The task of earth observing sensors is the mapping of surface properties. However, the surface information is masked, since the signal recorded by spaceborne optical sensors consists of several components and their magnitudes depend on atmospheric conditions, see Figure 1. In addition, topographic effects strongly influence the recorded signal. The objective of an atmospheric correction is the elimination of atmospheric and illumination effects to retrieve physical parameters of the earth's surface, e.g. surface reflectance, emissivity and temperature.
This information can be used for monitoring, change detection, surface-vegetation atmosphere transfer (SVAT) modeling, and surface energy balance investigations for climatic modeling and upscaling. Therefore, atmospheric correction is an essential part of preprocessing and a prerequisite for the derivation of certain value added products.
The method is mainly intended for small field-of-view sensors where the angular dependence of atmospheric parameters can be neglected. However, some wide field-of-view sensors such as IRS-1C/1D WiFS are also supported.
DN is the digital number recorded in a certain spectral channel, c0, c1 are the offset and gain of the radiometric calibration relating the DN to the at-sensor radiance L=c0 + c1*DN. The atmospheric correction process removes the signal components 1 and 3 from the total at-sensor radiance L, so radiance component 2 remains. This information is converted into a surface reflectance value.
More details on the processing (thermal bands, rugged terrain) can be found in the section "Method" (flat terrain and mountainous terrain ).
As an option the following value added products can be calculated: SAVI vegetation index, LAI, FPAR, albedo (wavelength-integrated reflectance 0.4-2.5 um), absorbed solar radiation flux. If the sensor has at least one thermal band and the boundary layer air temperature is specified then the following radiation and heat fluxes are included as additional output channels: difference of downwelling longwave thermal atmospheric flux and emitted surface flux, net radiation, ground heat flux, latent heat flux, sensible heat. All radiation and heat fluxes are given in Watts per square meter.
The ATCOR software was developed by DLR (German Aerospace Center) and the IDL version is licensed to ReSe for commercial marketing.