224 REFLECTION AND TRANSMISSION OF RADIO WAVES 



Actually, neither the antenna beam nor the pulse shape is rectangular, 

 and the scattering properties of the ground, even if they are area-extensive, 

 may vary with angle, so that a continuous transition from an inverse square 

 to an inverse cube relation takes place. More complicated situations occur 

 when one or more large individual scatterers are located within the illumi- 

 nated area. A more detailed discussion of this problem can be found in a 

 paper by Moore and Williams. ^^ 



4-14 SOLUTIONS TO THE CLUTTER PROBLEM 



Having considered the characteristics of radar targets and of sea and 

 ground clutter, we can now examine these together in order to find the most 

 favorable solution to the clutter problem. There is no unique solution, since 

 the factors involved depend on the operational problem and the limitations 

 placed on the radar parameters. A full discussion of all the considerations 

 and possible solutions is beyond the scope of this chapter, since the problem 

 involves the overall system design and operational philosophy. We shall 

 restrict ourselves to a consideration of certain features of the antisubmarine 

 warfare (ASW) problem, in order to bring out some interesting possibilities 

 based on sea clutter characteristics discussed in Paragraphs 4-10 and 4-11. 



In the first place, an early decision can be made regarding the polarization 

 of the antenna. Both theory and experiment show that sea clutter levels are 

 much lower on horizontal polarization than on vertical polarization. From 

 Fig. 4-37 it is seen that this can amount to 10 db or more. Hence, unless 

 the target shows a preference for vertical polarization by more than this 

 amount, horizontal polarization clearly is to be chosen. Furthermore the 

 discrimination based on target height, which will be discussed below, will 

 be achievable only with horizontal polarization. 



The following discussion will be based on a flat earth and will illustrate 

 the principles involved. The modifications necessary to take into account 

 the effect of the earth's curvature have been described in Paragraph 4-6. 

 These will affect the answer only quantitatively and will not change the 

 nature of the results. 



The primary mission of airborne radar in ASW is search; tracking is a 

 secondary mission. The object of system design and operation is to choose 

 the radar parameters so that the probability of detection is optimized. 

 Inevitably practical limitations will arise which restrict the ranges of certain 

 of the parameters. Ordinary (non-doppler) pulse radar will be considered 

 first, and then the additional improvement due to doppler radar will be 

 discussed briefly. 



8^R. K. Moore and C. S. Williams, Jr., Radar Terrain Return at Near-Vertical Incidence, 

 Proc. IRE ^5, 228-238 (1957). 



