30 



MEASURING THE RADIO REFRACTIVE INDEX 



Figure 2.5. Schematic diagram of the Navy Electronics Laboratory (Thiesen) system. 



sonde so that many more samples of each parameter were produced per 

 unit time. CHnger and Straiton [14] developed a radiosonde that com- 

 bines the parameters so that the output signal is in terms of the refractive 

 index (see fig. 2.4). Since the wet and dry terms are additive, a parallel 

 combination of independent conductances can be used. The dry term 

 sensor has a conductance proportional to temperature and inversely pro- 

 portional to pressure; the wet term sensor is such that it not only yields 

 a value proportional to the relative humidity, but also adjusts the relative 

 magnitude of the two terms. Thiesen [15] devised a similar radiosonde 

 utilizing two separate sondes on the same balloon (see fig. 2.5). The 

 temperature and humidity information are combined at the ground sta- 

 tion by means of an analog computer. For captive balloon application, 

 Hirao and Akita [8] and Crozier [9] developed similar systems using wet 

 and dry bulb thermistors to produce direct output in refractive index. 



2.3. Direct IVleasurement of the Refractive Index 



The resonant frequency of a microwave cavity is a function of its 

 dimensions and the refractive index of its contents. Hence, if a cavity is 

 open to the atmosphere, the resonant frequency changes as the refractive 

 index of the air passing through it changes. If a sealed reference cavity is 



