GEOMETRICAL LIMITS OF VISIBILITY 



189 



When both terminals of a path are elevated above '"'^-^ Height of Obstacle 



the ground (Figure 3), the horizon distance is 



__ _ As a first case, consider a smooth earth and two 



ch = ^2ka {^llh + V/zo), (6) terminals at the ground. The earth itself forms an 



+ 24 



+ 20 



+ 16 



in +12 



tn 1- 



Si 



O 2 



5° 



+ 8 

 + 4 



-20 

 -24 



J II 21 31, 10 20 ,2 12 22, t II 21 J II 21 31, 10 2 30, 10 2 30, 9 19 29 8 1 8 26, 8 1 8 28, 7 17 27 7 1 7 27, 



JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



SUN DATA FROM NAUTICAL ALMANAC 1945 

 Figure 1. Caloulation of solar azimuth. 



T 



FiGUEE 2. Geometry for horizon di.stance for zero 

 height transmitter. 



FiGUBK 3. Geometry for liorizon distance with ele- 

 vated transmitter. 



where again V2/>y( = 4,120 iii the metric system, obstacle which reaches its maximum height /?^ in the 

 If d is in statute miles and h in feet, middle of the path (Figure 4). By equation (2) 



rfz, = V2/11 + V2/i2 for fc = - • 



(7) 



h,„, = 



G-y 



d- 



(8) 



The relation between /)i, lu and d^ is graphically 2A-a Bka ' 



presented in Chapter 5 in the form of a nomogram, A point P on the ground at distances d' and d" from 



Figure 2. the two terminals (see Figure 4) has an elevation 



