Radar In World War Ii: The Battle Of The Beams

P.S. 175

Hart Deese

8J/3

January 1, 2007 Social Studies Exit Project: Radar and “The Battle of the Beams” In the 1920s, as airplanes increased in size, speed, and range, it was obvious that

they would be extremely important in future wars. Airplanes were fast, unhampered by ground terrain, and able to wreak havoc on enemy territory from above with powerful bombs. As British Prime Minister Stanley Baldwin observed in 1932, “the bomber will always get through” (Radar During World War II). But as good as they were, the bombers were always hampered by the darkness of night. During World War II, both Germany and Britain strived to reduce the difficulties of night bombing, and gain air superiority. At the beginning of World War II, British bomber pilots were well trained in celestial navigation for use in night bombing tactics, when pilots were unable to find their targets in the dark. The British were, however, disappointed to discover that this method was largely unsuccessful, with most bombs landing far from their intended targets. Before the war, the Germans were using a more effective method of night navigation, a radio signal broadcast along a narrow path that could be picked up by pilots, who could follow the signal to find the runway in low visibility conditions. This signal was generated via the Lorenz system, in which two broad signals (one mile wide at 200 miles range) were broadcast so that they overlapped forming a narrow region of “equisignal” that could be followed by pilots. This equisignal was produced by having one of the broad signals transmit dashes, and the other dots, so that pilots following the equisignal would hear a continuous tone, as the dots filled the silence between the dashes. As the war began, the German Luftwaffe adapted this technique for bombing purposes.

2 Two high-power Lorenz systems were set up across Germany, which could be rotated so that their signals crossed over a target in Britain. Bombers lined themselves up with one signal and followed it until they reached the second one, at which point they dropped their bombs. This system was called Knickebein (crooked leg) after the shape of the aerials used in the system. The two original Knickebein transmitters were located in Kleve and Bredstedt, Germany. When a deciphered “Enigma” message (intercepted by British intelligence June 12, 1940) first hinted of this system, there were many who believed that no beams could achieve such accuracy over the vast distances that would be required. As George Aylmore, who served in the RAF during WWII, explained, “There were those who were inclined to think if they could not do something, it couldn’t be done.” (The Good Guts) However, on June 22, 1940, British pilots with special equipment confirmed the accuracy of the beams, and Britain was forced to accept Knickebein as a reality. Britain codenamed Knickebein “headache.” Early countermeasures were nothing more than “simple noisemakers that transmitted an earful of mush on the beams’ frequencies and made it difficult for a bomber to pick up the intended signals,” (Buderi 196) but towards the end of August 1940, “headache” was forced to contend with more powerful systems codenamed “aspirins.” Aspirins were aerials that transmitted genuine-sounding dashes over Knickebein frequencies. The effect of these systems was that a German pilot on the true equisignal would hear a stronger dash sound, making him think that he needed to move farther into the dot region of the signal. If this system had been perfected, Britain would have actually been “bending the beams.” Britain’s false dashes, however, were never perfectly synchronized with the actual Knickebein dashes, and therefore at most their accomplishment was to confuse

3 German pilots and rob them of complete confidence in the beams. Regardless of what was actually happening, “legend grew up on both sides that [Britain was] genuinely bending the beams.” (Jones 128) Within just a few months, Britain had managed to “pull the Crooked Leg.” (Jones 105) On September 7, 1940, British cryptographers made a new break in Enigma traffic, pertaining to a Luftwaffe unit Kampf Gruppe 100, which was using equipment for a system code-named X-Gerät (X-Apparatus). Clearly the Germans had not stopped developing radar systems with Knickebein. By the 24th of September, British pilots had identified six beams independent of Knickebein systems, each of which the Germans had code-named for a river: Weser, Spree, Rhein, Elbe, Isar, and Oder. The six beams were arranged to guide bombers in a manner similar to, but more precise than Knickebein: two beams would be set up as director beams, one a fine beam aimed directly over the target, and the other a coarse one along the same line, with the purpose of making the narrower beam easier to find by specifying its general location. Crossing the fine director beam were four other beams, to be used in conjunction with a special timing apparatus. A German bomber would first encounter a “warning beam” about 70 km from the target, alerting him to set up the timing apparatus. The second beam came at 20 km, and was known as the Fore signal. At this signal, the pilot would start the timing apparatus. Next came the third and fourth beams, one being the main beam, and the other its backup, both at 5 km. Upon reaching this signal, the pilot would stop the apparatus, and if he had fed his correct altitude into it, the apparatus would by itself calculate when the bombs should be dropped. This system was incredibly accurate (all beams except the coarse director beam were broadcast at an accuracy of 10 yards at 200 miles), and much harder for the

