The Battle of the Beams

Riding the radio waves

World War II was not only an industrial war utilizing the entire industrial output of its belligerent countries, but also a scientific one, in which the newest advances of science and engineering were immediately placed in service of the war effort, leading to a breakneck speed of technological development. One example of this was the so-called "battle of the beams," a period that started with the first German attacks on Britain, and ended in May 1941, when Hitler finally gave up on the idea of beating Britain into submission and turned his attention to the newly opened Eastern Front and the Soviet Union. This peculiar battle revolved around a German plan to use radio to guide bombers to their targets in England at night, and the British attempts to thwart these efforts with the same technology.

Chain Home, the British early warning radar system, provided excellent information on incoming Luftwaffe attacks, and allowed the Royal Air Force (RAF) to launch accurate interceptor squadrons with the aid of the Dowding System (Read our earlier article). This eventually forced the Germans to evade intercepting fighters by resorting to the night bombing of targets. Navigating at night, however, had its own challenges, especially since German pilots were not trained to navigate by the stars. (British bomber navigators were, and Bomber Command thought they were doing an excellent job at finding their targets at night – until 1941, when a report revealed that British bombers were hilariously inaccurate at night.) The German solution to finding and hitting a target at night was to rely on technology, and guide the bombers to their goal by radio.

The Third Reich had already developed a radio-based guidance before the war, used by civilian aviation to land at night or in bad weather. The Lorenz system used two radio antennas, set up to the left and right of, and pointed down the runway. The two emitters were broadcasting very narrow, slightly conical beams. One played short Morse "dots" separated by longer pauses; the other long Morse "dashes" separated by short pauses. The two signals were coordinated so that only one of the antennas was emitting a beep at any given moment. If a plane coming in to land only heard dots, it was to the left of the approach path and only heard the dots; if it was to the right, it only heard the dashes; if it was just in the right spot in the middle, it received both signals, which melded into a single constant tone, the "equi-signal." The system had a range of about 30 miles (48 km).

The first version of the offensive guidance system was code named Knickebein, "crooked leg," due to the distinctive angled shape of the antennas it used. These antennas were built in pairs, like the ones used by the Lorenz system, with one broadcasting a narrow beam of dots and the other a narrow beam of dashes. They were, however, much larger and more powerful, so that the signals could be received by bombers flying high over England. Once the antennas were turned towards the target, the bomber followed the equi-signal, only moving away from the antennas, rather than towards them. A second pair of similar radio beams was also pointed towards the target, cast from a different location in Europe. Once the bomber flying down the path of the guidance beam hit this beam and heard its equi-signal, it was exactly where it needed to be and could drop its bombs. 

The British became aware of this new German system when a lucky RAF pilot managed to down a Heinkel He 111 bomber during a night raid, and a memo found onboard mentioned a "radio beacon Knickebein." Another He 111 from the same unit crashed in England a few days later, this one with the personal diary of a crewman also mentioning Knickebein. Intercepted German messages encoded by Enigma, already broken by the Allies, also mentioned "bombing beams."

The system was code named "Headache" by the British, and was investigated by Reginald Victor Jones, a physicist and military intelligence expert whose job was to study "new German weapons," both real and potential. Jones quickly found himself in the midst of a controversy. Frederick Lindemann, the head scientific advisor to the British government asserted that a radar beam-based guidance system could not work over such ranges, as the beams would not follow the curvature of the Earth. Physicist and engineer T. S. Eckersley argued that the beams would, in fact, bend. Prime Minister Winston Churchill ordered a radio-equipped plane to go up and try to find one of these beams if they really existed. The RAF didn't have equipment to detect the frequency used by Lorenz signals, and had to buy an American-made amateur radio receiver from a shop in London. The flight was almost cancelled when Eckersley withdrew his argument. Jones insisted on going ahead with the experiment, pointing out that it was ordered by Churchill, and threatening to tell the temperamental prime minister who canceled it.

The plane took off, and eventually did find a Lorenz beam. The crew followed it and ran into the crossbeam, right over the Rolls-Royce engine factory at Derby, the only facility producing the Merlin engines used in early Supermarine Spitfire (Read our earlier article) at the time, the intended target for an upcoming German raid.

The code name for fighting "Headache" was "Aspirin." At first, the British tried to distort the radio signals with diathermy sets, a piece of medical equipment used to generate high-frequency electromagnetic currents for therapeutic use; but a better solution was soon invented. Radio-equipped planes would search for the German beams. Once one was found, nearby radio stations would start broadcasting their own "dot" signals. At first, these timed to be slightly off compared to the real ones, so German aircrews wouldn't know which one was real. The fake signals were later tweaked to coincide with the real ones exactly, but spread along a wider area. German bombers could veer off course, and still hear the equi-signal, with the dots provided by the local radios, rather than the Knickebein antenna. The trick was so successful that the Germans started believing the British had some way of actually bending the radio beams. Some Luftwaffe bomber crews not only dropped their load in the wrong spot, but even got lost without a reliable beam, and ended up landing at an RAF base, believing they were back home.

The next generation of German radio-guidance systems, the X-Gerät ("X-Apparatus"), followed close on the heel of its predecessor, and was tested as early as December 1939. The new system involved not two, but four different beams code named after rivers. The main beam, Weser, was projected from near Cherbourg (Read our earlier article), pointed at the intended target. It used the same dots-and-dashes double beam system as Knickebein, only at a much higher frequency, which made it much more accurate: the equi-signal area was a mere 100 yards (91 m) across at a distance of 200 miles (320 km) from the radio station. The beam was, in fact, too narrow, and the bombers needed another, wider Knickebein beam just to help them find the Weser beam.

