Geoscience Reference
In-Depth Information
Table 3.2
RADAR wavelengths and frequency bands following the International Telecommuni-
cations Union (ITU) Radar Band Nomenclature (see
http://www radioing.com/eengineer/
bands.html
for alternative definitions)
Frequency
Wavelength
Frequency bands
Application
20-300 MHz
1-15 m
VHF
Windprofiler
400-900 MHz
30-70 cm
UHF (P-Band)
Windprofiler
1-2 GHz
15-30 cm
L-Band/UHF
Boundary layer windprofiler
2-4 GHz
7-15 cm
S-Band/UHF
Precipitation
4-8 GHz
4-7 cm
C-Band
Precipitation
8-16 GHz
2-4 cm
X-Band
Precipitation
16-20 GHz
1-2 cm
Ku-Band
Precipitation
35 GHz
8.5 mm
Ka-Band
Precipitation, clouds
75-110 GHz
3 mm
W-Band
Clouds
here,
P
0
denotes the emitted power (usually some hundreds of kW),
P
R
the received
power (minimum detectable power is about 10
−
14
W) and
P
bg
the power of the
background noise.
is the efficiency of the antenna,
A
the effective area of the
σ
antenna,
the scattering and absorption by particles soaring in the air,
r
the dis-
tance to the RADAR,
2
Z
the
τ
the pulse duration (typically half a micro second),
κ
water
2
ice
backscatter cross-section (
0, 17), and
c
the speed of light.
The ratio of the two terms on the right-hand side of the RADAR equation is called
signal-to-noise ratio (SNR). For the derivation of this equation Rayleigh scattering
has been assumed. Therefore, the wavelength of the RADAR
κ
=
0, 93,
κ
=
λ
must be much larger
than the diameter
d
of the precipitation particles (
λ
>10
d
for snow flakes). This equation gives an integral value over all precipitation
particles in the scattering volume. The range resolution
λ
>20
d
for water droplets and
r
is given by
=
τ
r
0.5
c
.
(3.2)
With pulse durations of half a micro second, this gives a range resolution of 75 m.
Because the received backscattered power is also proportional to the pulse duration,
a trade off between maximum range and range resolution has to be made.
Usually, RADAR beams are emitted nearly horizontally or at a small angle to the
horizontal plane. Therefore, the refraction of the beams is very important when com-
puting their path length. Refraction can only be neglected for vertical beams. The
magnitude of the refraction depends on the vertical gradient of the refraction index
n
of the atmosphere for electromagnetic radiation. This refraction index is mainly
a function of the temperature and moisture distribution. In order to get convenient
expressions, often a refractivity
N
is defined by
10
6
(
n
N
=
−
1).
(3.3)
For this refractivity, the temperature and moisture dependence can be expressed
as
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