Hi all
Please find the answer from Florian the S2 technical Manager from OPT-MPC project :
"Indeed, we have already identified a lack of information on this topic on Sentiwiki. A following text was already prepared and will be included on Sentiwiki soon:
Sun and viewing angles are provided in tile metadata. They are computed on a virtual grid projected on the tile ground surface. The top left node of the grid has the same coordinates as the top left corner of the tile footprint (top left corner of the top left pixel of all the rasters at different spatial resolutions). The grid contains 23 by 23 nodes, each node being separated by 5000 m. The grid fully covers the tile footprint. It should be mentioned that the nodes from the last line and last column of the grid are 200 m away from the bottom and right tile edges, respectively.
For each node of the grid, the zenith sun and viewing angles are computed based on the normal direction to the reference ellipsoid at the node coordinates. The azimuth sun and viewing angles are computed clockwise, based on the North direction at the node coordinates.
The viewing angles are provided for each spectral band and detector. An inverse location of each node of the grid to the detector/band viewing model is performed, so that only nodes lying inside the detector/band have their viewing angles computed. Therefore, if a node of the grid is located in an overlapping area between two consecutive detectors, two different values of viewing angles are assigned to this node, as being seen from two different viewing directions by two neighbouring detectors.
Both sun and viewing angles are provided in degrees."