REFRACTION OF RADIO RAYS 



165 



RADIO RAY 



Figure 4.56. Geometry of radio ray refraction. 



geometrical optics formula [45, 43], 



Ti,2 = — f cot 9 dn 



(4.41) 



under the assumption that dn/n = dn and 6 is the local elevation angle. 

 It is also assumed that n is spherically stratified and concentric with the 

 earth. The value of cot 6 is determined from Snell's law, and ti,2 is com- 

 pletely determined from a knowledge of the variation of n with height. 

 An empirical formula to describe the long-term average variation of n 



