A TERRAIN CLEARANCE INDICATOR 223 



There is in radio the corresponding phenomenon of an echo, an 

 electric-wave reflection. The velocity of a radio signal is so great 

 that an echo from the earth's surface is almost instantaneous; in fact, 

 the time interval is so small as to give rise to a problem in measuring 

 it. For instance, for heights less than a thousand feet the time to be 

 measured is less than two millionths of one second. 



The method used in the present instrument is extremely simple in 

 theory. A radio transmitter is provided on the airplane which sends 

 toward the earth a signal, the frequency of which changes at a definite 

 rate with respect to time. The signal is reflected by the earth and 

 returns as an echo after a time delay equal to twice the height, divided 

 by the velocity of propagation. During this interval the frequency 

 of the transmitter has changed and now differs from that of the echo 

 by an amount equal to the product of the rate of change of frequency 

 and the time of transit. The reflected wave is combined in the plane 

 receiver with some of the outgoing wave energy and the difference or 

 "beat" frequency is measured by a frequency meter. Since the read- 

 ing of the meter is that of the "beat" frequency, it is proportional to 

 the time delay of the echo and, hence, to height and thus can be 

 calibrated directly in feet. 



Early Efforts 



The evolution of this method is interesting because it illustrates 

 how one art is built upon another, and also the familiar story of separate 

 inventors arriving at the same answer almost simultaneously, actually 

 somewhat in advance of the existence of instrumentalities having the 

 characteristics required to make the invention practically serviceable. 



Many systems employing electromagnetic waves for the purpose of 

 indicating altitudes of an aircraft have been proposed.^ Among early 

 workers in this field who independently of each other were concerned 

 with methods involving frequency modulated waves were J. O. 

 Bentley ^ of the General Electric Company; Professor W. L. Everitt ^ 

 of Ohio State University and certain students in his department of 

 Electrical Engineering including the junior author* and M. W. 

 Hively; and the senior author. 



Under the direction of Professor Everitt, some experimental work 

 on the frequency modulation method, using wire lines, was undertaken 

 in the school year 1928-29. On the basis of this work a grant was 

 made by the Guggenheim Fund for the promotion of aeronautics and 

 an investigation was continued with experimental tests, during the 

 following school year under the auspices of the Ohio State Engineering 

 Experiment Station. The experiments were reported upon in the 



