P nLA d2 Ei Cos 0s LA a DN p

where, pi is the apparent reflectance in band i; Ei is the exo-atmosphere solar irradiance in band i, LA is the spectral radiance; d2 is the sun to earth distance correction factor; 0s is the solar zenith angle; DN is the digital number; a and P are the gain and off-set values obtained from the sensor calibration parameters.

The general term "narrow-band" include Landsat, IRS, AVHRR channels, while the general term "broad-band" include METEOSAT. The term "Planetary" and "Surface" refer to either albedo when they are calculated or measured from top of the atmosphere as seen by a satellite or at ground level without any intermediate atmosphere, respectively.

Several factors complicate the estimation of surface albedo from remotely sensed data-atmospheric effect, degree of isotropy of the surface and spectral interval of the narrow band interval (Brest and Goward, 1987).

Rugged terrain, the geometry between the sun, the surface orientation and the satellite sensor, which can vary from one pixel to another is a factor which makes the estimation of surface reflectance from remotely sensed data difficult.

Saunders (1990) suggested a detailed complex methodology to retrieve surface albedo from NOAA - AVHRR visible and infrared bands by considering Rayleigh scattering, aerosol scattering and gaseous absorption as the principal radiation attenuating mechanisms, wherein the accuracy of the atmospheric correction would be dependent on input profiles of atmospheric constituents.

Prasad et al. (1995) suggested a dark body radiance method for computation of albedo from NOAA - AVHRR data and found albedo values comparable to those retrieved by Saunders (1990) method. The methodology adopted by them is discussed below :

The albedo (A) is given by

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