the direction of the received signals at both A and B . A second method would 

 be to transmit signals from A and B and to measure their direction of arrival 

 at C. 



D . RF PROPAGATION 



The velocity of propagation of radio waves, in a perfect vacuum, is 

 independent of frequency and is equal to the velocity of light. Recent experi- 

 mental determinations have shown the values of free -space velocity to be in good 

 agreement. A number of these values have been combined in a least squares de- 

 termination which yields a value for this constant of C = 299, 792.9 ± km/sec. 



1 . INDEX OF REFRACTION 



Atmospheric conditions cause variations in the index of refraction and 

 thus in the velocity of propagation in free space. Therefore, range measure- 

 ments of high accuracy generally utilize a corrected velocity of propagation . 

 Measurements may be made over a base line of known length or the atmospheric 

 variables may be measured and the propagation velocity corrected with a com - 

 puted index of refraction: 



77.6 

 n = — =— 



p + 481 



»(l) 



(1) 



where n is the index of refraction, T is the temperature in K, p is the air 

 pressure in millibars, and w is the partial water vapor pressure, also in milli- 

 bars . 



The true velocity of propagation is then determined by the relationship: 



299 793 

 v - '■ km/sec (2) 



1+NlO"^ 



where N = refractivity = (n-l)lO . 



Atmospheric pressure, temperature, and humidity vary with location, 

 time, and height. However, except for the most accurate range and ship -posi- 

 tioning measurements, a homogeneous atmosphere and a resulting constant propa- 

 gation velocity are assumed. This assumption generally produces a negligible 

 error . 



Zvthuv 3l.littlf,3lnr. 



