The OLCI imaging spectrometer has 21 narrow spectral bands in the visible and NIR part of the spectrum, whereas the SLSTR multispectral instrument has six broader spectral bands, covering a wider spectrum and extending beyond the SWIR into the LWIR part of the spectrum.

The 21 narrow OLCI bands are not contiguous, with many gaps falling into the spectral coverage of the SPOT VGT bands. As illustrated in the figure below, the following procedure is carried out to simulate the VGT bands.

1. Downscaling: atmospheric correction of the nadir channels in the SYNERGY collocated data applied to each pixel. Two methods can be used, defined using a specific switch:
   1. the surface reflectance is computed during SYN Level 2 processing and considered as input.
   2. an atmospheric correction algorithm, similar to the one included in SYN Level-2 atmospheric correction is used. A standard continental aerosol model with aerosol optical thickness from climatology is assumed.
2. Hyperspectral interpolation: from the set of OLCI and SLSTR atmosphere-corrected surface reflectance, a hyperspectral surface reflectance spectrum is constructed by means of linear interpolation.
3. Hyperspectral upscaling: the hyperspectral surface reflectance is transported to TOA. In order to revert the atmospheric correction performed in the up-scaling step, it is essential that the reverse correction is carried out with the same aerosol model and aerosol optical thickness as used for the upscaling.
4. Hyperspectral filtering: the hyperspectral TOA spectrum is weighted with the spectral response functions of the VGT bands, resulting in a set of simulated VGT TOA reflectances.

Logical flow of VGT Band Mapping (VBM)