4-10] SEA RETURN 211 



4-10 SEA RETURN23 



In detection or tracking of targets on or near the surface, it is necessary 

 to be able to distinguish the target from the background clutter due to 

 reflections from the surface itself. For example, the AEW system discussed 

 in Chapter 2 was required to distinguish between sea return and target 

 echoes. In order to design a radar for such an application, the mechanism 

 of sea return and its relation to radar and tactical parameters must be 

 understood. 



In operations over water, this sea return or sea clutter is caused by all 

 elements of the surface within a resolution element of the radar. Since this 

 surface area is a function of pulse length and antenna beamwidth, the radar 

 area of such a target complex is a function of range and some of the radar 

 parameters. If the area of surface illuminated by a resolution element is 

 not too small, then the return can be considered to be from scatterers 

 uniformly distributed over this area, or area extensive. It is then convenient 

 to use a quantity, the radar area per unit area of sea surface, usually denoted 

 by 0-°, which is independent of the radar parameters. Then the radar area 

 (J is 



a = a^A, (4-59) 



where A is the physical area of a resolution element. For pulse radar 

 with pulse length r and azimuth and elevation beamwidths 4> and 6, 

 respectively, where A is the smaller of the two values: 



A = m li^ii = R^L {6 small) (4-60a) 



A = i?2$e/sin d {6 large) (4-60b) 



withL = Tf/2. 



The characteristics of sea return depend on a number of parameters. 

 These are the depression angle, polarization, frequency, and the condition 

 (or "state") of the sea. The last quantity includes the many factors which 

 affect the contour of the surface, such as the wind (its speed, direction, and 

 duration), swell (wave systems generated by distant storms), currents, 

 shoals, breakers, and others. Large waves in themselves do not necessarily 

 produce strong clutter, since a heavy swell, with little or no wind blowing, 

 does not produce a high level of clutter. On the other hand, clutter springs 

 up suddenly with a sudden onset of wind, even before waves of appreciabel 

 height are built up. Thus clutter seems to be more intimately connected 

 with the secondary wave structure due to the local wind than with the 

 primary wave structure. Although the many factors which influence sea 



2^For a thorough discussion of the World War II investigations of sea return, see the account 

 by H. Goldstein in D. E. Kerr (Ed.), Propagation of Short Radio Waves, pp. 481-581, McGraw- 

 Hill Book Co., Inc., New York, 1951. 



