254 



TRANSHORIZON PARAMETERS 



X 

 (3 

 LJ 



X 



REFRACTIVE INDEX 

 Figure 6.15. Linear profile model of the n decrease across an elevated layer. 



On the other hand, the analysis assumes a smooth layer extending 

 horizontally at least over a distance x equal to the first Fresnel zone. 

 We have: 



X = ■\/2a\/a 



(6.6) 



where 2a = path length = 203 km for the Illinois paths. Hence x is of 

 the order of a few tens of kilometers. In addition, we have assumed that 

 the layer is horizontal and smooth over a distance x, thus neglecting con- 

 vergence. If we adopt the Rayleigh criterion, the height of the surface 



