atmosphere and sea surface (L,( x) in Eq. 1). The interactions within 

 the atmosphere consist of scattering by the air (Rayleigh scattering) 

 and by microscopic particles suspended in the air (aerosol scattering). 

 In principle this added radiance could be removed if the concentration 

 and optical properties of the aerosol were known throughout an image. 

 The aerosol, however, is highly variable and, unlike the Rayleigh 

 scattering component, its effect on the imagery cannot be predicted a 

 priori. Thus, only very general aspects of the aerosol properties can 

 be used in estimating its contribution. 



The basic approach to atmospheric corrections used in Gordon et al. 

 (1980) involves knowing the inherent sea surface radiance L (X) at one 

 position in the image. Briefly, the sensor radiance L t (x) is divided 

 into its components: L r (x), the contribution arising from Rayleigh 

 scattering, L (x), the contribution arising from aerosol scattering, and 



a 



t(x)L (x), the inherent sea surface radiance diffusely transmitted 

 (Tanre et al., 1979) to the top of the atmosphere, i.e., 



L t (x) = L r (x) + L a (x) + t(x)L w (x). (7) 



Note that it has been implicitly assumed that there is not direct sun 

 glitter in the field of view of the sensor. Also, photons reflected 

 from the sea surface (without penetrating) have been included in the 

 term 



L r (x) + L a (x). (8) 



Given L ( x) at one position, L (x) can be found there from Eq. 7. 

 w a 



Then if the normalized size frequency distribution and refractive 

 index of the aerosol (which define an aerosol 'type') are independent of 

 horizontal position over a significant portion of an image, the ratio of 

 aerosol radiances at two wavelengths x and x , 



S(X,X ) = L a U)/L a (X ) (9) 



will be position independent over the same portion of the image, even 

 though both L ( x) and L (X ) may vary. Gordon (1981a) has demonstrated 



B-12 



