The Naxos radar warning receiver was a World War II German countermeasure to S band microwave radar produced by a cavity magnetron. Introduced in September 1943, it replaced Metox, which was incapable of detecting centimetric radar. Two versions were widely used, the FuG 350 Naxos Z that allowed night fighters to home in on H2S radars carried by RAF Bomber Command aircraft, and the FuMB 7 Naxos U for U-boats, offering early warning of the approach of RAF Coastal Command patrol aircraft equipped with ASV Mark III radar. A later model, Naxos ZR, provided warning of the approach of RAF night fighters equipped with AI Mk. VIII radar. Background Prior to the introduction of the cavity magnetron, radar systems used traditional vacuum tube electronics and were limited to about 1.5 m wavelength in UK use, and as low as 50 cm in German systems. Both could receive the transmissions of their opposing radar systems and radar warning receivers were widely used by both sides in a number of roles. By 1942, the UK had made enough progress on the magnetron to begin introducing new radars using it, including the AI Mk. VIII radar for night fighters, ASV Mk. III radar for sea-surface search (anti-submarine) and the H2S radar for bomber guidance. None of the existing German receivers could operate at the magnetron's 10 cm wavelength, and the introduction of the ASV Mk. III, in particular, led to significant losses among the U-boat fleet during the summer of 1943. Before the magnetron had been deployed operationally, there was a great debate in the UK over whether or not Bomber Command should be allowed to use it. Unlike other types of tube electronics of the era, which are quite fragile, the main component of the magnetron is a large block of copper. If an aircraft carrying one were shot down and recovered, there was a very good chance the block would survive, at which point the secret would be revealed to anyone familiar with microwave techniques. This is precisely what occurred on the night of 2/3 February 1943, when the second mission to attempt to use H2S led to one of the Short Stirling bombers carrying it being shot down near Rotterdam. The magnetron was recovered and this Rotterdam Gerät (gadget, or device) led to the rapid formation of a study group to exploit it. The group first met at the Telefunken