194 



ECHOES AND TARGETS 



0.6 0.8 



TIME IN SECONDS 



Fi(iURE 0. Pulse-to-i)ulse record of tlie signal I'rciin Hlnck Woods, Bar Harljor. Film S7, .July 9, 1944. 

 S-band, prf 3:53 'a l)cr sec, wind speed 22 ni|)h. 



This parametei' was tlieu plotted against the wind 

 speed as shown in Figure 3. Tliere is a quite good 

 correlation between the amount of fluctuation as meas- 

 ured by this parameter and the speed of the wind. The 

 fluctuation is of the order of 0.8 db at mph, which 

 is almost as good as our steadiest signals. At the other 

 extreme the fluctuation is about 3.4 db at 30 mph. 

 There appears to be a rather sudden Jump in the fluc- 

 tuation at a wind speed in the neighborhood of 20 

 m]ili. Tills jump has been obsserved at other .seasons of 

 the year and is believed to be rather general. It is sig- 

 nificant that the wind speed at which the ,]"um]3 occurs is 

 roughly that at which large branches and small trees 

 begin to inove as a whole. 



The theoretical description for a simple picture of 

 ground clutter consisting of an assembly of random 

 seatterers (leaves, grass, etc.) plus a fixed signal 

 (rocks, trees, trunks) is not difficult to work out. When 

 the proportion of steady signal is small, the first prob- 

 ability distribution closely resembles that for purely 

 random seatterers. For a large ratio of fixed-to- 

 random signal the amount of fluctuation is greatly 

 reduced, and the first probability distribution tends 

 to a Gaussian curve about the average intensity. This 

 is illustrated in Figures 4 and 5. Figure 4 is a plot of 

 the experimentally determined first probability dis- 

 tribution for a signal from heavily wooded terrain on 

 S band at 25-niph wind speed. This has been fitted 

 by a theoretical curve for a ratio of fixed to random 



signal of — 0.1 db. 



Figure 5 shows the distrilnition for a similar type 

 of terrain but for a wind speed of 10 mph. Here the 

 results arc fitted to a curve for a ratio of fixed to 

 random signal of -I- 5 db. The most probable intensity 

 is no longer at zero, and the amount of fluctuation is 

 considerably reduced. 



Figure 6 is a plot of the intensity of some ground 

 clutter at high wind over a period of iy2 seconds as 

 ol>tained from a pulsc-to-pulse film. While the signal 

 changes quite rapidly, it is not nearly so fast as sea 

 return, for example (see Figure 7). 



01 23458789 10 II 

 V IN CYCLES PER SECOND 



Figure 7. Video frequency sjiectrmn for ground clutter, 

 Baker Hill, Maine. Film 103, wind speed 25 mph, prf 

 333)3 per sec, wavelength S band. 



