein.
When examining the wreck of a German bomber shot down over the British coast it was discovered that the Lorenz blind landing machine, standard equipment on all German bombers at the time, was far too sophisticated for its alleged purpose. By reverse engineering German equipment and test flights to find the Knickebein beam, it was determined that the German system worked at a frequency of 31.5 MHz.
The British could now set upon jamming the transmission with hospital diathermy sets used to increase blood flow through body tissue with the help of an electric current. They also started transmitting a "surplus of dots" to confuse German pilots, which had the adverse effect that the disorientated planes jettisoned their bombs wherever they happened to be.
The Germans soon realised that the Knickebein system was useless and came up with another solution, the X-Gerät. With the invasion of France they could now post their transmitting stations right on the Channel coast. The X-Gerät used one lead beam and 3 cross beams. The lead beam, called Weser and transmitted from Cherbourg, designated flight path, while the crossbeams sent from Calais warned of the decreasing distance to target. The first cross beam, Rhine, set the crew on alert, while crossbeams Oder and Elbe started the bombing clock.
The technical apparatus to receive the X-Gerät signals was not readily available, and therefore only the bombers of Kampfgruppe 100 were fitted with the necessary equipment. These planes served as pathfinders leading regular bombers to the target by lighting it up with flares and incendiary bombs. Another downed Heinkell 111 allowed the British to examine this new equipment and again come up with a way to jam its frequencies, though not in time to prevent the devastating raid on Coventry. They also managed to broadcast false Elbe signals to entice the German bombers to release their payload prematurely.
The last German attempt at a night guiding system gave birth to the Y-Gerät. This system worked with a single beam that was received and rebroadcast to the ground station by the lead bomber. The phase difference between the original and the rebroadcast signal allowed for calculating the distance and speed of the plane. As a coincidence, this system worked at the same frequency (45MHz) as the yet unused TV station of the Alexandra Palace. All the British had to do was turn this transmitter on to overload and confuse the Y-Gerät. At this point the Germans abandoned their attempts to lead their bombers against England with radio guidance.