THEORY AND RESULTS 



151 



RADIO RAY 



Figure 4.42. Geometry of radio ray refraction. 



where ua is the index of refraction at the top of the duct, ta is the distance 

 from the center of the earth to the top of the duct, and no is the index of 

 refraction at the earth's surface, it is seen that dht is uniquely determined 

 from the surface gradient of A'^ with height inside the duct, provided the 

 height of the duct is known. 



If one desires to know the distance, dh, for a particular height, h, from 

 the reflection point of the ray in the duct (point B in fig. 4.41), where dh 

 is such that 



(4.34) 



