(even hours). One version that kept an image alive about a second before fading, proved to be a useful addition to the evolution of radar. This slow-to-fade display tube was used by air traffic controllers from the very beginning of radar. Airborne Intercept The Luftwaffe took to avoiding intercepting fighters by flying at night and in bad weather. Although the RAF control stations were aware of the location of the bombers, there was little they could do about them unless fighter pilots made visual contact. This problem had already been foreseen, and a successful programme, started in 1936 by Edward George Bowen, developed a miniaturized RDF system suitable for aircraft, the on-board Airborne Interception Radar (AI) set (Watson-Watt called the CH sets the RDF-1 and the AI the RDF-2A). Initial AI sets were first made available to the RAF in 1939 and fitted to Bristol Blenheim aircraft (replaced quickly by Bristol Beaufighters). These measures greatly increased Luftwaffe loss rates. Later in the war, British Mosquito night intruder aircraft were fitted with AI Mk VIII and later derivatives, which with Serrate allowed them to track down German night fighters from their Lichtenstein signal emissions, as well as a device named Perfectos that tracked German IFF. As a countermeasure, the German night fighters employed Naxos ZR radar signal detectors. Air-Surface Vessel While testing the AI radars near Bawdsey Manor, Bowen's team noticed the radar generated strong returns from ships and docks. This was due to the vertical sides of the objects, which formed excellent partial corner reflectors, allowing detection at several miles range. The team focussed on this application for much of 1938. The Air-Surface Vessel Mark I, using electronics similar to those of the AI sets, was the first aircraft-carried radar to enter service, in early 1940. It was quickly replaced by the improved Mark II, which included side-scanning antennas that allowed the aircraft to sweep twice the area in a single pass. The later ASV Mk. II had the power needed to detect submarines on the surface, eventually making such operations suicidal. Centimetric The improvements to the cavity magnetron by John Randall and Harry Boot of Birmingham University in early 1940 marked a major advance in radar capability. The resulting magnetron was a small device that generated high-power microwave frequencies and allowed the development of practical centimetric radar that operated in the SHF radio frequency band from 3 to 30