re-aligned with the first, the bombs were released. The clock mechanism was co-ordinated with the distances of the intersecting beams from the target so the target was directly below when the bombs were released. Y-Gerät was an automatic beam-tracking system and the most complex of the three devices, which was operated through autopilot. The pilot flew along an approach beam, monitored by a ground controller. Signals from the station were retransmitted by the bomber's equipment, which allowed the distance the bomber had travelled along the beam to be measured precisely. Direction-finding checks also enabled the controller to keep the pilot on course. The crew would be ordered to drop their bombs either by a code word from the ground controller or at the conclusion of the signal transmissions which would stop. The maximum range of Y-Gerät was similar to the other systems and it was accurate enough on occasion for specific buildings to be hit. British countermeasures 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 a search which discovered that Luftwaffe Lorenz receivers were more than blind-landing devices. Jones began a search for German beams; Avro Ansons of the Beam Approach Training Development Unit (BATDU) were flown up and down Britain fitted with a 30 MHz receiver. 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. 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 became known as meaconing using masking beacons (). Up to nine special transmitters directed their signals at the beams in a manner that subtly widened their paths, making it harder for bomber crews to locate targets; confidence in the device was diminished by the time the Luftwaffe was ready to conduct big raids. German beacons operated on the medium-frequency band and the signals involved a two-letter Morse identifier followed by a lengthy time-lapse which enabled the Luftwaffe crews to determine the signal's bearing. The meacon system involved separate locations for a receiver with a directional aerial and