I write this to underscore the information in the Tables here
Processing Baseline - Sentinel Online and in the L1C and L2A Data Product Quality Reports - Sentinel Online
Along with several other evolutions, Processing Baseline 04.00 L1C (TOA) and L2A (BOA) products (TTO 25th of January, 2022) now contain an Offset in the metadata. This offset has been included to accomodate ‘noisy’ pixels over dark surfaces, that can sometimes present negative radiometric values at L1B and L1C.
Prior to PB 04.00, these negative values were clamped to a predefined range (e.g. [1-32767]), thereby leading to a loss of information. In order to avoid this, and following an analysis by expertise, a radiometric offset has been introduced which shifts the range. The conclusions of the analysis resulted in the following specifications for the Offset:
- At L1B, RADIO_ADD_OFFSET = -100
- At L1C, RADIO_ADD_OFFSET = -1000
- At L2A, RADIO_ADD_OFFSET = -1000
SNAP is User-configurable for this change; the Radiometric Offset can be added in SNAP by selection of the relevant radio button (Tools > Options > Sentinel-2 Reader > Add negative radiometric offset (L1C -L2A) ):
Additional information can be found in the [Sentinel-2 Products Specification Document (PSD)](Sentinel-2 Products Specification Document (PSD) - Sentinel Online .)
I hope this information is of benefit to you.
OPT-MPC S2 Technical Manager
I posted a question yesterday (Clamp negative reflectance to 0 (NODATA) for Baseline 04.00 with new OFFSET correction?) that perhaps you could help me with (pert of it repeated below).
While comparing images between the current Baseline 04.00 and earlier Baseline versions I subtract the OFFSET value Baseline 04.00 images depending on product. This sometimes leads to, as expected, negative values in the resulting raster.
My question is: What is the correct way to present these values if clamping them to a floor of 0? Should they be regarded as NODATA (value 0) - Implying missing or faulty data? Or should they be viewed as a minimum value (like 1)? The reasoning being that these have a registered value that is out of bounds, but assumed to be approximately 0.
I would suggest that they should be regarded as NO DATA (value 0). This would at least flag them as unreliable. This is based on my understanding of negative reflectance values being linked to the Atmospheric Correction processing step. And thus they are an unwanted contribution to the output. So making them 0 identifies them as non-nominal, and helps serve as an avoidance measure. Or a flag of doubt. Giving them a value of 1 would potentially attribute them some relevance, and may serve to confuse.
I hope this helps.
OPT-MPC S2 Technical Manager
Thanks for your swift response @Jan,
Is this perhaps something that could be clarified in the “Sentinel-2 Products Specification Document” or some related documentation?
No worries. We are currently undertaking a wholesale update of Sentinel Online, so I will suggest that the content is updated to reflect these changes.
As far as the PSD goes, it’s really more a description of the Product levels and their packaging (and associated schemas and metadata), so the specifics of the Radiometric Offset aren’t in that remit. But I’ll mention it to the L2A expertise to see if it can be included in the ATBD .