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=== Sonic altimeter === In 1931, the US Army Air corps and General Electric together tested a '''sonic altimeter''' for aircraft, which was considered more reliable and accurate than one that relied on air pressure when heavy fog or rain was present. The new altimeter used a series of high-pitched sounds like those made by a bat to measure the distance from the aircraft to the surface, which on return to the aircraft was converted to feet shown on a gauge inside the aircraft cockpit.<ref>[https://books.google.com/books?id=9ycDAAAAMBAJ&pg=PA35 "Meter Gives Elevation"], ''Popular Science'', March 1931</ref>
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