VIM'I.K: \ TIO^. TO (;U(U M) HADVU EQUIPMENTS 



107 



o 

 A 



APPARENT ALTITUOE 



TRUE ALTITUDE 

 HORIZON PLANE 



TRUE EARTH 



TROPICAL ATMOSPHERE DRY Al». ABOVE 



25 50 75 



RANGE IN MILES 



25 50 75 



RANGE IN MILES 



100 



Figure 1. Maximum errors in absolute altitude due to atmospheric refraction. A. True earth radius. B. 4/3 earth 

 radius. Target assumed to be at true angle of zero degrees. 



tion for a range of 29,000 yd and an initial angle of 

 sight 10° are 0.5 mil. A maximum error is obtained 

 at 0.9 mil. For an initial angle of sight of 20° the 

 maximum error is about 0.6 mil as compared to an 

 error in a standard atmosphere of 0.4 mil. Errors in 

 azimuth and range are negligible. 



Early Warning Heightfinding Radar 



Military characteristics call for the following 

 tolerances in heightfinding radars. 



Set Freq. Band Accuracy Required 



AN/CPS-4 S 1,000 ft in absolute altitude and 



500 ft in relative altitude at 45 



miles range, preferably 90 miles. 



AN/CPS-6 S 1,000 ft in absolute altitude and 



500 ft in relative altitude at 75 



miles range, preferably 100 miles. 



AN/TPS-10 X 1,500 ft in absolute altitude and 



500 ft in relative altitude at 50 



miles range. 



Absolute Altitude 



Figure 1A indicates the errors in absolute altitude 

 for different air masses on the assumption that the 

 target is at a true angle of zero degrees. Thus at a 

 range of 75 miles the error in elevation is 940 ft in 

 a standard atmosphere and 1,880 ft in a tropical 

 atmosphere with dry air above. Figure IB depicts 

 the errors on the assumption that the standard 

 atmosphere correction (% earth radius) is applied. 

 Thus with the % earth radius correction the error 

 remains under 1,000 ft at 75 miles. However, these 

 atmospheric conditions represent normal conditions 

 so that in specific meteorological situations the error 

 may exceed 1,000 ft in 75 miles especially in the 

 trade wind regions. 



Since the maximum error occurs at a true angle 

 of 0°, these errors in absolute altitude for a range of 

 75 miles are tabulated for true angles of 0° to 3°. 



