Radar System Engineering

f$Ec.16.6] INTERNAL CLUTTER FLUCTUATIONS 643
and half the pulse length, as shown in Fig. 1618a. The individual
targets may consist of rocks, tree trunks, tree branches, leaves, etc.
Some of these, such astherocks, prefixed. Others, such as the branches
and leaves, move in the wind. In Fig. 16. 18b, the vector R, represents
the contribution from the fixed targets and Rz that from the moving
targets, and R is the resultant signal. At the next pulse, R2 changes to
R;, and the resultant to R’. The small vector r is the pulse-to-pulse
change.
Individual moving targets in the illuminated area are so numerous
that even the largest issmall in size compared witht heirs urn. Also the
phases of their echo signals are completely independent. Under these
two conditions, the vector R2 has a probability distribution like that of
statistical noise-that is, the end of thevector hasa Gaussian distribution
about the point O’. Thepulse-to-pulse change r likewise has a Gaussian
distribution.
The final result connecting r and R2 is the following:
(r),~, = $, (R,),m, (6)
wheref, is the PRF, A the wavelength, and k a factor that deper.ds on the
wind speed and type of terrain. Measurements of the ratio of (r),m to
(R),~ have been made by H. Goldstein.’ The following table gives
typical values extrapolated from the experimental figures which were
obtained using a repetition rate of 333/see. The voltage ratio of fixed to
variable clutter is also given. In the case of sea and storm echoes, there
is no fixed component.
TABLE162.-Typical,
nlEASCREDVALUESOFC;LUTTEEFLUCTUATION
Kind of clutter
I
A,
cm
Voltage ratio of rms Total echo/rms Buctuation,
fixed ~omponent to ] db
rms variable com- 1
ponent
PRF = 500 1000 2000
Thunderstorm.
Sea echo . . . . . . . . . . . . . . .
Sea echo . . . . . . . . . . . . . . .
Wooded terrain (wind
45mph) . . . . . . . . . . . ..l
Wooded terrain (wiud
25rnph) . . . . . . . . . . . . .
Wooded terrain (wind
25 mph)... . .
Wooded terrain (wind
ly mph). . . . . . . . . . .
9.2
92
3.2
32
3.2
9.2
9.2
0
0
0
0