356 



REFRACTION AND REFRACTIVE INDEX MODELS 



From the experimental data which are available at the present time 

 it may be concluded that: 



(a) Radio range and elevation angle errors can be predicted from the 

 surface value of the radio refractive index, and the accuracy obtained will 

 be generally commensurate with the estimates of residual errors made 

 from theoretical ray-tracing considerations. 



(b) The functional dependence of either angular refraction or range 

 errors on the surface value of the refractive index as derived from the 

 CRPL standard A'' profile sample may be applied to arbitrary locations 

 or climates without noticeable decrease in accuracy over that obtained 

 with a sample from the location under consideration. 



(c) The effects of horizontal inhomogeneities of the refractive index, 

 which certainly must have been prevalent over the transmission paths for 

 which experimental data have been presented, do not appear to introduce 

 any bias or additional residual variance into the values of observed re- 

 fraction variables over those j^redicted from surface observations. 



Table 8.7. Comparison of slopes for independent check 

 Predicted slope at 0o=2O mrad: 0. 358 mrad/iVs 





-Nsr- 



S. E. 



8.4. Correction of Atmospheric Refraction 

 Errors in Radio Height Finding 



8.4.1. Introduction 



As a radio ray passes through the atmosphere, the length and direction 

 of its path varies with the radio refractive index. Uncorrected radar 

 output determines the position of a target by a straight line path at con- 

 stant velocity. The difference between the straight path and the actual 

 path results in an error which becomes increasingly significant as the 

 distance to the target increases. The height error (the component of the 

 position error normal to the surface of the earth) constitutes over 95 per- 

 cent of the total error. Until recently, the range of height finding equip- 

 ment was sufficiently limited so that the refraction errors could be either 

 neglected, or approximated by the effective (four-thirds) earth's radius 

 correction [27]. 



