92 TECHNICAL SURVEY 
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. j 
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. 
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. 
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 thé 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 dificult model may take a crew of men several 
days to a week, as a model should be accurate. 
Once completed, all possible sites or aspects from a 
plane or ship may be readily examined. Models of 
enemy areas may be used to predict the coverage 
of possible enemy sites, and evasive action may be 
planned. The RPD is well suited for training and 
briefing of air personnel. Kits are provided contain- 
ing the light source, supports, etc. Darkroom facilities 
are required, and special processing of films is used 
to secure more realistic pictures. 
The supersonic method requires a model made of 
sand, glass beads, etc., to be used under water. Such 
models are much easier to construct than the RPD 
type. Supersonic gear is used to send out pulses 
which are reflected like radar pulses and the echo 
is picked up and presented on a PPI scope. Photos 
may be taken of the scope picture, and the method 
may also be used for training and briefing. Special 
equipment is required, but the models may be made 
easily and the presentation is obtained direct on the 
PPI scope without further processing. This method 
is well adapted for training, as flight, changes in 
altitude, etc., may be simulated readily by movement 
