and in some cases, the bombing was seen to fall as far as from the target. Radio electronics promised some improvement and the Telecommunications Research Establishment (TRE) developed a radio navigation system called "Gee" and then a second known as "Oboe". Both were based on transmitter stations in the UK which sent out synchronized signals. In the case of Gee, an oscilloscope in the aircraft measured the time difference between two signals to determine location. Oboe used a transponder in the aircraft to reflect the signals back to the UK where operators carried out the same measurements on much larger displays to produce more accurate values. In both cases, the ground-based portion of the system limited range to a line-of-sight, about for aircraft flying at typical mission altitudes. This was useful against targets in the Ruhr, but not the heart of Germany. Taffy Bowen had noticed during his early 1.5 m wavelength AI radar experiments before the war that the radar returns from fields, cities and other areas were different. This was due to geometry; objects with vertical sides, like buildings or ships, produced much stronger returns than flat objects like the ground or sea. During early tests of the AI system, the operator would often see coastlines at very long distances, and the development team used this as an ad hoc navigation system on several occasions. Bowen had suggested developing a targeting radar based on this principle, but the matter had been forgotten. In 1940, John Randall and Harry Boot, PhD students at the University of Birmingham, devised a new microwave-frequency vacuum tube known as the cavity magnetron that output thousands of watts of radio signal at 3 cm wavelength. At this wavelength, the antennas were only a few centimeters long, making radar much easier to fit to an aircraft. The mapping idea resurfaced in March 1941 when Philip Dee's group was developing a new AI radar, christened "AIS" in reference to its "sentimetric" wavelength. During tests in a Blenheim, the team noticed the same sort of effects Bowen had earlier. However, the set's wavelength, over ten times shorter than the original 1.5 m AI sets, provided much greater resolution and allowed them to pick out individual objects on the ground. Work begins In October 1941, Dee attended a meeting of the RAF Bomber Command where the night targeting issue was discussed. Dee mentioned the recent discoveries using AIS.