This was the British overall designation of the electronic measures to establish the nature of and then degrade the capabilities of the German ‘Knickebein’ radio navigation aid (1940/41).
British efforts to block the 'Knickebein' system were slow to start. The electronic intelligence group at the Air Ministry, led by R. V. Jones, knew of the 'Knickebein' system initially because a downed German bomber’s Lorenz navigation system was analysed by the Royal Aircraft Establishment and established as far too sensitive to be just a landing aid; moreover, transcripts of secret recordings of captured German aircrew indicated this may have been a bomb aiming aid. Prime Minister Winston Churchill had also been given 'Ultra' intelligence from decrypted Enigma messages that mentioned 'bombing beams'.
When Jones mentioned the possibility of bombing beams to Churchill, the latter ordered further investigation in 'Headache'. Many Air Ministry personnel did not believe that the system was actually in use, however, and Frederick Lindemann, the leading scientific adviser to the government, claimed that any such system would not be able to follow the curvature of the Earth, although T. S. Eckersley of the Marconi company had said that it could.
Eckersley’s claim was eventually demonstrated after Churchill ordered a flight to try to detect the beams. An Avro Anson was equipped with an American Hallicrafters S-27 amateur radio receiver, the only known receiver capable of receiving the 40 MHz signal at the time and in this instance operated by a member of the Y Service, the British signals intelligence organisation. The flight was nearly cancelled when Eckersley withdrew his assertion that the beams would bend round the earth enough to be received, but Jones pointed out that Churchill himself had ordered it and he would make sure that the prime minister would get to know who cancelled it.
The members of the Anson’s crew were told no specifics, and were simply ordered to search for radio signals around 40 MHz having Lorenz characteristics and, if they found any, to determine their bearing. The flight took off and eventually flew into the beam from Kleve in north-western Germany, and subsequently located the cross beam from Stollberg (its origin was unknown before this flight) in Schleswig-Holstein. The radio operator and navigator were able to plot the path of the beams and discovered that they crossed right over the Rolls-Royce engine factory at Derby, at that time the only factory producing the all-important Merlin aero engine. It was later realised that the argument over whether or the beams would bend round the Earth’s curvature was entirely academic as the transmitters were, more or less, in the line of sight to a bomber flying at high altitude.
British sceptics now argued that the system was proof that the German pilots were not as good as British bomber pilots who, they believed, had no need for such systems. When the fallacy of this argument was revealed by the Butt Report, which used aerial reconnaissance photographs to show that RAF bombing raids were only rarely, if ever, anywhere near their planned targets.
Efforts to negate the 'Knickebein' headache were aptly codenamed 'Aspirin', and were brilliantly simple. Initially, modified medical microwave equipment was used to transmit interference. Later, on nights when raids were expected, local radio transmitters broadcast an extra 'dot' signal at low power.The German practice of turning on the beams long before the bombers reached the target area aided the British effort, in which Anson aircraft fitted with receivers were flown around the country in an attempt to capture the beams' location, whose 'capture' was then be reported to nearby broadcasters.
The low-power 'dot' was initially broadcast essentially at random, so German navigators would hear two dots. This meant there were many equi-signal areas, and no easy way to distinguish them except by comparing them with a known location. The British broadcasts were later modified to send their 'dots' at the same time as those of the German transmitters, making it impossible to tell which signal was which. In this case the navigators would receive the equi-signal over a wide area, and navigation along the bomb line became impossible, with the aircraft drifting into the 'dash' area and no way to correct for it.
Thus the beam was seemingly 'bent' away from the target. Eventually, the beams could be inclined by a controlled amount which enabled the British to fool the Germans into dropping their bombs wherever the British wanted them. A side effect was that as the German crews had been trained to navigate solely by the beams, many crews failed to find either the true equi-signal or Germany again, and in the case of the latter eventuality some Luftwaffe bombers even landed at RAF bases, believing they were back in the Germany.