Geoscience Reference
In-Depth Information
”
(2.25, 5.5, 21, 43 cm) and a set of non-scanning radiometers (2.25, 6, 18, 21, 27 cm).
Active system includes airborne multichannel SAR system (3.9, 23, 68, 254 cm).
Successful tests of this equipment have been conducted in Russia, Ukraine, Uzbe-
kistan, Turkmenia, Moldova, Bulgaria, Cuba, USA (Krapivin and Sutko 2012).
Microwave radiometry or passive microwave remote sensing is based on mea-
surements of the natural electromagnetic radiation of objects and Earth covers in the
millimeter to decimeter wavelength range. Inside this band, the land surface radi-
ation is primarily a function of free water content in soil and also in
Passive system includes airborne scanning multi-channel radiometer
“
Radius
uenced by
other parameters of the soil-canopy system, such as above ground vegetation
biomass, soil density, water salinity and temperature of the system. Free water
content in soil is dependent on rainfall rate, arti
fl
cial watering, shallow groundwater
and processes of soil drying of at interface soil-atmosphere. Hence microwave
radiometry is an important tool for the assessment of soil hydrological regimes
(Shutko 1986, 1992; Krapivin and Shutko 2012).
The measure of the intensity of radiation in the microwave band is referred to as
a brightness temperature T
b
, which is a product of emissivity
, and thermodynamic
temperature T
e
, within the effectively emitting layer of the object: T
b
=
ʺ
T
e
. The
emissivity is a function of dielectric permittivity/permeability of the object/surface
of observation. For a land surface, the dielectric permittivity is
ʺ
first of all a function
of soil moisture. The higher a soil moisture content, the higher the permittivity of
soil, the lower the emissivity/intensity of radiation/brightness temperature of this
piece of land. For a water surface, the dielectric permittivity is
first of all a function
of electric conductivity of a water solution that is dependent on a concentration of
salts, acids, on a presence of oil
films and many other chemical substances. For
example, the higher salinity of water, the higher the dielectric permittivity of water
solution, the lower the emissivity/intensity of radiation/brightness temperature of
this water body (Gulyuev et al. 2005).
Within the 2 to 30 cm band, for air temperature t =10
C, the radiation
the sensitivity of radiation in the X-band (2
−
30
°
30 cm) to the
changes in free water content in bare soil, soil density, salinity and temperature of
the soil surface (Shutko 1986). These data show that the main parameter affecting
the intensity of a bare soil radiation, practically independent of spectral band, is the
soil moisture. Based on this sensitivity it is feasible to estimate the value of soil
moisture without a priori data on the soil parameters. It has also been seen that even
for rather high values of biomass, up to 2
−
3 cm) and L-band (18
−
3 kg/m
2
, the plant canopy is still
-
transparent in the decimeter wavelength range.
There are different sources of natural microwave radiation in microwaves, which
can in
fl
uence on accuracy of measurements: Galaxy,
ionosphere, atmosphere
(clouds), land. The minimum in
uenced by different sources of radio-noise is 2-cm
to 21-cm wavelength band, which is window of transparency at microwaves. The
centimeter and decimeter wavelength microwave radiometers are not in
fl
uenced by
the conditions of illumination, by presence of fog, smoke, clouds, and are very
useful for measurements in different meteorological conditions (Shutko 1992).
fl
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