Geology Reference
In-Depth Information
Visible
0.4-0.7
Midinfrared
0.7 -1.3
Thermal infrared
3-14
Photographic
infrared
0.7-1.3
8-14
Midinfrared
1. 3-1.3
3-5
10.5 -12.5
100%
O 2
H 2 O
O 3
O 2 O 3
CO 2
CO 2
H 2 O
CO 2
Absorption
0
0.1
0.2
0.3
0.4
0.6
0.8
1
1. 5
2
3456
8 0
12
15
20
30
Wavelength ( μ m)
Figure 7.36 Atmospheric absorption windows by different gases.
components may affect the spectral information of the
received radiation. The effects depend on the path length
of the radiation, its wavelength, the viewing geometry of
the sensor, and most importantly the atmospheric constit-
uents. In the case of optical sensors (where the source of
illumination is the sun), the atmosphere intervenes with
the path of both incident and reflected radiation. On the
other hand, only the path of the emitted radiation affects
observations for thermal infrared and passive microwave
sensors. Lack of understanding of the EM wave interac-
tions with the atmosphere will impact negatively on the
interpretation of the data. It is possible to use human intu-
ition to guide interpretation of remote sensing imagery
data in the VIS spectral region, but these are not as readily
available and reliable when it comes to the TIR or micro-
wave region. In short, for accurate retrieval of surface
parameters, the influence of the atmosphere should be
removed from the measured signal.
Absorption is the process by which radiation is
absorbed and converted into other forms of energy. In
general, the atmosphere absorbs 25% of solar radiation.
Atmospheric gases absorb radiation only at certain
wavelengths (shown roughly in Figure 7.36). Oxygen (O 2 ),
water vapor (H 2 O), and carbon dioxide (CO 2 ) absorb
radiation in the optical region, whereas ozone (O 3 ) and
CO 2 absorb radiation in the thermal infrared region.
While the volume fraction of CO 2 in the atmosphere is
only 0.035%, it is responsible for most of the absorption
of radiation in the infrared. At the same time it radiates
back some amount of infrared. For that reason CO 2 is the
prime greenhouse gas. The volume fraction of H 2 O varies
depending on the region and the weather conditions. It
can reach 4%. The integration of the water vapor con-
tained in a vertical column of the atmosphere is reported
to be 5 kg/m 2 in the polar regions [ Kern , 2001]. As shown
in Figure 7.35, absorption by atmospheric gases normally
limits the thermal remote sensing to two specific spectral
bands of wavelength, namely 3-5 and 8-14 μ m.
Scattering of EM waves is defined as the redirection of
energy when reflected off the surface. In general, the atmos-
phere scatters 6% of the incident solar radiation. While
surface scattering denotes the redirection of the incident
radiation as it strikes the surface, scattering in the atmos-
phere is defined differently. Depending on the diameter of
the scattering elements, atmospheric scattering refers not
only to the redirection of the radiation but also to the ran-
domness of emission after absorption of radiation. Three
types of scattering in the atmosphere are defined: Rayleigh,
Mie, and nonselective (Figure  7.37). Rayleigh scattering
occurs when the size of the scattering elements is very small
compared to the wavelength. Therefore, it is usually trig-
gered by atoms and molecules of gases. It is more effective
for the shorter wavelength of the optical spectrum (the
ultraviolet and blue bands). Rayleigh scattering is inversely
related to the fourth power of the radiation's wavelength.
Therefore, blue light (0.4 mm) is scattered 16 times more
Influences of Atmosphere and Clouds on Optical and
Infrared Observations The atmosphere influences the
observations through one or more of the following three
processes: absorption (attenuation), scattering, and emis-
sion of the radiation. The processes are triggered by
atmospheric gases as well as clouds. These processes are
described in details with their governing equations in
several topics [e.g., Elachi , 1988; Stephens ,1994; Marzano
and Visconti , 2003]. The task of accounting for these
processes in the remote sensing observations is called
atmospheric correction. This is perhaps the most impor-
tant preprocessing task of optical and infrared data
especially in the cases where multitemporal images are to
be compared and analyzed. The correction is not crucial
in the case of microwave sensing since the atmosphere is
mostly transparent with respect to the microwave signal.
The exception is the effect of water vapor and cloud
liquid water contents on emitted radiation at high micro-
wave frequencies (>37 GHz).
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