SYN Level-2 processing aims to combine information from the OLCI and SLSTR instruments to provide improved data for land surface analysis. It is divided into two main steps.
- The first step makes use of the enhanced spectral and angular capabilities of a combined instrument to propose a product with atmospherically corrected surface reflectance and corresponding information on aerosol properties.
- The second allows processing of data from SLSTR and OLCI to provide comparable products to SPOT-VGT to allow continuity of data delivery to the existing user community of SPOT-VGT. SYN Level-2 processing aims to output the SY_2_SYN package. This processing can be formulated as one of multivariate optimisation subject to multiple constraints. Given a set of satellite TOA reflectances and an initial estimate of atmospheric profile, the corresponding set of surface reflectances is estimated. Application of the observed set to the estimated set of reflectances results in an error metric, where a lower value of the metric corresponds to a set of surface reflectances (and hence atmospheric profile) that is more realistic. These two steps are repeated with refined atmospheric profiles until convergence at an optimal solution. This entire process is divided into four main sub-steps:
- Averaging of collocated TOA radiance over N pixels squared, to minimise the effect of errors in image collocation, whist retrieving aerosol within the spatial scale of aerosol properties.
- Pixel-wise retrieval of aerosol properties, itself divided into two main steps (atmospheric correction of averaged TOA radiance using an estimate of aerosol optical thickness and error metric calculation using spectral and bidirectional surface reflectance models).
- Interpolation of aerosol properties retrieved for the averaged grid onto the collocated grid.
- Atmospheric correction of collocated TOA radiance using the retrieved aerosol properties. VGT Continuity processing aims to output the SY_2_VGP, and the SY_2_VGK products. It is divided into four substeps:
- Band mapping to simulate the SPOT-VGT spectral bands (four bands) at top of atmosphere from the OLCI and SLSTR bands.
- Pixel flagging, indicating land, water, cloud cover, ice/snow and cloud shadows are transferred from SYN L2 datasets
- Projection from the ortho-geolocation grid to the SPOT VGT plate-carrée grid. At the end of this sub-step, the SY_2_VGP product can be written.
- Atmospheric correction, similar to that used in SYN processing, and computation of the NDVI. This last step aims to produce the VGK product.
A last step aims to define and write the SYN Level-2 products.