4 British to foil. Countermeasures against X-Gerät were called “bromides,” and appeared in October 1940. They employed basically the same principles used to sabotage Knickebein. The difficulty with X-Gerät, however, was that the system was still able to function with only three (out of the usual five or six) beams in operation. Therefore, on a typical night, assuming the Enigma messages had been cracked to reveal the courses of the beams, Britain had to jam as many as four beams, a difficult task in the few hours between decoding and the actual raid. X-Gerät was used to great effect (over 1000 people killed or seriously injured) in the bombing of Coventry on the night of November 14, 1940, which the Germans codenamed the “Moonlight Sonata.” On October 6, 1940, British intelligence caught wind of a third radar system, Y-Gerät, code-named “Wotan” after the Norse god. Searching for clues as to the nature of Wotan, they realized that the god had only one eye. One eye must signify one beam, revealing the nature of the system. Y-Gerät made use of a single beam, broadcast on line with the target. Bombers were equipped with an instrument that constantly rebroadcast the signal to the ground station emanating it. Measurement of this relay time allowed the ground station to determine distance and guide the bombers accurately. Unfortunately for the Germans, their choice of signal frequency was not ideal. Y-Gerät operated at 45 Mhz, the same frequency as the BBC television transmitter at Alexandra Palace in north London. The British countermeasure against Y-Gerät, code-named “Domino” and effective in late January 1941, made exquisite use of this German miscalculation, and worked as follows: the BBC transmitter, closer to the incoming bombers than the ground station in Germany, was able to receive the bombers rebroadcast signal and rerebroadcast it to the bomber, which interpreted it as a signal from the ground station, and

5 sent it back out again. This unforeseen third step in the relay was significant enough to throw Y-Gerät completely off track, and eventually, by “turning up the volume,” Britain was able to make the whole system ring with feedback. Britain’s complete sabotage of Y-Gerät was enough to shake German confidence in radar systems to its foundations.

By February 1941, the Battle of the Beams was as good as won. All three German systems had been countered, and British radar systems and tactics were improving. This became apparent as “the Luftwaffe was…not only tending to miss its targets, but it was beginning to encounter losses on a potentially prohibitive scale.” (Jones 179) As the German bombings of Britain decreased, the war in the air finally began to move towards the German home front. On July 14, 1941, Winston Churchill declared, “From now on we shall bomb Germany on an ever-increasing scale, month by month, year by year, until the Nazi regime has either been exterminated by us or — better still — torn to pieces by the German people themselves.” (Great Aviation Quotes) Both German and British forces knew that air combat was to be the deciding factor of the Battle of Britain, and the winning of the Battle of the Beams was a crucial step in Britain’s path to air superiority.

6 Bibliography Buderi, Robert. The Invention That Changed the World. New York: Simon & Schuster, 1996. It chronicles the transformation of the cavity magnetron into a potent military weapon, RADAR. Buderi's book is praised as the first "history of radar that even-handedly credits both the American and the British contributions in World War II." Fisher, David E. A Summer Bright and Terrible. Emeryville, CA: Shoemaker & Hoard, 2005. A well-crafted narrative of the Battle of Britain, told while following the career of Air Chief Marshal Lord Dowding of the RAF. While not the most exhaustively researched book, it is very readable and informative material. Jones, Reginald Victor. The Wizard War. New York: Coward, McCann & Geoghegan, 1978. An insightful first-person account of Britain's battle against German advances in radar technology, or as Churchill called it, the Wizard War. Written by "the man who broke the bloody Beam," it is one of the most critically acclaimed accounts of the Battle of the Beams. Latham, Colin, and Anne Stobbs. Radar: A Wartime Miracle. Gloucestershire: Sutton, 1997. Book of firsthand accounts from various RAF personnel regarding radar. "Battle of the Beams." Wikipedia. 24 Nov. 2006. Wikimedia. 28 Dec. 2006 . Though not extensively researched, the article is well written and concise, providing a factual overview of the topic. “The Good Guts.” WW2 People’s War. 1 Jan. 2007 A firsthand account of World War II through the eyes of George Aylmore, who served in the RAF Bomber Command. "Great Aviation Quotes." Sky God. 28 Dec. 2006 . Database of aviation quotes, including many from WWII from both key Nazi and Allied figures.

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"Radar During World War II." IEEE Virtual Museum. 2006. 28 Dec. 2006 . A brief overview of the use of radar in World War II, as well as important advances in radar technology during the war. Second World War Encyclopaedia. 28 Dec. 2006 . An encyclopedia of the weapons, tactics, vehicles, people and forces of World War II.