l'KKMAISHNT ECHOES 



141 



be visible except by diffraction, and targets above 

 the contours are in line of sight, and receive direct 

 radiation. For each azimuth and the corresponding 

 angle of sight ( Figure 36) the ranges are plotted for 

 various contour heights as 1,000, 5,000, 10,000, and 

 15,000 feet. Where these coverage contour lines are 

 close together the shielding is good but the coverage 

 is poor; where the lines are widely separated, the 

 shielding is weak, and toward the sea there is no 

 shielding except by the horizon. 



With the coverage contour diagram superimposed 

 on a map, the peaks exposed to radiation may be 

 noted. The extent of the echoes due to these peaks 

 depends on the horizontal radiation pattern and 

 pulse width. The horizontal beam width is only a 

 very rough measure of the width of an echo, and 

 some other angle usually between the half-power 

 points and the nulls will determine the echo width. 

 The angle may be estimated by considering the range 

 and size of the peak. The extension of the echo in 

 range will be at least as great as the pulse width in 

 miles, which as it appears on the indicator is about 

 0.1 mile per usee. Actual echoes are usually much 

 wider than this, as all of the exposed hill sends back 

 an echo. 



The echoes are then sketched in, based on inspec- 

 tion of the profiles. The plotter's judgment is a very 

 important factor, but the following rules may be 

 used as a guide. 



1. Shade in a circle for the main pulse several 

 miles wide, depending on the pulse width and local 

 return. 



2. Consider each profile in turn and for each peak 

 or hillside in front of the shielding plot an echo on 

 the main and all sidelobes. 



3. A series of sharp hills within the shielding 

 region should be plotted as a single echo rather than 

 a number of echoes. 



4. The inner edge of an echo should be at the same 

 range as the hill, and its extension depends on the 

 slope of the hill and the pulse width, which may be 

 several miles with some sets. 



5. In case of doubt plot the echo. 



6. Peaks beyond the shield may be in the diffrac- 

 tion region and the relative intensity of the radiation 

 at these peaks will then be obtained from Figure 40 

 as described above. 



7. If the mountain is large enough to intercept 

 several lobes, the interference effects may be ignored. 

 The echo strength may be estimated roughly as 

 proportional to the cross-sectional area of the moun- 



tain, the relative intensity of the radiation from 

 Figure 40, and the inverse square of the distance. 

 For side and back lobes an additional factor is 

 required. 



8. The 1 to 1 ,000,000 scale map should be carefully 

 checked to make sure that no peaks are missed in 

 between the azimuth considered or at extreme ranges. 



In the above method much is left to the judgment 

 of the plotter but it will be found that with experience 

 a reasonably good estimate of permanent echoes may 

 be made from a map. 



Example 8. Profile Method. A detailed example of 

 a difficult site will be worked out, and comparison 

 will be made with the actual recorded echoes. The 

 site selected is that of Figure 35. The characteristics 

 of the SCR-270B radar are given in Table 2. 



Table 2. Type SCR-270B. Characteristics of antenna 

 pattern. 



Other characteristics of this set are as follows: 



Pulse width 



Nominal range 



Sweep sector 



Elevation: center of antenna 



30 jusec = 3 miles 

 150 miles 

 185° to 290° 

 387 ft 



From these data may be calculated the relative 

 echo strengths of mountains at various distances 

 and the relative side and back echoes. A reference 

 value of 1.0 is taken for the main echo from a typical 

 mountain 100 miles distant, and the relative intensity 

 from Figure 40 is taken equal to 1.0. It is estimated 

 that all echoes whose strength compared to the 

 reference value is over 0.25 will be strong enough to 

 obscure targets. Thus the back echo of a mountain 

 10 miles away in a diffraction region where the 

 relative strength is 0.5 would have an echo value 

 of (100/10) 2 X 0.5 X 0.04 = 2.0 and should be 

 plotted since it exceeds 0.25. 



A table may be constructed for the main, side, 

 and back lobes (M, S, B) for various distances and 

 degrees of diffraction to show which echoes should 

 be plotted. Table 3 is such a table, corresponding to 

 a reference strength equal to 0.25. This table will 

 apply only for the conditions of this example. 



