60 



TROPOSPHERIC REFRACTION 



N 



40 



2 4 6 8 10 



Altitude Above Mean Sea Level in Kilometers 



Figure 3.3. Typical N versus height distrihulions. 



One might wonder, in the light of the data of figure 3.3, why the effective 

 earth's radius approach has served so well for so many years. It appears 

 that this success is due to the 4/3 earth model being in essential agree- 

 ment with the average A^ structure near the earth's surface which largely 

 controls the refraction of radio rays at the small values of 6^ common in 

 tropospheric communications systems. 



It would seem that the deficiency of the effective earth's radius ap- 

 proach could be lessened by modifying that theory in the light of the 

 average A^ structure of the atmosphere. An indication of the average A^ 

 structure was obtained by examining a variety of N profiles which were 

 carefully selected from 39 station-years of individual radiosonde obser- 

 vations to represent the range of A'^ profile conditions during summer and 

 winter at 13 climatically diverse locations. The results of this examina- 

 tion are given in table 3.1. 



