98 TECHNICAL SURVEY 
Ficure 45. Predicted permanent echoes. (Example 8.) 
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 
there is trapping, and the size of this echo indicates 
the unusual weather conditions at the time the data 
were taken. This echo is due to 2 mountainous island 
260 miles away and about 5,000 ft high. There is 
no shielding except from the curvature of the earth. 
The relative intensity compared to the free space 
intensity computed from the formulas for the dif- 
fractive region (not given here) is 0.5 per cent: This 
echo would not ordinarily be plotted in spite of the 
large area of the mountain side. 
In correlating the predicted and actual fixed echo 
diagrams, Figures 45 and 35 respectively, it will be 
noted that the degree of success achieved depends 
on the effort expended. Numerous small echoes were 
not predicted, but these are unimportant from an 
operating standpoint. “‘Permanent’’ echoes vary over 
wide limits with changes in weather conditions and 
efficiency of the equipment so that only a fair 
agreement should be expected in their predictions. 
Microwave Permanent Echoes 
With microwave equipment a simple analysis of 
the terrain is generally sufficient. The beam may 
be treated virtually as a searchlight, as the back 
radiation and diffraction effects are small. Trapping 
is likely to be severe and in some regions it is the 
controlling factor. Sea or land clutters are important 
and the extent of such echoes may be estimated 
from equation (16). 
Microwave sets because of. their narrow beam- 
width, high resolution, and PPI presentation are 
well adapted to navigational uses. Coastlines may 
be readily identified, and ships near land may 
accurately determine their position. Over land it is 
frequently difficult to correlate a PPI picture with 
a map. In many cases it may be very desirable to 
be able to locate terrain features accurately. 
