A TERRAIN CLEARANCE INDICATOR 



227 



indicating frequency meter circuits " of the condenser charge and dis- 

 charge variety have been described in the technical Hterature. 



The operation of the system can be understood more easily by 

 reference to Fig. 2. The variation of the transmitter frequency with 



signals: 

 direct reflected 



'MAX 



RADIO 

 FREQ, 



LOW 



FREQ. 







^^ 



1. 



2Fm 



— KEY — 

 H = HEIGHT OF AIRPLANE 

 C = VELOCITY OF PROPAGATION OF 

 RADIO WAVES 



^=TIME DIFFERENCE 



FrT1=N0. OF CYCLES OF FREQ.- 



MODULATION PER SECOND 



2AFFm= RATE OF CHANGE OF 

 TRANSMITTER FREQ. 



Fd =FREQ. DIFFERENCE IN DETECTOR 



OUTPUT = 2H (2 AF F^) = 4AFFmH 

 C ^ ^ C 



± 



:v: 



v: 



T TIME 



Fig. 2 — Operating theory. 



time is indicated by the soUd sawtooth Hne.* The value of the ordi- 

 nate of this curve at any point is the transmitter frequency for the 

 corresponding time. The frequency is varied from Fmin. up to Fmax. 

 and back Fm times per second, so the rate of change of frequency is 

 2AF Fm when AF is substituted for T^max. — -^min. The linear fre- 

 quency variation shown, while ideal, is not essential for the successful 

 functioning of the apparatus. The dashed sawtooth line represents 

 the variation with time of the frequency of the echo signal from the 

 earth's surface. This curve is displaced to the right by a time equal 

 to twice the height divided by the velocity of propagation, or, in other 

 words, the time it took the radio signal to go down to the earth and 



* A simple harmonic wave that changes in frequency from instant to instant is no 

 longer a single frequency but a series of discrete frequency components. In the 

 present instance, the number of cycles of frequency modulation per second is small 

 compared to the transmitter frequency swing, so the spectrum occupied by the signal 

 is substantially that of the swing itself. 



