232 



TRANSHORIZON PARAMETERS 



1945 



1946 



Figure 6.4. Field reception of W2XMN on 42.8 Mc/s at Needham, Mass., 1945-1946. 



(After Pickard and Stetson, 1947). 



signal levels on VHF paths of comparable length, frequency and angular 

 distance. 



The primary purpose of a radio-meteorological parameter is to provide 

 the best estimate of the statistical distribution of field strength (in terms of 

 hourly, daily, weekly, or monthly median values as required on a specific 

 path). The reliability of the parameter must be judged solely in terms of 

 this requirement, and care must be exercised in assessing the value of any 

 given parameter in t^rms of data obtained over limited intervals of time 

 or from restricted geographical areas. Discussions later in the chapter 

 will consider to what extent it is possible to develop a parameter which, 

 in addition to being statistically reliable, is also characteristic of the 

 physical structure of the atmosphere. 



The present practice in applying radio-meteorological parameters con- 

 sists in determining an average signal level for a given distance and then 

 adjusting this average level for climatic and seasonal differences by refer- 

 ence to the changes in some function of the refractive index of the atmos- 

 phere. We may express this procedure mathematically, for a linear 

 regression model, thus: 



E = b ■ f{n) + a 



(6.1) 



where E is the field strength, and b is the regression coefficient expressing 



