A Terrain Clearance Indicator

HE problem of an altimeter for aviation has engaged the attention of many inventors and experimenters for twenty years or more. As a result, about every conceivable fundamental method of attacking the problem, by the utilization of acoustic or electric phenomena, is disclosed in the art, including the many U. S. patents on the subject. The familiar aneroid altimeter has reached a high degree of perfection and enables the pilot to maintain level flight at any desired altitude but it gives no clue as to the variation of the elevation of the terrain beneath. The pilot has to know his position at all times and perform a mental calculation, in order to know his height above the ground at any given moment. A number of airplanes have drifted off their normal courses and have crashed on higher ground. An altimeter based upon the use of a sound echo is subject to two fundamental limitations. The first of these limitations is the extremely high noise level produced by the airplane's motors and propellers, which tends to submerge the relatively weak echo at heights of more than a few hundred feet. The second is that the speed of sound is not enough greater than the speed of airplanes. At a height of one thousand feet approximately two seconds are required for a sound to travel to the ground and return. In this time interval a modern airplane would travel six hundred feet and the clearance may have changed materially. * Read before the Institute of Aeronautical Sciences at the Chicago meeting, November 19, 1938, and to be printed in the Journal of the Institute.