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
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
5.2.2 Dark-target atmospheric correction
5.2.3 Direct digital number to reflectance transformation