478 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
OISTANT ECHO 
RECEIVED 
Figure 8. A typical permanent echo diagram for a VHF radar 
Prediction of Permanent Echoes 
Permanent echoes may be determined by several 
methods: (1) tests with the radar at the site; (2) radar 
planning device [RPD]; (8) supersonic method; and 
(4) profile method. 
The feasibility of moving the radar to the site to 
determine the permanent echoes is dependent on the 
portability, accessibility, etc. Echoes obtained with 
one type of equipment may be very different from 
those of another type of radar with a different an- 
tenna directivity, frequency, and range. 
The RPD technique requires construction of a relief 
model of the terrain considered. A small light source 
is used to simulate the radar transmitter and the 
echoes are plotted as a result of a study of the areas 
illuminated. This method is useful for short ranges 
and microwaves where the diffraction and side and 
back lobe radiation are small. Construction of a 
fairly difficult relief model may take a crew of 
specially trained men several days to a week, as the 
model should be accurate. Once completed, all 
possible sites or aspects from a plane or ship may be 
‘readily examined. Photographic and darkroom fa- 
cilities are required and also kits containing a light 
source, supports, etc. 
The supersonic method uses a relief model under 
water. Supersonic gear is used to send out pulses 
which are reflected like radar pulses and an echo is 
picked up and presented on a plan’ position indi- 
cator [PPI] scope. Photos may be taken of the 
scope or it may be used directly to train operators 
and for briefing. Considerable equipment is re- 
quired, but the construction of the models is com- 
paratively simple. 
The profile method involves a study of topo- 
graphical maps and plotting of the echoes according 
to their visibility and the amount of diffraction. A 
fairly difficult site may be handled in perhaps eight. 
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, prediction of permanent echoes is sim- 
pler and the profile method may be worked out in 
a few hours. 
Prediction by the Profile Method 
The discussion here refers chiefly to VHF (1to10m) 
radars in a mountai ious terrain, but the methods 
have general application. The principal requirements 
are topographic maps of the surrounding area with 
a scale of one or two miles to the inch and a contour 
interval of 20 feet, although intervals up to 100 feet 
may be used. Regional aeronautical maps with a 
Lee) 
KILOMETERS 
Fieure 9 Typical profile. 
Oe. 
