The ATCOR 3 Method

Processing of bands in the solar region (400 - 2500 nm)

The total signal at the sensor consists of four components: path radiance, reflected radiation from the viewed pixel, scattered radiation from the neighborhood, and terrain radiation reflected to the pixel (see Figure 1). The atmospheric conditions (water vapor content, aerosol type, visibility) for a scene can be estimated using the SPECTRA module. Then, the surface reflectance spectrum of a target in the scene can be viewed as a function of the selected atmospheric parameters. It can be compared to typical library spectra.


Figure 1 : Schematic sketch of radiation components for a rugged terrain.

Component 1 : path radiance: radiation scattered by the atmosphere (photons without ground contact).
Component 2 : reflected radiation from the viewed pixel.
Component 3 : adjacency radiation: ground reflected from the neighborhood and scattered into the view direction.
Component 4 : terrain radiation reflected to the pixel (from opposite hills, according to the terrain view factor).
Only component 2 contains information from the viewed pixel.

The reflectance calculation is performed iteratively in 5 or 6 major steps: Note: if the adjacency range R is selected as R=0 then steps 2 to 5 are omitted.

Processing of bands in the thermal region (8 - 13 um)

This section applies to Landsat-4/5 Thematic Mapper band 6, Landsat-7 ETM+ band 6, ASTER, and possible future thermal band sensors. It is similar to the flat terrain case (see Figure 2 of Method ATCOR2). However, the elevation dependence of path radiance, atmospheric transmittance, and downwelling thermal flux is taken into account.
For a sensor with n thermal channels there are n equations with n+1 unknowns, namely the n surface emissivities plus a surface temperature. So, the system of equations is always underdetermined. Several possibilities exist to address this problem. In the ATCOR model, the user can select a constant surface emissivity, e.g. 0.98 (TM band 6: 10.5-12.5 um, ASTER band 13: 10.3-11.0 um) for a scene or a surface cover dependent value.

After the emissivity selection the surface radiance can be calculated for the chosen atmospheric conditions (water vapor, aerosol content), which is converted to a surface temperature using a 2nd order polynomial approximation of the radiance-temperature relationship. For multi-band thermal data the normalized emissivity method (NEM), adjusted NEM, and in-scene atmospheric correction (ISAC) can also be selected.

Accuracy of the Method

The accuracy of the method depends on several factors :

In the thermal region, the surface temperature retrieval additionally depends on the appropriate surface emissivity map. If the deviation of the true surface emissivity from the assumed emissivity is less than 0.02, then the temperatures will be accurate to about 1-1.5 K. Larger deviations will occur if the emissivity estimate is not close to reality. As a rule of thumb, a surface temperature error of about 0.5-0.7 K per 0.01 emissivity error for surface temperatures is much higher than the boundary layer air temperature.

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last modified: DS, 18.10.14