Chapter 8. Applications of Tropospheric 

 Refraction and Refractive Index Models 



8.1. Concerning the Bi-Exponential Nature of 

 the Tropospheric Radio Refractive Index 



8.1.1. Introduction and Background 



The recent explosive growth of space science and telecommunications 

 has spurred the development of new models of the tropospheric radio 

 refractive index to account for the systematic refraction of radio waves 

 and the calculation of theoretical radio field strengths at satellite heights. 

 The simple exponential model has been found to represent, to a first 

 approximation, the average refractive index structure within the first few 

 kilometers above the ground for the United States [1],^ France [2], and 

 Japan [3]. All of the above investigations have reported varying degrees 

 of departure of the atmosphere from this model and Misme [4] has en- 

 deavored to delineate the regions of the world where the exponential 

 model is most applicable, although subsequent analysis of several types 

 of data has shown this model to be more generally applicable than at first 

 sight and not unreasonable for use even in arctic and tropic locations [5]. 



If one considers that A^ is composed of a dry term, 



and a wet term, 



(8.1) 



,^ = -^^g X 10'. (g^2) 



then one may consider the height variation of each term separately. We 

 shall examine the possible advantages of a model of the form 



N(z) = Do exp {-^} + Wo exp {-^} (8-3) 



to describe the average decrease of A'^ with height, where Do and Wo are 

 the values of the dry and wet comi)onents at the earth's surface and Hd 



* figures in brackets indicate the literature references on p. 373. 



311 



