372
S. Paloscia
thermal infrared, and the microwave band (1 mm-30 cm). In the visible (VIS) and nearinfrared bands (NIR) (0.4-3.0 /Lm) solar radiation is partially reflected and partially absorbed
by the earth's surface: whereas in water the visible radiation can penetrate even for several
meters, in solids it is absorbed only for a very thin layer a few microns deep. In these regions
of the spectrum remote sensing information is obtained by measuring the solar radiation and
refers to the surface of observed bodies. In both thermal infrared (TIR) and microwave (MW)
regions sensors are sensitive to the energy emitted from natural bodies due to a thermal effect
and can measure their surface temperature; nevertheless in the microwave range the emitted
and reflected radiation depends much more on the characteristics of the surfaces and therefore
can give significant information concerning some geophysical parameters, namely soil moisture,
biomass, ice depth, snow water equivalent, etc. Moreover, microwaves have a penetration power
much higher than shorter wavelengths and thus they are the only way for investigating subsurface characteristics. It is known that the atmosphere strongly attenuates all the " x and
UV radiation, it is transparent in the VIS range and, except for some bands, has a still high
absorptance in the infrared region. Moreover, energy propagation in the different bands depends
on meteorological conditions in that clouds strongly absorb visible and infrared radiation up to
millimetric waves (>. < 1 cm), while they influence much less the propagation of microwaves.
This fact alllows the use of microwave sensors when it is necessary to operate independently
from meteorological conditions.
Remote sensing sensors are substantially of two types: passive and active. To the first group
belong the sensors (radiometers, spectrometers) able to measure the radiation coming from
the observed surfaces: solar radiation (in the VIS-NIR range) and radiation directly emitted
from the surface for thermal effect (in the TIR-MW range). To the second group belong the
sensors (radar, laser) which have their own illuminating source able to emit in various bands
of the spectrum. The most largely used active sensor is the Side Looking Radar (SLAR) in the
two versions: Real Aperture Radar (RAR) and Synthetic Aperture Radar (SAR). Both sensors
use very short pulses to obtain a good resolution across the flight line. In a RAR longitudinal
resolution is determined by the physical dimensions of the antenna and the observation distance,
whereas for SAR the resolution is independent from the distance in that the motion of the
platform is used to obtain a long synthetic antenna. SAR has performances higher than RAR,
however it is a highly complicated sensor both for the system and for the signal processing.
Electromagnetic parameters usually measured by remote sensing sensors are the following:
- Radiance (Wm- 2 ster- I ):
It is the radiant flux per solid angle unit which passes through a surface coming from a direction,
divided by the element of the surface itself projected on the plane perpendicular to this direction.
Since the radiance varies with the wavelength a spectral radiance has been defined (Wm- 2 ster- I
m- I ). The concept of spectral radiance corresponds to that of brightness (Wm- 2 ster- I H- 1 )
used in microwave radiometry where it more usually refers to spectral variations to frequency
instead of wavelength.
- Reflectance:
It is the ratio between the radiant flux coming from reflection or scattering from an element
of the surface and the incident flux on the element itself. In the microwave technique the
characteristics of scattering of a target are expressed from the radar cross section (m 2 ), i.e.
the area intercepting that quantity of power which, if it was isotropically scattered, would
produce an echo equal to the one received from the object. When large objects are observed,
the normalized radar cross section is preferably introduced, defined as the radar section per
unit of surface area.
- Emittance / Emissivity:
Since natural bodies are not perfect emitters, their radiance IS reduced by a factor, called
S. Paloscia
thermal infrared, and the microwave band (1 mm-30 cm). In the visible (VIS) and nearinfrared bands (NIR) (0.4-3.0 /Lm) solar radiation is partially reflected and partially absorbed
by the earth's surface: whereas in water the visible radiation can penetrate even for several
meters, in solids it is absorbed only for a very thin layer a few microns deep. In these regions
of the spectrum remote sensing information is obtained by measuring the solar radiation and
refers to the surface of observed bodies. In both thermal infrared (TIR) and microwave (MW)
regions sensors are sensitive to the energy emitted from natural bodies due to a thermal effect
and can measure their surface temperature; nevertheless in the microwave range the emitted
and reflected radiation depends much more on the characteristics of the surfaces and therefore
can give significant information concerning some geophysical parameters, namely soil moisture,
biomass, ice depth, snow water equivalent, etc. Moreover, microwaves have a penetration power
much higher than shorter wavelengths and thus they are the only way for investigating subsurface characteristics. It is known that the atmosphere strongly attenuates all the " x and
UV radiation, it is transparent in the VIS range and, except for some bands, has a still high
absorptance in the infrared region. Moreover, energy propagation in the different bands depends
on meteorological conditions in that clouds strongly absorb visible and infrared radiation up to
millimetric waves (>. < 1 cm), while they influence much less the propagation of microwaves.
This fact alllows the use of microwave sensors when it is necessary to operate independently
from meteorological conditions.
Remote sensing sensors are substantially of two types: passive and active. To the first group
belong the sensors (radiometers, spectrometers) able to measure the radiation coming from
the observed surfaces: solar radiation (in the VIS-NIR range) and radiation directly emitted
from the surface for thermal effect (in the TIR-MW range). To the second group belong the
sensors (radar, laser) which have their own illuminating source able to emit in various bands
of the spectrum. The most largely used active sensor is the Side Looking Radar (SLAR) in the
two versions: Real Aperture Radar (RAR) and Synthetic Aperture Radar (SAR). Both sensors
use very short pulses to obtain a good resolution across the flight line. In a RAR longitudinal
resolution is determined by the physical dimensions of the antenna and the observation distance,
whereas for SAR the resolution is independent from the distance in that the motion of the
platform is used to obtain a long synthetic antenna. SAR has performances higher than RAR,
however it is a highly complicated sensor both for the system and for the signal processing.
Electromagnetic parameters usually measured by remote sensing sensors are the following:
- Radiance (Wm- 2 ster- I ):
It is the radiant flux per solid angle unit which passes through a surface coming from a direction,
divided by the element of the surface itself projected on the plane perpendicular to this direction.
Since the radiance varies with the wavelength a spectral radiance has been defined (Wm- 2 ster- I
m- I ). The concept of spectral radiance corresponds to that of brightness (Wm- 2 ster- I H- 1 )
used in microwave radiometry where it more usually refers to spectral variations to frequency
instead of wavelength.
- Reflectance:
It is the ratio between the radiant flux coming from reflection or scattering from an element
of the surface and the incident flux on the element itself. In the microwave technique the
characteristics of scattering of a target are expressed from the radar cross section (m 2 ), i.e.
the area intercepting that quantity of power which, if it was isotropically scattered, would
produce an echo equal to the one received from the object. When large objects are observed,
the normalized radar cross section is preferably introduced, defined as the radar section per
unit of surface area.
- Emittance / Emissivity:
Since natural bodies are not perfect emitters, their radiance IS reduced by a factor, called