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BELL SYSTEM TECHNICAL JOURNAL 



radio receiver is connected through a similar coaxial line to a second 

 antenna similarly located but arranged in such a way that a minimum 

 of direct signal is received from the transmitting antenna and as much 

 echo as possible from the ground. The direct and reflected signals are 



TRANSMITTER 



FREQUENCY ( 

 MODULATOR I 



OSCILLATOR f 



(ultra-high FR£Q.)l 



FREQUENCY- I 



MODULATED J 



SIGNAL 1 



Fig. 1 — Overall system. 



applied to a detector circuit in the receiver. The output of this de- 

 tector is a signal of a frequency equal to the instantaneous difference 

 existing between the direct and the reflected signals and is proportional 

 to the height of the plane above the terrain. This signal is ampHfied 

 by the receiver and applied to a frequency meter or counter circuit 

 which is so designed that a current proportional to the frequency and, 

 hence, to the height flows through a meter calibrated in feet and 

 located on the airplane's instrument panel. A number of types of 



