Page 93 - Radar Technology Encyclopedia

clutter, homogeneous clutter, land 83

ground clutter (see land clutter). The average reflectivity observed by a radar at low graz-
, is dominated by the clutter
(
¤
ing angle, y < y = l 4ps )
Homogeneous clutter is distributed clutter for which the c h
propagation factor F :
c
amplitude pdf is approximately Rayleigh with constant mean
4
value over many clutter cells of the observing radar. A thresh- s F = ( g sin y) F » ( gy) y y)
4
0 4
(
¤
c
c
c
old set by a cell-averaging CFAR processor can control false
which is a strong function of frequency. The strong depen-
alarms from such clutter with minimal losses. DKB 4
dence of F for a particular clutter cell on the heights of clut-
Ref.: Nitzberg (1992), pp. 213–216. c
ter sources within that cell causes a broad spread of
The clutter horizon refers to the range beyond which clutter amplitudes from cell to cell, typically described by a Weibull
is masked by the earth’s surface. For surface clutter, the hori- or log-normal distribution. Weibull spread parameters, a = 4
zon range R is set by the height h of the radar antenna above to 5, or log-normal standard deviations s = 12 to 15 dB, are
h
r
y
the surface: normally observed in land clutter near the horizon (see clut-
R = 2kah ter (amplitude) distribution.)
h r
Typical values of land clutter reflectivity are shown in
6
where ka is the effective earth’s radius (8.5 ´ 10 m for stan- Fig. C26, based on parameters of Table C2 with low grazing
dard refractive conditions, using the four-thirds radius angle values calculated for S-band.
approximation, k = 4/3). For atmospheric clutter, it is
20
R = 2ka( h + h )
h r mc
where h is the maximum altitude of the clutter source. 10 Mountains, urban
mc
When ducting conditions are present (k >> 4/3), surface
0
clutter may be visible at ranges more than 400 km from radars Wooded hills
at the surface. DKB 10
Ref.: Barton (1988), pp. 127–128. Reflectivity in dB 20
clutter improvement factor (see clutter attenuation, MTI
30
improvement factor). Rolling hills
Farmland, desert
Interclutter visibility (ICV) is “a measure of radar capability 40
to detect targets between points of strong clutter by virtue of Flatland
50
the ability of the radar to resolve the areas of strong and weak Smooth surface
clutter” (see also clutter (amplitude) distribution). DKB 60
0.1 1 10 100
Ref.: IEEE (1993), p. 665; Skolnik (1990), p. 15.3 Grazing angle in degrees
Land clutter is the echo from a land surface, characterized Figure C26 Land clutter reflectivity for different surfaces,
0
by its surface reflectivity, s or g, or by RCS values of dis- with propagation calculated for S-band.
crete clutter sources. Using the constant-g model (see surface
The velocity spectrum of clutter from trees and other
clutter), for angles below y (at which specular reflection vegetation depends on the wind velocity. Early measurements
s
becomes significant), values of g for different land surfaces
matched the spectrum with a Gaussian function having a stan-
are approximately independent of radar frequency and are
dard deviation s = 0.1 to 0.3 m/s. More recent measure-
vt
shown in Table C2, along with approximate values for the
ments using equipment with linear dynamic range more than
rms height deviation s . 60 dB show exponential spectra:
h
Table C2 1 æ v ö
Sv () ---exp –= ----- v ³, 0.2 m/s
Land Clutter Characteristics a è a ø
Rms Height where a is the -4 dB spectral width, ranging from 0.04 m/s in
Type of
g 10 log g Deviation, light air (2 to 4 m/s) to 0.06 m/s in breezy air (5 to 6 m/s) and
Surface
s (m) 0.09 m/s under windy conditions (8 to 12 m/s). These spectra
h
Mountains 0.32 -5 100 are shown in Fig. C27, normalized such that the total power is
unity.
Urban 0.32 -5 10
Measurements in which spectral density as a function of
2
3
Wooded hills 0.10 -10 10 frequency f was reported to fall off as 1/f or 1/f , giving
Rolling hills 0.063 -12 10 very broad spectra at levels from -40 to -60 dB from the
peak, can be attributed to receiver nonlinearity (in some
Farmland,
0.032 -15 3.0 cases, log receivers were used). DKB
desert
Ref.: Long (1975); Barton (1964), Ch. 3; Skolnik (1970), Ch. 25; Nathanson
Flatland 0.01 -20 1.0 (1990), Ch. 7; Billingsley, J. B., and Larrabee, J. F., “Measured Spectral
Extent of L- and X-Band Radar Reflections from Wind-Blown Trees,”
Smooth surface 0.0032 -25 0.3
MIT Lincoln Lab. Proj. Rpt. CMT-57, 6 Feb. 1987.