I'KHMWUM KCI-IOKS 



143 



RANGE IN MILES 



Figure 43. Profiles for Example 8. 



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4 5 6 



RANGE IN MILES 



Figure 44. Profiles for Example 8. 



considered an effective shield because of the large 

 diffraction around such obstacles. The map is 

 inspected between the azimuths used and the hori- 

 zontal limit of shielding of a ridge noted. Thus the 

 shielding ridge on 120 degrees (Figure 44) is found 

 to drop off at 138 degrees. From the curves of 

 Figures 38 and 39 are read the ranges for h 2 — h r = 

 1,000, 5,000, 10,000, and 15,000 ft for the line-of- 

 sight angles at various azimuths. These points are 

 plotted on Figure 42; they are connected by heavy 

 dashed lines and are the coverage contours. 



In Figure 45 are plotted the predicted echoes. It 

 will be noted that the shielding to the east is very 

 good and most of the mountains are not visible. To 



the north the numerous mountains are unshielded 

 and give rise to many echoes which extend into the 

 search sector to the west. The islands are inherently 

 bad and cannot be shielded without drastic loss of 

 coverage. In some cases, as along azimuth 345°, 

 ridges which cause large echoes shield more distant 

 ridges. The broken terrain in this region is taken to 

 give one large echo rather than a number of small 

 echoes. In most cases the simple rules for plotting 

 echoes may be applied directly. 



Where diffraction is involved the procedure should 

 be more detailed. In Figure 43 azimuth 20° will be 

 examined to determine the visibility of the hill at 

 4.65 miles. The following data are obtained from 

 the profile. 



hi = 387 ft; d\ - d' 2 = 1.45 miles 

 h, = 550 ft; d't = 3.20 miles 



H = 425 ft; d'i = 4.65 miles 



From equation (8) : 



4.65 X 550 - 3.20 X 387 , 4.65 X 3.20 



4.65 - 3.20 

 = 917.2 ft . 



From equation (10) : 



425 - 917.2 



- + 



5,280 X 4.65 



X 57.3 = -1.15° . 



From Figure 40 the intensity is found to be 15 

 per cent. At the very short range of this hill a strong 

 echo would be expected at this intensity, and all 

 lobes would be plotted. 



At 138.5° azimuth and 160 miles is a 10,000-ft 

 mountain (not shown in any figure). The data for 

 this case are: 



h = 387 ft; d'i - d', = 0.27 miles 



&2 = 380 ft; d'i = 160 miles approximately 



H = 10,000 ft; d\ = 160 miles approximately 



, 160 X 380 - 160 X 387 160 X 160 

 0.27 " 2 



16,950 ft. 



10,000 - 16.950 

 5,280 X 160 



X 57.3 = -0.472°. 



From Figure 40 the relative intensity is 43 per 

 cent. A main lobe echo is plotted on the second sweep 

 at 10 miles since the first sweep is only 150 miles. 



In Figure 35 is shown a large echo at 110 miles 

 from 175 to 242 degrees. This is received only when 



