A Terrain Clearance Indicator * 



By LLOYD ESPENSCfflED and R. C. NEWHOUSE 



There is described a radio altimeter that gives continuously on 

 the plane a measurement of the separation between the plane 

 and the earth's surface or projections therefrom. There is pro- 

 jected from the plane and reflected from the earth back to it a 

 very short radio wave, the frequency of which is continuously 

 swung back and forth. The returned wave is thereby made to 

 differ from the outgoing wave in frequency by an amount that is 

 proportional to the echo path; and the difference or "beat" fre- 

 quency is indicated on a frequency meter calibrated in feet of 

 separation. The paper outlines some of the early efforts in this 

 field, some of the technical problems involved, the theory of the 

 system and the practical experimental results that have been 

 obtained. 



Introduction 



'T^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 perfec- 

 tion and enables the pilot to maintain level flight at any desired alti- 

 tude 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 

 ofif 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 ex- 

 tremely high noise level produced by the airplane's motors and pro- 

 pellers, 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. 



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