PERMANKNT ECHOES 



19a 



10.4.4 



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]; (3) 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 \A'ith 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. 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. 



200 



SITE 



4 6 8 10 12 



KILOMETERS 



Figure 9. Typical profile. 



Kits are provided containing the light source, sup- 

 ports, etc. Photographic and darkroom facilities 

 are also required. 



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. 



10.4.5 Prediction by the Profile Method 



The discussion here refers chiefly to VHF (1 to 10 m) 

 radars in a mountainous 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 

 scale of about 1 inch to 16 miles and 1,000-ft 

 contours are suitable for checking distant echoes. 



From the maps, profiles are prepared for various 

 azimuths about the radar station. The first mile or so 

 should be plotted accurately; at greater distances, 

 the critical points, such as hills and breaks, should 

 leceive the most attention. On each profile is drawn 

 the tangent line from the center of the antenna to 

 the point on the profile which determines the shield- 

 ing, as in Figure 9. The angular elevation a of this 

 line of sight is marked on the diagram. If a is nega- 

 tive, the profile should be checked out to the radar 

 horizon to obtain the correct shielding angle. On a 

 plane earth diagram, the line of sight is actually 

 curved, but for distances up to 10 miles it may be 

 taken as straight with small error. The height 

 difference, with a in radians, is then ecjual to 



/i2 — hi = dtana. (18) 



When the distance is larger so that earth curvature 

 has to be taken into account, to the above expression 

 for the height difference ho — hi must then be added 

 the amount by M-hich the earth is sloping eway over 

 the distance d. This amount is d-/2ka, and the 

 complete expression for /^o — hi becomes 



7,2 - hi = dtana + ~. (19) 



2ka 



For easier handling of this equation, a set of 

 curves may be drawn where hi — hi is plotted against 

 d for various constant values of a. These curves 

 may then be used to determine the height of the 



