138 



SITING AND COVERAGE OF GROUND RADARS 



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 

 of the sonar head. 



In general the profile method should be used on 

 long waves or on microwaves where only a few sites 

 are being considered. It is well adapted for the 

 estimation of nonstandard atmospheric effects. For 

 air- or ship-borne radar the RPD or supersonic 

 methods are convenient because of the large number 

 of aspects involved. It may be noted that the latter 

 two methods should not be considered more exact 

 than the profile method, as the principle of similitude 

 does not apply unless all elements including the 

 wavelength are changed in proportion. The principal 

 difficulty is to secure a source which has the same 

 radiation characteristics as the antenna system. 



15.5.6 Prediction by Profile Method 



The profile method will be described in detail. The 

 discussion will refer chiefly to VHF radars in a 

 mountainous terrain, but the methods have general 

 application. The principal requirements are topo- 

 graphic maps of the surrounding area with a scale 

 of 1 or 2 miles to the inch and a contour interval 

 of 20 ft, although intervals up to 100 ft may be 

 used. Maps with a scale of about 20 miles to the 

 inch are needed for checking distant echoes. Regional 

 aeronautical maps, with a scale of about 1 inch to 

 16 miles and 1,000-ft contours, are suitable as the 

 height of prominent peaks is indicated. 



From the maps, profiles are prepared for various 

 azimuths about the radar station. The first mile or 

 so should be plotted accurately, and at greater 

 distances the critical points such as hills and breaks 

 should receive the most attention. A convenient 

 scale is 2 miles to the inch for range and 500 ft to 

 the inch for elevation. The distances to which the 

 profile should be plotted is a matter of judgment, 

 but it should be extended to perhaps 20 miles, or 

 further if there is doubt. 



On each profile is drawn the tangent line from the 

 center of the antenna to the point on the profile 

 which determines the shielding, as in Figure 36. 



1000r 



500 



SITE 



4 6 



RANGE IN MILES 



Figure 36. Typical profile. (Note: 7 in degrees). 



This is the line-of-sight curve; it is drawn for each 

 azimuth, and the vertical angle y is marked on the 

 profile. If the angle is below the horizontal it is 

 negative, and caution must be used on high sites 

 not to exceed the limiting shielding angle of the 

 radar horizon. This is given by the expression 



y = -0.0108 V2hi , (45) 



where y is the angle between the effective horizon 

 and the horizontal at the antenna in degrees and hi 

 is the height of the center of the antenna in feet. 

 The line of sight is actually curved, as explained 

 in the section on visibility problems, but for ranges 

 up to 10 miles the error in using a straight line is 

 small. For longer distances the dip QX as computed 

 from equation (5) should be considered. More con- 

 venient for this purpose are the curves of the line 

 of sight for various angles which are calculated from 

 Figure 37. Standard refraction is taken into account 

 by use of % a instead of a for the earth's radius, 



| a = 1.33 X 3,960 = 5,280 , 



h = 5,280d tan y + 



(46) 



2' 



with hi and h 2 in feet and d in miles. Above 10°, or 

 where the shielding is distant, equation (8) should 

 be used. 



