Geoscience Reference
In-Depth Information
Radar (SAR) survey or active microwave remote sensing is based on mea-
surements of the parameters of arti
cially radiated electromagnetic signals scattered
by the Earth
s covers and objects. Due to high spatial resolution radar systems are
very important instruments of remote sensing from space. Previously studies of
scattered signal from soil and vegetation were conducted using on-ground and
airborne scatterometers and radars. The results of these studies are the dependences
of scattered signal on soil moisture, soil surface roughness, vegetation parameters,
etc. (Chukhlantsev and Vinokurova 1991; Chukhlantsev 1992).
Theoretical models for electromagnetic waves scattering by bare soil are based
on Kirchhoff approximation. Different expressions were obtained for the back-
scattering coef
'
s
but most of them may be written as
cient
r
s
¼ he
h
F
ð
klSin
#Þ
R
s
Cos
2
# þ rð
h
2
h ¼ 4k
2
2
Cos
2
r
Þ;
r
#;
ð
9
:
24
Þ
where
is the standard deviation of rughness height, l is the roughness correlation
length, k is the wave number in free space (2
˃
ˀ
/
ʻ
),
ˑ
is the nadir viewing angle, R is
the soil re
ectivity (Chukhlantsev 1992).
Analysis of formulas (
9.24
) showed that
fl
The sensitivity of the electromagnetic wave scattering by bare soils to re
fl
ec-
tivity variations of bare soil surface doesn
t depend on roughness parameters,
wavelengths and observation angle; dynamic range of scattering by bare soils is
no more 9 dB, when re
'
fl
ectivity varies from 0.05 (very dry soil) to 0.4 (very
high soil moisture); 6
ectivity may be obtained with an
accuracy of soil backscatter measurements of 1.5 dB
8 gradations of re
fl
-
Maximal dynamic range of backscatter by bare soils is equal to about 18 dB,
when standard deviation of roughness height
˃
varies from 0.5 to 4 cm.
There are two independent parameters: the standard deviation of roughness and
the re
ectivity (soil
moisture) and roughness parameters, it is necessary to use measurements at
minimum 2 frequencies. The best separation of these parameters is observed for
observations at wavelengths satisfying the condition: h(
fl
ectivity. It is obvious, to separate and to estimate the re
fl
ʻ
1
)
≈
1, h (
ʻ
1
) <<1; that
produces wavelengths 10 and 24 cm.
ed by experimental data. Quantitative and quali-
tative analysis showed that radar data may be good tool for soil moisture mea-
surements. It is possible to obtain 6
These conclusions were veri
−
8 gradations of soil moisture for bare soil and 3
−
4 for soil covered by vegetation (Chukhlantsev 1992).
It has theoretically been shown and experimentally proven in investigations
(Kutuza et al. 2000) and in the studies conducted by other teams that active and
passive systems are sensitive to soil moisture changes and to changes in surface
roughness, peculiarities of relief, vegetation biomass and structure, and angles of
observation. In general, the terrain features, surface roughness, changes in relief,
and vegetation characteristics in
uence the radar return more than natural emitted
radiation, other conditions being equal.
fl
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