184.108.40.206.7.2 Atmospheric correction over land (step 2.6.23)
This scheme is detailed in RD 8, sections 2.15, 2.17. The input of the algorithm are TOA reflectances, corrected for gaseous absorption and stratospheric aerosols, above all valid land pixels. The outputs of the algorithm are:
- Rayleigh corrected reflectances for all pixels;
· Dense Dark Vegetation (DDV) flag for all pixels;
· aerosol model index and Angström exponent for pixels flagged as DDV;
Processing of the atmospheric correction over land is performed in 13 MERIS bands (basic set of 15 bands described in section 2, minus the O2 absorption band at 761.25 nm (band 11) and H2O absorption band at 900 nm (band 15)).
220.127.116.11.7.2.1 Rayleigh Correction Processing (step 2.6.15)
Rayleigh correction processing is organised in several steps. First the Rayleigh reflectance is computed for every 4x4 pixels window. Then, Rayleigh transmittance and Rayleigh spherical albedo are computed for every 4x4 pixels window. Then, the TOA apparent reflectance corrected for gaseous absorption r*ng is corrected for Rayleigh contributions for each pixel in order to derive the top of aerosol reflectance rtop. The 4x4 pixels windows do not overlap.
18.104.22.168.7.2.2 Dense Dark Vegetation (DDV) Screening (step 2.6.13)
DDV screening consists in flagging pixels identified as DDV by comparing a spectral index, the Atmosphere Robust Vegetation Index (ARVI), to a tabulated threshold which depends on Earth location and on the date of data acquisition. The ARVI is built using MERIS bands 3 (442 nm), 7 (665 nm) and 13 (865 nm).
22.214.171.124.7.2.3 Aerosol above DDV (step 2.6.17)
Aerosol type and optical thickness are estimated for DDV pixels. The same set of aerosol models is used for all pixels in a 32x64 pixels window (both in RR and FR); these windows do not overlap.
Using the aerosol parameters, we can estimate the aerosol reflectance ra at MERIS bands 12 (775nm) and 13 (865nm), and the corresponding optical thicknesses. The alpha product expresses the spectral dependence of the optical thickness: .