On separate evenings in the spring of 1940, two German bombers were shot down over Britain. Contents from the wreckage of each Heinkel 111 included a small scrap of paper on which were written Lat/Long coordinates and the word Knickebein - German for “bent leg”.
Surviving airmen from one of the crashes were interrogated and revealed nothing – until the Brits left the room, allowing the German airmen to talk amongst themselves. . .and the hidden microphone in the room with them.
Confronted separately, they gave details of Knickebein – a navigational system that used intersecting radio beams to guide bombers to their target.
Although a much more complex version of this method guides airplanes to landing destinations today, Knickebein represented the first use of such technology to guide planes to a target zone away from an airfield. The Brits code-named this system “Headache”.
The wreckage from the bombers was examined again, this time with extra attention given to the radio equipment. The undamaged radios from the bombers were found to have been tuned to various frequencies around 30 MHz. In the meantime, one of the captured airmen, designated A231, drew a sketch of a transmitting tower and described its associated transmitter’s modulation scheme.
With this information, the RAF realized that they had two choices – they could either jam the signal outright or they could manipulate it in such a way as to cause erroneous navigational information to be derived from it. Of course they chose the latter, which meant that they would need to disect the signal in detail in order to know how transmit their own Knickebein-like signal .
This was the job of “Y Service” (mostly pre-war hams) and to do this they were furnished with the only commercial radios capable of receiving in the desired range available at that time – Hallicrafters S27′s. Four British aircraft outfitted with S27′s made up the top-secret 109 Squadron. Several ground stations also monitored for Knickebein/Headache: Bawdsey, West Beckham, Ottercops and Dover.
The ground-based S27′s didn’t operate as you might expect, say, with an indoor location and an external antenna. Transmission line losses at the then-high frequency of 30 MHz required that the heavy Hallicrafters be lugged up a tower and operated at the antenna, coastal gales or no.
What they learned was that Knickebein transmitted a signal that the pilot would hear as a steady tone if he were in the “eqi-signal area” of the beam – in other words, dead center & flying along its path. Too far to the left, he would hear dots; too far to the right, dashes.
Once the pilot neared his target, the bomber’s radioman would tune to another frequency – 31.5 or 33.0 MHz – and receive the signal from another transmitter. Two steady tones from each transmitter indicated their intersection and that they were over their target.
Armed with this information, the Brits began transmitting properly synchronized dots on 30, 31.5 and 33 MHz using 150-watt medical transmitters commandeered from hospitals, the result being that German pilots continued to “correct” by turning away from the dots.
Many became hopelessly lost and ran out of fuel. One bomber crashed offshore, its crew stunned to hear English being spoken by authorities who came out to investigate the crash. They had thought they were wading ashore in Spain. Oops.
Knickebein didn’t transmit all the time…only when a bombing mission was underway. Therefore the Brits couldn’t either and had to constantly monitor the 10m band for activity. This program of detection and “disinformational navigation” was named Aspirin.
As the war progressed, German pilots became more adept at discerning Knickebein signals from Aspirin signals. And, as happens in every major war (Cold or Hot), technology progresses at an accelerated rate – Knickebein was eventually replaced with X-Gerat (74 MHz), with an accuracy of 12.5 feet at 100 miles from the transmitters, which was used to devasting effect in the bombing of Coventry.
The British response was code-named Bromide – a more potent analgesic than Aspirin.
Check out 7:10 into this video for an explanation of how Knickebein guided the pilots:
And finding the beam on the Hallicrafters at 5:45 into the video:
British countermeasures against Knickebein (01:45)