4-10] SEA RETURN 213 



As was shown in Paragraph 4-5, 

 vertical polarization produces a 

 much stronger field on and just 

 above a reflecting surface than does 

 horizontal polarization. Hence the 

 scattering elements of the sea sur- 

 face are more strongly illuminated if p^^. 4-35 Possible Geometry of Reflec- 

 vertical polarization is used so that ted Wave from Sea Surface, 

 sea clutter at low angles is much 



stronger with vertical than with horizontal polarization, assuming that the 

 same for both polarizations. 



Because of the presence of reflected waves, the appropriate radar equa- 

 tion for sea clutter is obtained from Equations 4-1 and 4-59: 



where F is a suitable average value of F'^. For a uniform distribution of a 

 with height above the surface, Katzin^^ gives 



F = 6, R< Rt (4-62a) 



F=6{Rt/R)\ R> Rt (4-62b) 



where Rt is the transition range between the R~^ and R~^ regions. The 

 simple plane surface reflection theory for a surface with a reflection coeffi- 

 cient of — 1 gives 



Rt = 5hH/\ (4-63) 



where h is the radar height and H the height of the top of the target, which 

 here is to be interpreted as the height of the wave tops above the equivalent 

 reflecting plane. Since wave heights themselves are distributed in a 

 statistical manner, and the location of the equivalent reflecting plane is not 

 known, an empirical relation must be deduced from experiment. A limited 

 amount of experimental evidence suggests the relation 



Rt = 2A//i/io/X (4-64) 



in which //i/iu is the crest-to-trough wave height exceeded by 10 per cent 

 of the waves (a unit frequently used by oceanographers). 



A further consequence of the reflection interference phenomenon at very 

 small depression angles is that the return no longer remains "area exten- 

 sive." The appearance of the sea clutter on an A scope then breaks up into 

 a series of discrete echoes or "spikes" which appear much like individual 

 targets. These can persist at fixed ranges for periods of a number of seconds. 

 Fig. 4-36 shows an example of this. Spikiness is explainable by the com- 



