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In 1928 commercial aviation in the United States had developed to the point that the urgent need for additional facilities which would permit all-weather flight was evident to everyone. In anticipation of this need, the aircraft industry had undertaken the development of new and more precise aircraft instruments, including the sensitive altimeter, the artificial horizon, and the directional gyro. The aural radio range was beginning to be installed along the lighted airways and the National Bureau of Standards had completed initial development of the visual type range beacon. The aircraft radio group at the National Bureau of Standards, headed by Harry Diamond under the supervision of Dr. John H. Dellinger, undertook to coordinate these new facilities in an effort to solve the problem of flight during poor visibility. With General (then Lieutenant) James Doolittle as pilot, an installation at Mitchel Field in 1929 of the low-power visual range beacon and low-frequency marker beacon, together with the new flight instruments permitted Doolittle to effect hooded landings. It was evident that additional information was necessary. This was achieved by the development of the landing beam or glide path. Operating on a frequency of about 100 mc, it provided a signal which informed the pilot as to his location with reference to a predetermined descent path. Combined with the radio range beacon, instrument landings now became possible.