In June 1940, a German prisoner of war was overheard boasting that the British would never find the Knickebein, even though it was under their noses. The details of the conversation were passed to an RAF Air Staff technical advisor, Dr. R. V. Jones, who started an in-depth investigation which discovered that the Luftwaffe's Lorenz receivers were more than blind-landing devices. Jones therefore began a search for the German beams. Avro Ansons of the Beam Approach Training Development Unit (BATDU) were flown up and down Britain fitted with a 30 MHz receiver to detect them. Soon a beam was traced to Derby (which had been mentioned in Luftwaffe transmissions). The first jamming operations were carried out using requisitioned hospital electrocautery machines. A subtle form of distortion was introduced. Up to nine special transmitters directed their signals at the beams in a manner that widened its path, negating its ability to accurately locate targets. Confidence in the device was diminished by the time the Luftwaffe decided to launch large-scale raids. The counter operations were carried out by British Electronic Counter Measures (ECM) units under Wing Commander Edward Addison, No. 80 Wing RAF. The production of false radio navigation signals by re-transmitting the originals was a technique known as masking beacons (meacons).

In 1940 a German prisoner overheard boasting that the British would never find what system?