ECHOES AND TARGETS 309 
200 
100 
R-F INTENSITY 
°o 0,2 04 0.6 os TF) 12 
TIME IN SEGONDS 
Ficure 6. Pulse-to-pulse record of the signal from Black Woods, Bar Harbor. Film 87, July 9, 1944. 
S-band, prf 333% per sec, wind speed 22 mph. 
ors random signal of + 5 db. The most probable intensity 
0.9 is no longer at zero, and the amount of fluctuation is 
ae considerably reduced. 
ag Figure 6 is a plot of the intensity of some ground 
Ap clutter at high wind over a period of 14% seconds as 
obtained from a pulse-to-pulse film. While the signal 
F(y) 0-8 changes quite rapidly, it is not nearly so fast as sea 
0.4 return, for example (see Figure 7). 
0.3 The frequency spectrum can be obtained from these 
0.2 data as in the previous’ case. Figure 7 shows the video 
ol frequency spectrum obtained under the same condi- 
oh tions as Figure 4 for high winds. The spectrum ex- 
PNIGICHESIPERISeCoND tends as high as 12 c. Figure 8 is a plot of the fre- 
Ficure 7. Video frequency spectrum for ground clutter, quency spectrum obtained from the same film as in 
Baker Hill, Maine. Film 103, wind speed 25 mph, prf Figure 5. Here at a wind speed of 10 mph the fre- 
333% per sec, wavelength S band. quency spectrum docs not extend beyond 2 ec. 
1,0 
ce Targets Viewed over Water 
0.8 In addition to the sources of fluctuation described 
0.7 above, echoes from targets viewed over water will 
change due to the varying reflection from the surface 
of the sea. Some English investigations have shown 
F(Y) 0.8 that at high angles of incidence the amount of reflec- 
tion can often change quite rapidly. However, there 
of is another effect due to reflection from the sea surface 
0.3 which is of a much longer period, namely, tidal varia- 
: ae tions. 
Some of our earliest work consisted of monitoring 
sO! 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 
Ficure 8. Video frequency spectrum for ground clutter. 
Mt. Penobscot, Mt. Desert Island. Film 82, wind example is the case of two standpipes on Strawberry 
speed 10 mph, wavelength S band, prf 33314. Hill on Nantasket Peninsula, which when viewed 
°o af 0A a6 OB 10 121A 16 L6 20 
V IN GYGLES PER SECOND 
