Chapter 2. Measuring the Radio 

 Refractive Index 



2.1. The Measurement of the Radio 

 Refractive Index 



The radio refractive index is defined as the ratio of the speed of propa- 

 gation of radio energy in a vacuum to the speed in a specified medium. 

 The radio refractive index may be measured directly if the measuring 

 instrument is sensitive to the speed of propagation. Refractive index is 

 measured indirectly by measuring temperature, pressure, and humidity 

 with subsequent conversion to the refractive index as indicated in the 

 previous chapter. The direct method employs radio frequency refrac- 

 tometers to determine the refractive index; the indirect method, standard 

 weather observations. The direct method appears preferable because 

 accuracy is dependent upon a single sensor, rather than three and, of 

 course, because A'^ values are obtained directly. However, refractometers 

 are relatively complex and expensive devices requiring no small degree of 

 skill to maintain. Hence, as yet, refractometers are not in general use, 

 or even available in sufficient quantity to permit large-scale mapping of 

 refractive index structures. The bulk of the synoptic and climatological 

 mapping of refractive index is still based on the indirect method of 

 measurement. The laborious task of converting the measured parame- 

 ters to refractive index has been somewhat alleviated by the development 

 of special analog computers [1]' and digital computers. 



2.2. Indirect Measurement of the Radio 

 Refractive Index 



The accuracy of the determination of the refractive index from standard 

 weather observations has been discussed in chapter 1. Figure 2.1 illus- 

 trates the degree of accuracy to be expected in the measurement of the 

 refractive index as a function of the accuracies of the sensors for sea-level 

 conditions. It is assumed that the errors are additive. Planes of equal 



' Figures in brackets indicate the literature references on p. 45. 



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