5.2 Atmospheric Correction of Remotely Sensed Data
Atmospheric correction is a major issue in visible or near-infrared remote sensing because the presence of the atmosphere always influences the radiation from the ground to the sensor.
The radiance that reaches a sensor
can be determined by
Normally Lmax, Lmin and
DNrange are known from the sensor manufacturer or operator.
However, Ls is composed of contributions from the target, background and the atmosphere (Figure 5.5):
Figure 5.5 Target, background and scattered radiation received by the sensor.
As introduced before, the atmosphere has severe effects on the visible and near-infrared radiance. First, it modifies the spectral and spatial distribution of the radiation incident on the surface. Second, radiance being reflected is attenuated. Third, atmospheric scattered radiance, called path radiance, is added to the transmitted radiance.
Assuming that Ls is the radiance received by a sensor, it can be divided into LT and LP
LS = LT + LP (1)
LT is the transmitted radiance.
LP is atmospheric path radiance.
Obviously, our interest is to determine LT.
For a given spectral interval, the solar irradiance reaching the earth's surface is
EG =
where ES is the solar irradiance outside the atmosphere,
Ti atmospheric transmittance along the the incident direction,
i incident angle
Ed diffuse sky irradiance
Surface can be either specular or diffuse. Most surfaces can be considered as approximate diffuse reflectors at high solar elevations, i.e. when i is small.
If the surface is assumed to be a
perfect diffuse reflector i.e. the Lambertian case, the ratio of
the radiation reflected in the viewing direction to the total
radiation into the whole upper hemisphere is given by 
.
Based on Lambertian assumption,
where 
is the target reflectance, Te is the
transmittance along the viewing direction. Therefore in order to
quantitatively analyze remotely sensed data, i.e. to find r, atmospheric
transmittance T and path radiance Lp have to be
known.
5.2.1 Single scattering atmospheric correction
