and concludes that the worst-case built-in scenario was on the order of 9%, but this might be as high as 10% when flying over rough terrain like the built-up areas of cities. During early flights of the system, it was noticed that the pattern of the returns as seen on an oscilloscope was distinct for different types of terrain below the aircraft. This opened the possibility of all sorts of other uses for the same technology, including ground-scanning and navigation. However, these concepts were not able to be explored by Bell at the time. Use as general purpose radar It had been known since the late 1800s that metal and water made excellent reflectors of radio signals, and there had been a number of attempts to build ship, train and iceberg detectors over the years since that time. Most of these had significant practical limitations, especially the use of low-frequency signals that demanded large antennas in order to provide reasonable performance. The Bell unit, operating at a base frequency of 450 MHz, was among the highest frequency systems of its era. In Canada, the National Research Council began working on an airborne radar system using the altimeter as its basis. This came as a great surprise to British researchers when they visited in October 1940 as part of the Tizard Mission, as the British believed at that time that they were the only ones working on the concept. However, the Canadian design was ultimately abandoned in favour of building the fully developed British ASV Mark II design, which operated at much higher power levels. In France, researchers at IT&T's French division were carrying out similar experiments when the German invasion approached the labs in Paris. The labs were deliberately destroyed to prevent the research falling into German hands, but German teams found the antennas in the rubble and demanded an explanation. The IT&T director of research deflected suspicion by showing them the unit on the cover of a magazine and admonishing them for not being up-to-date on the latest navigation techniques. Applications In civil aviation Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules) and automatic landings, allowing the autopilot to know when to begin the flare maneuver. Radar altimeters give data to the autothrottle which is a part of the Flight Computer. Radar altimeters generally only give readings