Knickebein (navigation)
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Knickebein ("crooked leg" in German, but also the name of a magic raven in a German fairy tale) was a radio navigation system used by the Luftwaffe early in World War II to aid bomber navigation. It was replaced with the X-Gerät (navigation) system.
Lorenz directed an aircraft down a line, so two crossed beams could be used to fix a single spot. Several systems based on this idea were studied through the 1930s. Lorenz had a range of about 30 miles, enough for blind-landing but not good enough for bombing raids over England. In addition the beams of Lorenz were deliberately set wide enough that they could be easily picked up at some distance from the runway centerline, but this meant their accuracy at long ranges was fairly limited. This was not a problem for blind landing, where the distance covered by the fan-shaped beams decreased as the airplane approached the transmitters, but for use in the bombing role this would be reversed and the system would have maximum inaccuracy over the target.
For bomber use the modifications to Lorenz were fairly minor. Much larger antennas with considerably smaller beam angles were set up, and broadcast power was increased considerably. The first two of these new Knickebein transmitters were set up in northern Germany near the border with Denmark, and another to the south near France. The two beams could be pointed to cross over the target, and the bombers would fly into the beam of one and ride it until they started hearing the tones from the other (in another radio). When the steady "on course" sound was heard from both radios, they dropped their bombs. Using Knickebein required nothing more than the installation of a second Lorenz receiver.
Knickebein was used in the early stages of the German night-bombing offensive, and proved to be fairly effective. However the tactics for using the system in a widespread bombing effort were not yet developed, so much of the early German night bombing offensive was limited to area bombing anyway.
Efforts in England to stop the system took some time to get started. British intelligence at the Air Ministry, led by R. V. Jones, were aware of the system due to a number of intercepts and captured documents from downed bombers. However many in the Air Ministry didn't believe they used such a system, and Lindemann claimed that any such system would not be able to follow the curvature of the Earth anyway. Both claims were proven untrue when one enterprising British intelligence officer flew a light plane equipped with an amateur radio into one of the beams and picked up the signals. At this point the same sceptics then started pooh-poohing the system as proof that the German pilots weren't as good as their own, who could do without such systems. However it was Lindemann himself who proved this wrong, when his "photoflash" systems started returning photographs of the RAF bombing raids, showing that they were rarely, if ever, anywhere near their targets.
Efforts to block the Knickebein were brilliant in their simplicity. On nights where raids were going up, local radio transmitters broadcast the "dot signal" at low power. This meant that as the bombers flew over England, almost any point at all would make the "on course" steady signal, and the planes would drift far to the left of their line. The crews knew something was wrong, but the nature of the problem was a mystery. Eventually a German radio engineer flew on one mission and, finding himself many miles from the target, concluded that the British had invented a system for bending the radio beams. They gave up on Knickebein soon after.
