346 



REFRACTION AND REFRACTIVE INDEX MODELS 



ELEVATION ANGLE, 9^, DEGREES 



26° 40" 55° 65° 



Figure 8.20. Comparison of measured total atmospheric refraction of 1.85 cm radio 

 waves at Cedar Rapids, Iowa, with values predicted from Ns. 



what one would expect to observe if all of the measured values of refrac- 

 tion were increased by a systematic calibration error of about 50 /xrad 

 over their correct values. The standard deviations in figure 8.20 tend 

 to flatten out at high elevation angles, an effect which is to be expected 

 theoretically as pointed out previously. At any rate, the largest differ- 

 ence between the observed data and the predicted curve in figure 8.20 

 at elevation angles over 30°, is only about 50 /urad or 10 sec of arc (the 

 angular diameter of the planet Mars at its average distance from the 

 earth is 10 sec of arc, an angle not discernible to the naked eye). Al- 

 though this discrepancy might be significant militarily, it is only about 

 0.5 percent of the diameter of the target sun and is probably near the 

 limit of accuracy of the equipment used. 



