Page 96 - Radar Technology Encyclopedia

86 clutter, sea clutter spectrum

Ref.: Barton (1988), Ch. 3; Skolnik (1990), Ch. 13; Nathanson (1990), Ch. 7; values of h may increase by several decibels, before drop-
v
Currie (1992), pp. 168–180; Morchin (1993), pp. 70–80. ping to the value for rain. DKB
Ref.: Atlas (1964), pp. 371–372.
to 20 log (r /b
0 0)
0
The clutter spectrum in velocity or doppler frequency is the
result of relative motion between the radar antenna and the
Reflectivity, , in dB(m 2 /m 3 ) s 0 22.5 l= 8.6 mm 23 cm where v is the mean clutter velocity relative to inertial space,
7.5
clutter sources. For clutter that fills the radar beam, the mean
velocity will be
15
a v cos–
b
v =
v cos
p
c
c
v is the mean radar platform velocity, a is the angle between
p
= 33 cm
30
2 cm
the radar beam and the clutter velocity vector, and b is the
37.5
tor. The corresponding doppler shift is f =
, where l
2v l¤
d
is the wavelength. Airborne clutter moves with the local wind
45 3.2 cm 5.3 cm 10 cm l angle between the radar beam and the platform velocity vec-
velocity, while sea clutter has an average velocity between 1/
52.5 8 and 1/4 of the wind velocity.
The spectrum, W(f) in frequency or W(v) in velocity, can
60 usually be approximated as Gaussian in shape:
0.1 1 10 100
Grazing angle in deg 2
æ f ö
Wf () W exp= 0 – ç --------- ÷
2
Figure C29 Reflectivity of a medium sea for different è 2s ø
radar bands. c
æ v 2 ö
Sidelobe clutter refers to clutter illuminated and received Wv () W exp= – ç --------- ÷
0 2
from beyond the antenna mainlobe. The total power of side- è 2s ø
v
lobe clutter in a range cell containing mainlobe clutter is nor- with standard deviation (clutter velocity spread)
mally a small fraction of the mainlobe clutter. However, when
there is relative motion between the radar antenna and the 2 2 2 2
s = s + s + s + s
clutter sources, the velocity spread of sidelobe sources may v va vt vs vp
exceed the width of the clutter rejection notch, leaving side- where s is the antenna scanning component, s is the clut-
vt
va
lobe clutter as the principal source of clutter residue. This is a ter turbulence component, s is the wind shear component,
vs
critical problem in medium- and high-PRF airborne radars and s is the platform motion component. The correspond-
vp
when detecting receding targets. Here, the sidelobe clutter ing clutter spread in frequency is s = 2s /l . Velocity spectra
v
f
power must be integrated over sidelobes in two-coordinate of five types of clutter are shown in Fig. C30, and reported
angle space of the lower hemisphere, producing a clutter values of spectral spread are shown in Table C5.
spectrum extending from -2v /l to 2v /l , where v is the
p p p The antenna scanning component is directly proportional
velocity of the radar platform and l is wavelength. A similar to the angular velocity of the beam, or inversely proportional
problem applies to shipborne radar, with lower values of v .
p to the observation time t the dwell time of the beam:
o
DKB
s = 0.13 l t ¤( )
Ref.: Skolnik (1990), Ch. 16. va o
The value will apply to mechanically scanning antennas and
Snow clutter is slightly less intense than rain clutter for the
also to electronically scanned antennas that are moved from
same precipitation rate (when r is defined in terms of the
pulse to pulse, rather than step-scanned on a burst-to-burst
water content of melted snow). For dry snow, the relation-
basis.
ships for reflectivity as a function of precipitation rate are
The clutter turbulence component is dependent on the
1.6
r – 14 internal motion of the clutter, induced by the wind. Typical
h = -------- ´ 3 ´ 10
v 4
l values for precipitation or chaff are 1 to 2 m/s; for sea clutter
1/8 of the wind speed (see land clutter for data on the spec-
Z = 500r 1.6 trum of trees and foliage). Bird clutter can be considered as
For snow of unknown characteristics, values approxi- having a total velocity spread s = 5 m/s, corresponding to a
vt
mately twice those given in these equations appear reason- 30 m/s band of speeds relative to the air.
able, while for aggregate snow the constant changes from 500 The wind shear component of clutter velocity spread, for
to 2,000, and the exponent from 1.6 to 2.0, giving a reflectiv- clutter filling the elevation beam, is given by
ity greater than that for rain.
A particularly intense reflection comes from the altitude s vs = 0.3R q k cos a
c e sh
layer in which snow melts into rain (the “bright band”). Here,