22
H. Rott
Propagation in the Visible and Solar Infrared. The main factor for
atmospheric attenuation of visible light is scattering by air molecules and by
aerosols. Molecular scattering can be modelled accurately using the Rayleigh
scattering approximation (Eq. 2.15). The atmospheric optical thickness, T)"
due to Rayleigh scattering is proportional A -4. This means that molecular
scattering in the atmosphere is very important at short wavelengths (UV and
visible blue) and of little relevance in the infrared.
The magnitude and angular distribution of scattering by aerosols are
highly variable, depending on total aerosol content and on the size distribution, the dielectric properties, and the shape of the particles. Due to this
variability, effects of aerosol scattering are difficult to correct. Because the
aerosol particles are much larger than the air molecules, aerosol scattering is
less dependent on A. In an atmosphere with average turbidity, aerosol scattering dominates over molecular scattering for A ;::: 0.5 /-lm.
In the infrared region the absorption due to various gases (primarily H20
and CO2 ) dominates over the scattering losses. The spectral curve of irradiance at the earth's surface in Fig. 2.6 was calculated with the computer code
LOWTRAN-7 (Kneizys et al., 1988), assuming a standard atmosphere and
clean air (tropospheric) aerosol. In the infrared the observation of the surface
is limited to the atmospheric window regions between the absorption bands.
The main window regions in the solar IR are at A < 1.1/-lm, 1.2 ::::; A ::::; 1.3/-lm,
1.5::::; A ::::; 1.7/-lm, and 2.0::::; A::::; 2.3/-lm.
Figure 2.7 illustrates the different contributions to the reflected spectral
radiance, L oo ,)" observed by a satellite sensor:
(2.17)
where L p ,)' is the radiance scattered within the atmosphere in direction of
the sensor (the path radiance), L r ,), is the radiance coming from the target
at the earth's surface with reflectance r s ,)" and L~,), is the reflected radiance
from the areas adjacent to the target. For quantitative analysis the spectral
surface reflectance, r s,)', is required which represents the ratio of the reflected
radiance at the surface, L s ,)" over the global irradiance (the sum ofthe direct,
Edir,)" and diffuse irradiance, Edi!,),):
(2.18)
L s ,), is attenuated in the atmosphere with the optical thickness T),(O, (0); at
the sensor arrives
(2.19)
From Eq. 2.17 to Eq. 2.19 it is obvious that information on atmospheric
scattering and absorption properties is required for calculating the surface
reflectivity from satellite measurements. The radiative transfer code 68 enables the calculation of the surface reflectance from satellite measurements for
H. Rott
Propagation in the Visible and Solar Infrared. The main factor for
atmospheric attenuation of visible light is scattering by air molecules and by
aerosols. Molecular scattering can be modelled accurately using the Rayleigh
scattering approximation (Eq. 2.15). The atmospheric optical thickness, T)"
due to Rayleigh scattering is proportional A -4. This means that molecular
scattering in the atmosphere is very important at short wavelengths (UV and
visible blue) and of little relevance in the infrared.
The magnitude and angular distribution of scattering by aerosols are
highly variable, depending on total aerosol content and on the size distribution, the dielectric properties, and the shape of the particles. Due to this
variability, effects of aerosol scattering are difficult to correct. Because the
aerosol particles are much larger than the air molecules, aerosol scattering is
less dependent on A. In an atmosphere with average turbidity, aerosol scattering dominates over molecular scattering for A ;::: 0.5 /-lm.
In the infrared region the absorption due to various gases (primarily H20
and CO2 ) dominates over the scattering losses. The spectral curve of irradiance at the earth's surface in Fig. 2.6 was calculated with the computer code
LOWTRAN-7 (Kneizys et al., 1988), assuming a standard atmosphere and
clean air (tropospheric) aerosol. In the infrared the observation of the surface
is limited to the atmospheric window regions between the absorption bands.
The main window regions in the solar IR are at A < 1.1/-lm, 1.2 ::::; A ::::; 1.3/-lm,
1.5::::; A ::::; 1.7/-lm, and 2.0::::; A::::; 2.3/-lm.
Figure 2.7 illustrates the different contributions to the reflected spectral
radiance, L oo ,)" observed by a satellite sensor:
(2.17)
where L p ,)' is the radiance scattered within the atmosphere in direction of
the sensor (the path radiance), L r ,), is the radiance coming from the target
at the earth's surface with reflectance r s ,)" and L~,), is the reflected radiance
from the areas adjacent to the target. For quantitative analysis the spectral
surface reflectance, r s,)', is required which represents the ratio of the reflected
radiance at the surface, L s ,)" over the global irradiance (the sum ofthe direct,
Edir,)" and diffuse irradiance, Edi!,),):
(2.18)
L s ,), is attenuated in the atmosphere with the optical thickness T),(O, (0); at
the sensor arrives
(2.19)
From Eq. 2.17 to Eq. 2.19 it is obvious that information on atmospheric
scattering and absorption properties is required for calculating the surface
reflectivity from satellite measurements. The radiative transfer code 68 enables the calculation of the surface reflectance from satellite measurements for