Since January 25th 2022, the Sentinel 2 products, both level L1C and level L2A, are processed according to baseline N0400. A description of the associated changes is available here.

In particular, with that baseline, an offset has been added to the image reflectances.

Details of the implementation for the L1C products:

Provision of negative radiometric values (implementing an offset):A radiometric offset will be added up to the image reflectance at Level-1C. The dynamic range will be shifted by a band-dependent constant, i.e. RADIO_ADD_OFFSET. From the user’s point of view, the L1C Top of Atmosphere (TOA) reflectance (L1C_TOA) shall be retrieved from the output radiometry as follows:

The offset will be reported in a new field in the General_Info/Product_Image_Characteristics section of the Datastrip and User Product Metadata. This evolution allows avoiding the loss of information due to clamping of negative values in the predefined range [1-32767] occurring over dark surfaces.

• Digital Number DN=0 will remain the “NO_DATA” value
• For a given DN in [1;215-1], the L1C TOA reflectance value will be: L1C_TOAi = (L1C_DNi + RADIO_ADD_OFFSETi) / QUANTIFICATION_VALUEi

Details of the implementation for the L2A products:

Provision of negative radiometric values (implementing an offset): Similarly to Level-1C, the dynamic range will be shifted by a band-dependent constant, i.e. BOA_ADD_OFFSET. From the user’s point of view, the L2A Bottom of Atmosphere (BOA) reflectance (L2A_BOA) shall be retrieved from the output radiometry as follows:

The radiometric offset value will be consistent with the one applied to the Level-1C product and will be reported in a new field in the General_Info/Product_Image_Characteristics section of the Datastrip and User Product Metadata. It is also noted that the percentage of negative surface reflectance pixels per band will be also reported in the L2A_QUALITY report in the QI_DATA folder of the tile.

• Digital Number DN=0 remains the “NO_DATA” value
• For a given DN in [1;215-1], the L2A BOA reflectance value will be: L2A_BOAi = (L2A_DNi + BOA_ADD_OFFSETi) / QUANTIFICATION_VALUEi

According to this change, new global attributes have been added to the netcdf products available on sattellitdata.no: QUANTIFICATION_VALUE and RADIO_ADD_OFFSET for the L1C and BOA_QUANTIFICATION_VALUE and BOA_ADD_OFFSET for the L2A.

Unfortunately, these attributes have only been added to the production on August 29th 2022. Products generated between 25th January and 28th August did not at first include those attributes. All those products have now been reprocessed to include the additional attributes.