FLUCTUATIONS OF RADAR ECHOES 



193 



1.0 

 0.9 

 0.8 

 0.7 

 0.6 

 F{t/) 0.5 

 0.4 

 0.3 

 0.2 

 0.1 

 



20 40 60 80 100 



V IN CYCLES PER SECOND 



120 



Figure 2. Experimentally deteriniiied frequency spec- 

 trum for sea echo. 



tweeii the width of the spectra is 2.4 compared to 

 2.88 for the ratio of wavelengths. This discrepancy is 

 probably drie to the crudity of the measurements on 

 X band". 



Ground Clutter 



The ordinary ground clutter consists of echoes from 

 a variety of types of targets: earth, rocks, trees, 

 branches, bushes, leaves, grass. Our present conception 

 is that tJie fluctuation in ordinary ground clutter arises 

 from the motion of leaves and branches in the wind, 

 changing the phase patterns in a manner somewhat 

 similar to that for random scatterers. There will in 

 addition be a relatively steady signal from fixed ob- 

 jects such as rocks and tree trunks. 



We have obtained much qualitative evidence for 

 this picture, but it is difficult to obtain quantitative 



I eo - 



&DB 



WIND SPEED IN MPH — » 



Figure 3. Fluctuation in signal from Blue Hills versus 

 wind speed. S band, 10 a.m. April 24, 1944 to 11 a.m. 

 April 25, 1944. 



I 



FuaiRE 4. First probability distribution, Baker Hill, 

 Maine. Wind speed 25 mph. Curve: theoretical, fixed 

 to random signal = 1 db. la = average intensity. 

 Histogram: experimental results. Film 103, 3,000 

 pulses, S band. 



data because the wind speed at the target is not usually 

 kno^sTi. Fortunately, in the Boston area tlie largest 

 ground signal is due to the Great Blue Hills, which 

 is the site of the Blue Hill Observatory. It is thus 

 possible to obtain data on the wind speed at the target. 

 We monitored the signal from Blue Hills for a 24-hr 

 period in April of 1944. Movies were taken of the 

 A scope at regular intervals. During the period of 

 observation the wind speed varied between 30 mjili 

 and dead calm. To interpret the data a somewhat crude 

 parameter was defined as a measure of the amount of 

 fluctuation. Tlie change in the signal strength from 

 one frame to the next (0.06 sec) was measured and 

 averaged over 200 frames. 



T_ _ 

 lo 



Figure 5. First probability distribution, Mt. Penobscot, 

 Maine. Wind speed 10 mph. Curve: theoretical, fixed 

 to random signal = -|- 7.2 db. h = average intensity. 

 Histogram: experimental results. F'ilm 82 (16 fr per sec), 

 400 frames, S band. 



