Page 89 - Radar Technology Encyclopedia

clutter, atmospheric clutter, cloud 79

usual sources include precipitation (rain, snow, hail), aurora, Aurora clutter. The aurora consists of ionized gases in the E
and refractive index anomalies sometimes called “angels” region of the atmosphere, excited by charged particles from
because of their uncertain origin. Chaff, the man-made equiv- the sun. Radar echoes are observed primarily in the VHF and
alent of rain clutter, is discussed separately, although its low UHF bands and can take either of two forms. Discrete
model parameters are similar to those of atmospheric clutter. echoes, observed at night, are extended vertically at a specific
When a meteorological radar is considered, the precipitation range, while diffuse echoes are observed during daylight and
actually constitutes the target, and the RCS calculations will are extended horizontally along the E-layer. Both types have
yield target, rather than clutter, RCS. However, the following the fluctuating characteristics of complex targets and are dis-
discussion applies the term clutter without regard to the radar placed and spread in velocity. DKB
objective. Ref.: Skolnik (1962), pp. 621–624.
The RCS of this type of clutter is calculated using the
Bistatic clutter is clutter illuminated from the direction of the
volume of the clutter cell V and the volume reflectivity h v transmitter and scattering in a different direction to a receiver.
c
(see volume clutter). The reflectivity of precipitation
The bistatic angle is defined as the angle between the trans-
depends on radar wavelength and the dielectric constant of
mitting and receiving paths, varying from zero for the mono-
the precipitation, according to the relationship
static radar to p for inline forward scattering. The bistatic
5
p 2 surface or volume reflectivity at small bistatic angles for clut-
h = ----- K Z
v 4 ter composed of scattering elements smaller than one wave-
l
length is approximately the same as the monostatic
2
2
where l is radar wavelength, K = (m - 1) / (m + 2) is the
reflectivity. Except at angles near forward scatter, bistatic
refractive index factor), m is the complex index of refraction,
clutter reflectivity can be approximated by the monostatic
and Z is the average value of the sixth power of droplet diam- reflectivity at the bisector of the bistatic angle. For surface
2
eter (called the radar reflectivity factor). Values of |K| are
clutter in a vertical plane containing the two paths, bistatic
near 0.93 for raindrops and 0.2 for ice crystals and snow. A reflectivity can be evaluated (Barton, 1985) as
3
6
typical model giving a value for Z in mm /m is
1
b 0 æ y + y 2 ö 0
------------------- +
Z = ar s = gsin è 2 ø s f
b
where a and b are empirically determined constants and r is
where y and y are the grazing angles of the two paths, and
2
1
the precipitation rate. Values used in this relationship are
2
given below for different types of precipitation. Note that 0 æ r 2 æ ( y + y )ö
1
2
0 ö
s = ----- exp – ç --------------------------- ÷
these values, expressed in the conventional radar meteorolog- f è b ø è 4b 2 ø
0
ical units, must be multiplied by 10 - 18 to find the reflectivity 0
2
3
in m /m . The result, when using values of a and b predicting is the specular reflectivity for a surface having reflection
3
6
z in mm /m , is coefficient r and rms slope b / 2 . DKB
0
0
5 Ref.: Barton, D. K., “Land Clutter Models for Radar Design and Analysis,”
p 2 – 18 b 2 3
×
h = ----- K × a 10 r m ¤ m Proc. IEEE 73, no. 2, Feb. 1985, pp. 198–204; Willis (1991), Ch. 9;
×
v 4
l Nathanson (1991), pp. 342–349.
chaff clutter (see CHAFF.)
(See also ANGEL ECHO, aurora clutter, cloud clutter,
rain clutter, snow clutter.) DKB Cloud clutter at microwave and lower frequencies is usually
produced by precipitation droplets, although these may be
Ref.: Atlas (1964), pp. 371–374; Bogush (1989), p. 30; Sauvageot (1992),
pp. 111–122. suspended by updrafts, producing no precipitation at the sur-
face. In millimeter-wave bands, droplet sizes normally asso-
Clutter attenuation (CA) is defined as the ratio of processor
ciated with nonprecipitating clouds are more detectable. The
input clutter-to-noise ratio (C/N) to output ratio (C/N) . In a reflectivity for the average cloud conditions is shown in
o
i
pulsed doppler processor, the input ratio is defined for a filter
Fig. C17, as a function of water droplet density, for different
centered on the clutter doppler frequency, and the output ratio
radar bands.
for a filter centered on the target doppler frequency. Hence,
The reflectivity as a function of cloud density is most
for pulsed doppler radar, CA will vary with target velocity.
conveniently expressed in terms of the parameter Z, the sum-
For MTI radar, the definition given above implies normaliza-
mation of the sixth power of droplet diameters within one
tion to the noise gain of the processor. An older definition of
cubic meter of the cloud (see atmospheric clutter), but the
CA in MTI radar using a delay-line canceler is “the ratio of
usual relationships of Z with precipitation rate are invalidated
clutter power at the canceler input to clutter residue at the
by updrafts. Instead, a relationship with condensed water den-
canceler output, normalized to the attenuation for a single 3
sity, M in g/m , may be used:
pulse passing through the unprocessed channel of the can- 2
Z = 0.048M (average clouds)
celer.” (See also MTI.) DKB 2
Z = 8.2M (advection fog)
Ref.: Barton (1988), p. 254; Schleher (1991), p. 108; IEEE (1993), p. 157. 1.96
Z = 391M (cumulus at onset of rain)
These may be compared with