Til K CALCULATION OK VKKTICAL COVERAGE 



145 



he treated virtually as a searchlight, as bhe back 

 radiation and diffraction effects are small. Trapping 

 is likely to be severe and in some regions it is the 

 controlling factor. Sea or land clutters are important 

 and the extent of such echoes may be estimated 

 from equation (16). 



Microwave sets because of their narrow beam- 

 width, high resolution, and PPI presentation are 

 well adapted to navigational uses. Coastlines may 

 be readily identified, and ships near land may 

 accurately determine their position. Over land it is 

 frequently difficult to correlate a PPI picture with 

 a map. In many cases it may be very desirable to 

 be able to locate terrain features accurately. 



The presence of some distinctive echo is of great 

 assistance in orientation of the picture, but scope 

 distortions and the nature of the echoes cause much 

 confusion. It is therefore desirable to be able to 

 correct the distortions and to be able to prepare a 

 radar map which shows the terrain features likely 

 to contribute to the observed pattern. 



The PPI distortions are due to the beamwidth, 

 range marker errors, and nonlinear sweep. The 

 width of the beam causes objects to appear wider 

 than they are, as discussed in preceding sections. 

 The range marker errors may be determined by 

 calibration with a precision-type calibrator. By 

 preparing a cardboard scale to line up with the range 

 pips the correct ranges of echoes may be obtained. 

 Because the sweep usually takes about 15 ^sec to 

 attain a steady speed the pattern is displaced inward 

 with respect to the map. This may be compensated 

 in part by adjusting the centering control so that 

 at least one of the range markers is moved out 

 radially to its true range. The pattern will then 

 show a central hole, and the first half mile will be 

 displaced from its true position, but the pattern 

 as a whole will be more accurate. 



For construction of the radar map it is desirable 

 to have topographic sheets of a scale of 1 to 20,000 

 which show modern structures. Aerial photographs 

 are also useful. Map matching is done by adjusting 

 the sweep length and centering controls with major 

 changes in scale made photographically. To eliminate 

 detail of little interest it is desirable to ink in only 

 those contours which correspond to equal increments 

 of radar range based on the curved surface of the 

 earth. That is, the retraced contour intervals should 

 form a sequence of squared numbers (1, 4, 9, 16, 

 25 ■ ■ • n 2 ), for example, 20, 80, 180, 320, and 500 ft. 

 The amount of distortion to introduce into the radar 



map is obtained from the range correction scale and 

 the shift of the PPI center. For each azimuth 

 considered the map is shifted to compensate for 

 the centering error, and the corrected range scale 

 is used to lay off distance. 



150 THE CALCULATION 



OF VERTICAL COVERAGE 



15.6.1 



Introduction 



The computation of vertical coverage diagrams in 

 the optical region consists essentially of adding two 

 vectors, the contributions of the direct and reflected 

 waves, which have been modified by earth curva- 

 ture, antenna directivity, etc. The actual computa- 

 tion of the contours of constant field strength tends 

 to be laborious because of the implicit nature of the 

 parameters. The problem may be formulated in a 

 rigorous, general manner, but the solution is likely 

 to be unwieldy. 



For field purposes where high accuracy is not 

 required, a method of computing vertical lobe 

 patterns is desired that is direct, does not require 

 excessive calculations, provides a simple physical 

 interpretation of terrain effects, and is flexible. The 

 methods presented here are designed to meet these 

 requirements, and the computer may readily accom- 

 modate the labor of calculations to the required 

 accuracy and the complexity of the problem. 



The path difference of the direct and reflected 

 rays, the distance of the reflection point, and the 

 vertical angle are functions of each other, while the 

 reflection coefficient, the divergence factor, and other 

 factors depend on the vertical angle. It is therefore 

 desirable to examine the problem in a general way 

 to determine what simplifications may be introduced. 



With microwaves the reflecting surface must be 

 quite smooth to be effective. Thus by equation (16) 

 for the S band and an angle of 1 degree the roughness 

 must be less than 15 in. if the reflection is to be of 

 much assistance. The rolling character of sea waves 

 makes a substantial variation in signal strength so 

 that the reliable range is only sl : ghtly greater than 

 that of the direct wave alone. Also highly directive 

 antennas are commonly used with microwave radars. 

 These factors reduce the magnitude of the interfer- 

 ence effects. The fineness of the structure of a micro- 

 wave pattern and the relatively weak reflection 

 effects commonly encountered therefore render it a 

 useful approximation to deal with the direct wave 

 pattern only for most purposes. 



