THE FREQUENCY DEPENDENCE OF SEA ECHO 



195 



The i'l-equeiiLy spcttnim can be obtained from these 

 data as in the previous case. Figure 7 shows the video 

 frequency spectrum oljtained under the same condi- 

 tions as Figure 4 for high winds. The spectrum ex- 

 tends as high as 13 c. Figure 8 is a plot of the fre- 



02 0.4 0.6 ae IJO LZ I.4 1.6 

 V IN CYCLES PER SECOND 



1.8 2.0 



Figure 8. Video frequency spectrum for ground clutter. 

 Mt. Penobscot, Mt. Desert Island. Film 82, wind 

 speed 10 mph, wavelength S band, prf 333 '3. 



queucy spectrum obtained from the same film as in 

 Figure 5. Here at a wind speed of 10 mph the fre- 

 quency spectrum does not extend beyond 2 c. 



12.1.4 Targets Viewed over Water 



In addition to the soui'ces of fluctuation described 

 above, echoes from targets viewed over water will 

 change due to the varying reflection fronr the surface 

 of the sea. Some English investigations have shown 

 that at high angles of incidence the amount of reflec- 

 tion can often change quite rapidly. However, there 

 is another effect due to reflection from the sea surface 

 which is of a much longer period, namely, tidal varia- 

 tions. 



Some of our earliest work consisted of monitoring 

 the signal from a number of isolated targets viewed 

 over water over a period of many tidal cycles. It was 

 found that quite a large number of echoes showed a 

 definite correlation with the tide. One very striking 

 example is the case of two standpipes on Strawberry 

 Hill on Nantasket Peninsula, which when viewed 

 from Deer Island in Boston Harbor showed a 15-db 

 variation with tide. Their range is 10,000 yd, and the 

 targets are about 60 ft high. Under these conditions 



the targets subtend more than two lobes of the inter- 

 ference pattern at S band. Since the effect of the tidal 

 change is to move this lobe structure up and down by 

 10 ft. it is difficult to believe that a change as large as 

 15 db could be thus produced. However, one can break 

 up the returned signal into a number of separate sig- 

 nals differing as to whether they suffer two reflections 

 on the surface of the sea or one reflection or go directly 

 to and from the target. While the amplitude of each 

 of these signals does not vary much with tide, their 

 relative phases do, and the total signal can still change 

 considerably in amplitude because of the interference. 

 A similar set of measurements was made on a corner 

 reflector mounted on a small island in Boston Harbor. 

 Here the corner reflector although only 6,000 yd away 

 acts essentially as a point target. The agreement with 

 the theory for a point target is quite good. 



Our results emphasize the extreme caution that must 

 be employed in the use of standard targets to monitor 

 radar performance. They are just the type of targets 

 which are normally chosen in the field, and obviously 

 their variations with the tide make them entirely un- 

 suitable for the purpose. It may be possible to find tar- 

 gets whose echoes are sufficiently steady so that they 

 can be used for monitoring. However, they cannot be 

 found without the use of such test equipment as would 

 obviate the need for standard targets. 



12.2 THE FREQUENCY DEPENDENCE 

 OF SEA ECHO" 



As the power and frequency of radar sets continue 

 to increase, and the size of the target to be detected 

 decreases, the presence of sea echo becomes of ever 

 greater operational significance. It acts as a built-in 

 jammer, blanketing and obscuring the desired signals. 

 Despite this growing practical importance the basic 

 phenomena of sea echo have not yet been established. 

 Certainly, the fundamental mechanism responsible 

 for the signal is not yet known. Various conflicting 

 theories have been jDroposed. It has been suggested 

 that scattering from drops of sjDray is the cause of 

 the echo. iVnother hypothesis is that of reflection or 

 diffraction from the large surfaces of the waves them- 

 selves. Still other theories have been advanced at one 

 time or another. 



WlTatever the size of the scatterers, the power re- 

 ceived at the radar can be described by a common 

 formula. Consider some particular scatterer, say the 



•"By H. Goldstein, Radiation Laboratory, MIT. 



