I'KKMANENT KCHOKS 



137 



other echoes, (2) separated from the transmitter by 

 rough Donreflecting land, (3) a single distinct target 

 such as a steel tower, (4) weak in response, that is, 

 comparable to that of a distant aircraft. 



The range of echoes which come in on the second 

 or third pulse may be estimated by adding one or 

 two times the length of the range scale to the 

 observed range plus an allowance for the return 

 trace time, usually several miles. To determine by 

 test which sweep an echo is associated with, the 

 pulse repetition rate should be changed, and the 

 shift in range of the echo observed. Thus if the range 

 scale is 200 miles long and the pulse rate is reduced 

 10 per cent, then a target at 250 miles which had 

 been appearing at 48 miles would shift to an indi- 

 cated range of 23 miles and could thus be distin- 

 guished from a 48-mile target that would shift to 

 43.2 miles. 



Frequency-sensitive permanent echoes are not 

 suitable for checking range accuracy. The frequency 

 changes from maximum to minimum return are 

 usually too small to be detected on a frequency 

 meter, so that frequency-sensitive echoes are recog- 

 nized chiefly by their unsteady appearance. 



Azimuths may be determined to best accuracy 

 by "beam splitting." This consists in turning the 

 antenna slightly to one side of the maximum until 

 the signal decreases to a predetermined level. The 

 antenna is then turned past the maximum until the 

 same level is reached and the two azimuths are 

 averaged. When checking azimuth accuracy the 

 possibility of horizontal diffraction due to a nearby 

 hill should be considered. 



1554 Shielding 



The principal device for control of fixed echoes is 

 shielding. This means that the antenna is to be sited 

 in such a way that distant hills are screened by a 

 local obstruction. A local echo at say 3 miles, is 

 combined with the main pulse or ground return, 

 and the distant echo is weakened or eliminated 

 entirely. In operating regions the loss of coverage 

 may be more serious than the permanent echo, so 

 shielding should be used with caution. 



Rear areas which are not scanned should be well 

 shielded so that back and side echoes do not interfere 

 with targets in important tactical regions. Operation 

 over such shielded sectors would be limited to high 

 targets. 



Construction of artificial shields made of poultry 



netting has been suggested in some cases, where the 

 back radiation and side lobes were relatively strong. 

 The very large size of such structures ordinarily 

 renders them impractical. Most of the antennas 

 using parabolas have a small back radiation, and 

 permanent echo problems are much simpler. 



In special cases it may be desirable to eliminate 

 a particular echo from some obstacle without using 

 shielding. This may be done by constructing a target 

 of sheet metal on the side of the obstacle, spaced so 

 that the target echo and obstacle echo are about 

 180° out of phase. This requires accurate alignment 

 of the target (so that it is normal to the radiation to 

 within 5° or less) and close control of the frequency. 

 It is also necessary that the area be adjusted so that 

 the response of the target and obstacle are equal. 



io o.o Prediction of Permanent Echoes 



Permanent echoes may be determined by several 

 methods : 



1. Tests with the radar at the site. 



2. Profile method. 



3. Radar planning device [RPD]. 



4. Supersonic method. 



The feasibility of moving the radar to the site to 

 determine the permanent echoes is dependent on 

 portability, accessibility, etc. Echoes obtained with 

 one type of equipment may be very different from 

 those from another type of radar with different 

 directivity, frequency, and range. 



The profile method, which will be described in 

 detail below, involves a study of topographical maps 

 and plotting the echoes according to their visibility 

 and the amount of diffraction. A fairly difficult site 

 may be handled in perhaps 8 man-hours. This method 

 is adapted to long-range, low-frequency radars where 

 diffraction and side and back lobe radiation are 

 important. On microwave equipment fixed echo 

 prediction is simpler and the profile method may be 

 worked out in a few hours. 



The RPD technique requires construction of a 

 relief model of the terrain considered. A small light 

 source is used to simulate the radar and the echoes 

 are plotted as a result of a study of the areas illu- 

 minated. This method is adapted for short ranges 

 and microwaves where the diffraction and side and 

 back lobe radiation are small. Construction of a 

 fairly difficult model may take a crew of men several 

 days to a week, as a model should be accurate. 