The other three beams were cast from near Calais, and intersected Weser at three different points. A bomber flying down the guidance beam would first hit the Rhine beam roughly 30 km (18.6 mi) before reaching the intended bomb release point; this warned the radio operator to set up his equipment. The plane next hit the Oder beam precisely 10 km (6.2 mi) before the release point. When this happened, a stop-clock automatic started counting the seconds. The bombers soon hit the last beam, Eder, 5 km (3.1 mi) after Oder and the same distance before the release point. When this happened, the stop-clock started counting back down – if the plane maintained a constant speed, the countdown ended after flying another 5 km, which put it right at the release point. Once time ran out on the clock, the bombs were released automatically.

The Luftwaffe didn't have enough high-frequency radio sets to equip all of its bombers. The sets needed to use the X-Gerät were only given to the experimental Kampfgruppe (later Kampfgeschwader) 100. They then flew ahead of bomber raids and used the system's high accuracy to drop flares and illuminate the target, allowing the other bombers to aim visually. Most bombs released this way fell within 100 yards of the target, which was considered an unlikely feat even during daylight raids at the time. The devastating November 1940 raid on Coventry demonstrated the effectiveness of the system. The practice of dropping flares first was adopted by the Allies, and eventually gave birth to the pathfinder units, who also dropped paratroopers to set up landing zones ahead of a large drop.

The X-Gerät proved a tougher nut to crack than its predecessor. The British first tried the same false signals to let the bombers veer off course, but this didn't work. A Heinkel He 111 that crashed along the English coast in November 1940 revealed why. The Germans used the 2,000 Hz frequency for guidance, while the British false signals were 1,500 Hz. This normally wouldn't have been a problem, as the two frequencies were considered to be close enough, but the wrecked plane's radio was equipped with a very sharp filter that only let 2,000 Hz signals through. The British adjusted their jamming frequency accordingly. This came too late to save Coventry on November 14, 1940, but in time to prevent a similarly accurate raid on Birmingham on November 19.

The eventual countermeasure to X-Gerät was not a false guidance beam, but a false Elbe beam. Once the British detected the Oder beam which started the countdown clock aboard the bomber, a beam identical to Elbe was broadcast, aimed so it would intersect the guidance beam not 5, but only 1 km after Oder. This meant that the clock would begin the final countdown early and release the bombs well short of their target. Since the system was fully automated, the German bomber crews could not override the false signal for the final countdown.

The third and final German attempt was the Y-Gerät ("Y-Apparatus"). Unlike its predecessors, this system used a single beam pointed at the target, with the same dash-dot modulation as previous ones. There were no intersecting beams, but there was a second radio transmitter at the same location, sending out a modulated signal. Once this signal reached a bomber mid-flight, an onboard transponder automatically returned the signal to the station. The station measured the time that passed between sending out the signal and receiving the reply, which allowed them to determine the distance between the station and the plane. This measurement, combined with the direction of the return signal, gave ground controllers a very accurate idea of where the plane was, and they could just send the bomber radio message with necessary course adjustments.

Unfortunately for the Germans, the British already had a countermeasure in place by the time the Y-Gerät came online – albeit only through sheer dumb luck. In November 1939, two months after World War II began, anti-Nazi German scientist Hans Ferdinand Mayer sent a seven-page message to the British embassy in Oslo, Norway (where he was at the time), giving details of several military technologies Germany was working on. One of these was the method the Y-Gerät used to determine the distance of the plane.

R. V. Jones also learned from Enigma decrypts that the Germans sometimes referred to this new system as Wotan, the German version of Odin, the one-eyed chief god of the pagan Germanic pantheon. Believing that the Germans were prone to give overly-descriptive code names to their projects, he deduced that the one-eyed deity referred to a system that only used a single radio beam.

This was a lucky guess based on incomplete information. Unknown to Jones, Wotan was originally the code for the four-beam X-Gerät, and the code for Y-Gerät was, in fact, Wotan II. Had he known this, he probably would not have assumed a single-beam system, and it would have taken him much longer to come up with a defense against it.

In another stroke of luck, the Y-Gerät operated at 45 MHz. The BBC television tower at Alexandra Palace, a sports and entertainment venue in North London, used the same frequency to broadcast for a few years until it was shut down due to the war. This tower was a ready-made countermeasure. The return signals broadcast by German bombers were received by the British, passed forward to the TV tower, and broadcast again from there. German ground controllers in Nazi-occupied Europe received two response signals, slightly offset from each other: one from the bombers, and the other from the British. With no way of telling which one was real, they couldn't determine the bombers' distance accurately enough.

Jones was fond of practical jokes and had the second signal broadcast at a low power first. Since the jamming was not obvious, German aircrews started blaming ground controllers for sending bad signals, while latter fired back with accusations that the bombers had loose connections. Jones then had the power turned up very slowly over time, leading the Germans to believe that the problem was something inherent with the system that was not noticed during testing. By the time the Germans discovered the true source of the wrong signals, those were being broadcast loud enough to make the entire Y-Gerät system ring with feedback, rendering it useless.

The Germans gave up on developing new guidance systems for nighttime raids. The Battle of Britain was lost, and the bombing of British cities and industries no longer held the same vital significance. In May 1941, Hitler's attention was already turning eastwards to the imminent invasion of the Soviet Union. A guidance system similar to the one used over England was experimented with on the Eastern Front but never deployed to full effect. The British, on the other hand, took the hard-learned lessons to heart and developed their own, more advanced guidance methods for their own raids over German in the later years of the war.

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