26 MEASURING THE RADIO REFRACTIVE INDEX 



device operating over a range of ±100 nibar with reference to an average 

 value with time constants of perhaps 5 msec. The capacitive micro- 

 phone is capable of accuracies of ±0.01 mbar and lag constants of several 

 milliseconds over a limited range (±2 mbar). The resolution diminishes 

 to about ±0.1 mbar over the normal range of variations of atmospheric 

 pressure. The capacitive microphone requires correction for significant 

 temperature variations. Strain-gage pressure transducers have approxi- 

 mately the same characteristics as capacitive microphones with some 

 degradation in lag constants. Differential strain gauge transducers oper- 

 ating over a range of 4 to 5 mbar may yield accuracies of several hundred- 

 ths of a millibar and are relatively free of temperature effects. 



Relative humidity reciuires the greatest care in measurement. The 

 common lithium chloride strip is accurate to within ±5 percent in relative 

 humidity. In general, the lag constant is of the order 8 to 10 sec for 

 temperatures in excess of °C (see sec. 2.5). Other sensors under de- 

 velopment show promise. The barium fluoride strip [5] yields accuracies, 

 hysteresis effects, and response times far superior to lithium chloride but 

 has the disadvantage of rapid aging. Phosphorous pentoxide sensors [6] 

 and aluminum oxide sensors [7] appear to be quite promising. The most 

 accurate method that is available for measuring water vapor pressure is 

 the wet-dry bulb technique using electrical thermometers (±0.02 mbar). 

 The lag constant will be relatively large, as it is a function of the wetting 

 properties of the wick, the rate of aspiration, and even the relative 

 humidity, and is thus recommended only for temperatures above —24 °C. 



Measurements up to several thousand feet can be made from towers or 

 with tethered balloons or a wiresonde. Only the sensors need be sent 

 aloft. All auxiliary devices are on the ground, and long cables transmit 

 the information from the sensors to the recording equipment [8, 9]. 



The radiosonde is in almost universal use for high-altitude measure- 

 ment of the meteorological parameters influencing the refractivity. There 

 are many models of the radiosonde, differing from country to country. 

 The principle of operation for present American radiosondes is illustrated 

 in figure 2.2. An aneroid capsule is used as the active element of a 

 baroswitch. The temperature sensor, humidity sensor, and a reference 

 resistance are alternately switched into the grid circuit of a blocking 

 oscillator as the baroswitch wiper moves under the action of decreasing 

 pressure. The blocking oscillator controls the pulse rate of the rf trans- 

 mitter; hence, the pulse rate of the transmission is indicative of the value 

 of the sensor being sampled. The number of the contact energized is a 

 measure of the pressure; the switching sequence permits identification 

 of temperature and humidity values. A constant rate of ascent of the 

 balloon is assumed so that the values of the parameters can be identified 

 with the proper altitude. 



