24 



MEASURING THE RADIO REFRACTIVE INDEX 



Figure 2.1. Accuracy of (he delermination of refractivity as a function of the accuracies 



of the meteorological sensors. 



accuracy are shown for accuracies of ±0.1, ±0.5, ±1.0, and ±2.0 A'^. 

 It can be seen that a measurement error of ±1.0 A'^ can be contributed 

 by each sensor if the sensor errors exceed ±0.8 °C for temperature, ±3.7 

 mbar for total pressure, and ±0.22 mbar for vapor pressure measure- 

 ments. It is apparent that extreme accuracy is required in the measure- 

 ment of the water pressure. The temperature can be measured easily to 

 within several tenths of a degree, the total pressure to within several 

 millibars, but the humidity is considerably less responsive to the same 

 degree of relative accuracy. If the wet and dry bulb hygrometer tech- 

 nique is used to measure the humidity with subsequent conversion using 

 either the psychrometric formula of Sprung- [2] or List^ [3], the degree of 

 accuracy is seen to be a function of temperature. For an error of no 

 greater that ±1.0 A^, the relative humidity must be accurate to within 

 0.33 percent at 35 °C, while at °C an accuracy of 3.0 percent is necessary. 



e = Cs — c , c IS a constant. 



566 



e = Cs — [fi(l + Co/u.)] p(AT'), Ci and c^ are constants. 



