36 MEASURING THE RADIO REFRACTIVE INDEX 



been made to convert them to airborne use. However, where vertical 

 gradients to high ahitude are desired, practical aspects of aircraft opera- 

 tion such as the turning radius and the angle and rate of climb or glide 

 are factors influencing how well the vertical gradients are estimated. 

 Considerable horizontal variation in N is often observed. In addition, 

 the time involved in ascent or descent is relatively long, and the measure- 

 ments are valid only under static conditions for the total time of measure- 

 ment. Due to expense and weight, conventional refractometers are not 

 economical or practical for use in balloon ascent, whether free or tethered. 

 This led to the development of several lightweight refractometers to be 

 used either in wiresonde, radiosonde, or dropsonde applications. 



Figure 2.9 is a block diagram of a lightweight (6 lb), expendable refrac- 

 tometer developed by Deam [37] and Deam and Cole [38] for use as a 

 balloon-borne unit or a dropsonde. The device is a Pound oscillator [31], 

 operating at nominal frequency of 403 Mc/s; the instantaneous frequency 

 is determined by coaxial cavity. The refractive index is sampled by the 

 cavity, and is reflected as a capacitance in the tuned circuit of the oscil- 

 lator. 



Operational tests indicate the electronics are sufficiently stable to pro- 

 duce the desired accuracy although the cavity size and mass appear to 

 introduce a sizable time constant. Present accuracy is estimated to be 

 better than zb5 A'^ units for a complete profile. 



The Hay refractometer [39] was developed as a compromise between 

 the microwave refractometer and the conventional radiosonde (see fig. 

 2.10). It lacks the accuracy of the microwave refractometer by an order 

 of magnitude but, weighing only 7 lb, it incorporates the lightweight 

 feature of the radiosonde. As a balloon-borne instrument it fills a need 

 for more accurate, faster probing of the refractive index at the higher 

 elevations. 



This refractometer is a 10-Mc/s oscillator whose frequency is deter- 

 mined by an air-sensing capacitor. A change in the refractive index of 

 the air passing through the capacitor is reflected as a frequency change 

 in the oscillator. The sensing capacitor is alternately switched with a 

 reference capacitor of identical design but which is protected from the 

 free atmosphere. The use of a reference capacitor diminishes the problem 

 of temperature compensation since both capacitors will be at very nearly 

 the same temperature. To further improve the temperature charac- 

 teristics, the 12 plates of each capacitor are made of invar and are sepa- 

 rated by 0.25-in. quartz spacers. The output of the 10-Mc/s oscillator 

 is doubled in frequency and transmitted to the ground station. 



