In July 1940 the German airforce began bombing Britain. This was met with polite disagreement on the British side — and with high technology, ingenuity, and improvisation. The defeat of the Germans is associated with anti-aircraft guns and fighter planes, but a significant amount of potential damage had been averted by the use of radio.
Night bombing was a relatively new idea at that time and everybody agreed that it was hard. Navigating a plane in the dark while travelling at two hundred miles per hour and possibly being shot at just wasn’t effective with traditional means. So the Germans invented non-traditional means. This was the start of a technological competition where each side worked to implement new and novel radio technology to guide bombing runs, and to disrupt those guidance systems.
Finding a Runway is Like Finding a Bombing Target
For the first four months of the campaign, Germany used a guidance system called ‘Knickebein’. This translates as ‘crooked leg’ and allegedly was derived from the shape of the transmitting antennas. In 1940, radio navigation was decades old technology and in wide-spread use. The Lorenz system for example was a landing aid that allowed pilots to precisely line up with a runway from as far as 30 miles away. Such systems were in international use since the mid thirties.
Lorenz beams works by emitting narrowly focused radio signals from a set of antennas positioned perpendicular to the runway. An approaching pilot would use his radio to listen to the signal that consisted of short tones with longer pauses (dots) when lined up to the left of the runway, and long tones with short breaks (dashes) when off to the right. When lined up with the runway, the pilot heard a continuous tone because the dot and dash signals overlapped in the middle. The system was easy to use and the required equipment was cheap, or even already in place, so it spread quickly.
Knickebein was little more than two Lorenz beams turned to eleven. The base stations were more powerful and the beams had a narrower focus, but the principle was the same. Two stations were used to mark a bombing target by intersecting their beams over it. A bomber crew then flew along one beam and dropped their bombs when they crossed the second one.
The system had several strong points: It was easy to use and relied on equipment that was already on board. This meant that it was cheap to roll out in large numbers and could not be identified for what it was when found by the enemy. One or more Lorenz receivers were just what you expected to find in a big plane. Furthermore even after getting hints about its underlying principle, the system was deemed impossible by parts of the British forces. The objections were based on the fact that the frequencies used for Lorenz beams were too high to propagate past the horizon and that thus no signal emitted from German or occupied soil could mark a target in Britain.
The test flight that proved the existence of Knickebein almost didn’t happen. Since airplanes were in short supply the hunt for the signal took place from an old training aircraft that was just about to be decommissioned. The plane had no suitable radio on board and was fitted with an American made amateur radio receiver from a shop in London. On June 21 a successful flight received the signal and proved long-range use of Lorenz was possible. Reception worked because at the operating altitude of a long range bomber, the horizon is far enough away that it becomes possible to pick up high frequency signals from across the Northern Sea.
Straightening The Crooked Leg
The British had figured out how the Germans were guiding their bombers and needed an immediate solution to put a stop to it. An obvious option would be jamming the signal, but the required radio transmitters to accomplish such a feat were hard to come by. The newly founded electronic countermeasures unit hit it off with a hack. The head of the unit learned that medical diathermy machines — used for high-frequency therapeutic heat — operated in the same radio band as Knickebein. The machines were commandeered from hospitals all over the country and moved into the paths of the beams when a raid was imminent. The diathermy sets produced a lot of radio noise that suppressed the beams. More sophisticated methods were developed soon but the repurposed medical equipment provided critical first aid.
Later on transmitters that broadcasted ‘surplus dots’ were rolled out. They caused great confusion among the attacking aircrews and succeeded in throwing them off their tracks. The effort to render Knickebein useless culminated in synchronizing the decoy transmissions with the German signal to ‘bend’ the beams and move the bombings away from their intended targets.
Whether or not these surplus dots were actually used is up for debate. Wikipedia says yes, while the person in charge of the operation has given a BBC interview in which he reports that the capability was in place but never used for unrelated operational reasons. This didn’t really matter because by now the Germans had realized that Knickebein had been rendered useless and came up with a new idea.
Fooling a Fully Automated System
The successor was the X-Gerät or X-device. On the transmitting side it was little more than a more elaborate version of Knickebein. X-Gerät again used one Lorenz beam to set the bomber’s course and then three cross beams to time the drop. Early British countermeasures consisted of emitting surplus dots as before, which this time showed little effect. The reason for this may have been arrogance. When analyzing the German signals, the modulation frequency of its tones was wrongly measured as 1.5 kHz. The correct frequency was 2 kHz. When the error became known, the British considered it irrelevant, assuming that a radio operator in a noise plane would be unable to tell the difference.
It took them several weeks and a downed German bomber to realize that with the X-Gerät there was no radio operator involved. The system used an optical indicator to keep the plane on track and an electromechanical calculator to automatically release the bomb load. These units featured electronic filters that completely removed the surplus dots from the received signal. With this new understanding, the surplus dots could be brought to some effect.
Additionally another system was rolled out, the ‘False Elbe’ (the four beams used by the German side all bore the names of rivers). The time between crossing the last two, ‘Oder’ and ‘Elbe’ signals set the bomb timer. This was fully automated, allowing no human override. So to protect the high value targets, the British operated their own station that projected a beam that would be interpreted as the ‘Elbe’ signal right behind the ‘Oder’. The result was an early drop and a thoroughly plowed countryside just south of Britain’s industrial centers.
What’s In a Name?
At this point it became clear that approaches based on Lorenz beams were obsolete. The Germans had to come up with a completely different solution. While the system was officially designated as ‘Y-Gerät’ this pinnacle of creative naming didn’t sit well with the guy behind it, so it also got the code name ‘Wotan’. This information leaked to the British and the fact that Wotan was another name for the one-eyed god Odin made them suspect that Y-Gerät would work with just a single beam instead of several like its predecessors did.
Based on what they knew about radar technology — which was a lot for that time, as the British were the first to use radar in a large scale — and the abilities of the other side, the British determined that Wotan was likely to be an active radar system. To keep track of a plane, it would be fitted with an automatic transponder. A pulse would be emitted from a ground station and the airborne unit would automatically send its response when the pulse was picked up. The distance traveled could be precisely determined by measuring the time between the pulse and the response. Together with the bearing of the response signal, the ground team was able to locate the plane and direct it to its target via radio.
The Y-Gerät was superior to Knickebein and X, in that it did not reveal the location of an intended target before a raid took place. On the other hand it was unable to direct a large number of planes at the same time. Also it was doomed from the beginning. Having gained understanding of the new system beforehand, combined with sheer luck, the British were able to mess with Wotan from day one. The carrier frequency used by the other side coincided with the one of a TV broadcast station that had been mothballed when the war broke out. To counter Wotan, the British transmitted their own responses when they received pulses, slowly increasing the transmission power over time.
With the system being new, the resulting inaccuracies were blamed on teething problems and lack of training. Not knowing that Wotan was being jammed, aircrews and radar operators accused each other of incompetence. The Y-Gerät was the last bomber navigation system the Germans brought to bear in the Battle of Britain.