212 



REFLECTION AND TRANSMISSION OF RADIO WAVES 



clutter make the phenomenon a complicated one, by now most of these 

 appear to be understood. 



The radar parameters which control sea clutter are the depression angle 

 (6), polarization, frequency, antenna beamwidths, and pulse length. The 

 last two have been discussed already and may be eliminated, when the 

 return is area extensive. The other three are interrelated. 



Since sea return is back-scattering from the surface itself, factors which 

 affect the illumination of the surface elements responsible for the back 

 scattering have an important effect on o-°. Polarization, frequency, and 

 wave height are such factors, and their effects are intertwined. Katzin^S-^ 

 showed that a number of their effects could be explained on the basis of an 

 illumination of the scattering elements which is the combination of direct 

 and reflected waves, similar to that above a plane reflecting surface. 



As was shown in Paragraph 4-5, interference between the direct and 

 reflected rays creates a lobe structure above the surface. Below the lowest 

 lobe, Pr oc R~^ for a single target. For an extended target distributed in 

 height from the surface upward, this relation still holds if the top of the 

 target is below the first lobe. At nearer ranges, where the target subtends 

 one or more lobes, the target in effect integrates the varying illumination 

 over it, so that the deep ripples of the lobe pattern are smoothed out and 

 Pr cc R"^. For pulsed radar, the illuminated area of the sea surface at small 

 depression angles is proportional to range, in accordance with Equation 



4-60a above, so that the reflection 



Log(R\/4/i) 



Fig. 4-34 Composite Plot of Sea-Clutter 

 Power at Three Frequencies and Six 

 Altitudes from 200 to 10,000 ft: Coor- 

 dinates Normalized to Test Interference 

 Mechanism. 



mechanism just described should 

 give an R~^ range variation at short 

 ranges, and an R~'' range variation 

 at long ranges. 



Fig. 4-34 shows a composite plot 

 of sea-clutter power measured with 

 horizontal polarization at various 

 frequencies and altitudes to test the 

 interference mechanism. Here the 

 measured points clearly define a re- 

 gion where Pr oc R~^, which changes 

 into one where Pr oc R~''. The 

 agreement with the type of behavior 

 just discussed lends strong support 

 to the reflection mechanism. Pre- 

 sumably reflection takes place froni 

 the region ahead of the wave crests 

 in the manner indicated in Fig. 4-35. 



^'•M. Katzin, "Back Scattering From the Sea Surface," IRE Cofitrn/ion Record, 3, (1), 

 72-77 (1955). 



25M. Katzin, On the Mechanisms of Radar Sea Clutter, Proc. IRE 45, 44-54 (1957). 



