although we knew its position within 200 yds. (approx. 310°T; 3,000 yds). 

 Both FTC and STC anticlutter circuits proved ineffective in this case. 

 Later the growler was sighted just off the bow. This small ice formation 

 was of sufficient size to inflict serious damage to a thin-hull vessel making 

 contact at moderate speed. Of interest here is the fact that the masking 

 effect from snow is less than that from rain as would be expected from 

 considerations of the low reflection coefficient for snow shown in figure 27. 



SEA RETURN 



General 



Although it has been established that icebergs are very poor reflectors 

 of radar, and that reflection might be further decreased by aspect and 

 subnormal propagation conditions which exist on the Grand Banks during 

 spring, it is well established that most icebergs do provide good targets 

 and during calm sea conditions some reliance can be placed on radar. 

 However, it appears that distinguishing small ice targets in heavy seas 

 might be the limiting factor in the reliability of radar as an instrument 

 for providing safe navigation through ice infested waters. The basic phe- 

 nomena of sea return or sea clutter have not yet been definitely estab- 

 lished; however, it is well known that sea echo from waves acts as a 

 built-in jammer, blanketing and obscuring small target echoes. Other 

 things being equal, the strength of the sea return depends upon the state 

 or roughness of the sea, which in turn depends largely upon the wind 

 force. As demonstrated in 1945 [5], the range of sea clutter on the scope 

 is very nearly directly proportional to the state of the sea and the wind 

 force. 



The radar cross section and echo strength of sea return are difficult 

 quantities to measure or compute because among other things the reflec- 

 tion surfaces extend from the ship to an indefinite range. The variation of 

 reflected power with range does not necessarily follow the same relation 

 as that for a ship or iceberg target as is apparent from the fact that the 

 reflection from waves is less at longer ranges due to a decrease in the angle 

 of incidence. In general, the decrease in sea return with range is more 

 rapid than that for other targets. 



Quantitative Measurements 



Of prime importance to this study was the quantitative determination 

 of the masking effect of sea return. Many mariners have reported that 

 ice targets have gone undetected due to masking by sea return but it 

 remained, however, to make quantitative measurements under controlled 

 conditions to definitely assess the importance of sea return. Numerous 

 measurements were made on all states of the sea from the maximum 

 range of sea return to as close as practical on the "A" scan. It was early 



84 



