ATMOSPHERIC CORRECTION ALGORITHM FOR NOAA-AVHRR PRODUCTS - THEORY AND APPLICATION

Investigation of the effect of atmospheric constituents on NOAA Advanced Very High Resolution Radiometer (AVHRR) visible and near-infrared data is presented. Part A describes the general remote sensing equation including scattering, absorption and bidirectional reflectance (BRDF) effects for the AVHRR solar bands. Part B specifically addresses the magnitude of the atmospheric effects for AVHRR solar bands with respect to their impact on the normalized difference vegetation index (NDVI) and the surface BRDF. Part C addresses possible approaches for acquiring atmospheric informations and illustrates examples of atmospheric correction of surface reflectance and NDVI. Invariant effects (ozone absorption and molecular scattering) and variant effects (water vapor absorption and aerosol scattering) are shown to dominate the atmospheric effects in the AVHRR solar bands. Because these components are a function of viewing and illumination geometry, their effects are highly variable and at times compensating. Decoupling the various components is shown to be appropriate allowing selective correction. Water vapor absorption is of major importance, affecting the apparent reflectance in the near IR by reducing it by 10-30%, depending on the amount of water vapor and the direction of illumination and observation. Ozone absorption can reduce the apparent reflectance in channel 1 by 5-15%. Molecular scattering can increase the apparent reflectance in channel 1 by DELTA(rho) = 0.02-0.07 depending on the direction of illumination and observation. In the red channel (channel 1) the effect of moderate haze is of the same magnitude as the reflectance of vegetated surfaces and the effect of dense haze, dust outbreaks, or smoke episodes can mask completely the surface properties. Molecular scattering can be easily corrected and should be a priority; ozone absorption, although less important, can be easily corrected also. Correction for water vapor absorption should also be made a priority for sparsely vegetated areas due to the large diurnal variations in content. The aerosol effect is highly variable as a function of location and time and, consequently, the required inputs are often unavailable. Its correction is important for derivation of absolute radiances and to correct the NDVI for densely vegetated areas. It can suppress the value of the true NDVI by 0.10.