9.2 Atmospheric Correction

9.2 Atmospheric Correction

The solar radiation is absorbed or scattered by the atmosphere during transmission to the ground surface, while the reflected or emitted radiation from the target is also absorbed or scattered by the atmosphere before it reaches a sensor. The ground surface receive not only the direct solar radiation but also sky light, or scattered radiation from the atmosphere. A sensor will receive not only the direct reflected or emitted radiation from a target, but also the scattered radiation from a target and the scattered radiation from the atmosphere, which is called path radiance. Atmospheric correction is used to remove these effects(see Figure 9.2.1 and Figure 9.2.2).

The atmospheric correction method is classified into the method using the radiative transfer equation, the method using ground truth data and other methods.

a. The method using the radiative transfer equation
An approximate solution is usually determined for the radiative transfer equation. For atmospheric correction, aerosol density in the visible and near infrared region and water vapor density in the thermal infrared region should be estimated. Because these values cannot be determined from image data, a rigorous solution cannot be used.

b. The method with ground truth data
At the time of data acquisition, those targets with known or measured reflectance will be identified in the image. Atmospheric correction can be made by comparison between the known value of the target and the image data (output signal). However the method can only be applied to the specific site with targets or a specific season.

c. Other method
A special sensor to measure aerosol density or water vapor density is utilized together with an imaging sensor for atmospheric correction. For example, the NOAA satellite has not only an imaging sensor of AVHRR (Advanced Very high Resolution Radiometer) but also HIRS (High Resolution Infrared Radiometer Sounder) for atmospheric correction.