l2gen version consistency for re-processing full dataset for MODIS and VIIRS

[andrei_mitrich]

Dear Forum experts,

We are processing L1A data for MODIS, VIIRSN and VIIRSJ1 with SEADAS l2gen, to L1C, which effectively means L2 with all gains set to 1 for use in processing with the POLYMER atmos correction that has its own SVC gains. We have noticed that in NASA reprocessings (say, r2018, r2022) all available data are processed with the same, latest version of software, say, R2022.1, then for later processing, newer data inputs, the latest available version is used, say, 9.7-V2024.7, 9.11-V2025.5 etc). Our tests show that different versions of software could lead in L1C data differences about 1%, in some examples, with following processing to Level2 magnify the differences by ~10, i.e., differences are roughly within 10%, in some examples - even higher. Effectively, this means we are observing 10% and above errors. Then two questions arise:
Which way is best/correct/recommended:

1) applying always the same l2gen version, as at SVC processing, even if newer versions appear;
2) regardless version used for SVC (i.e., at re-processing), always use the newest available - exactly what NASA is doing in operational processing;

Suspecting the way (2) would produce less accurate results, why NASA follows this way of processing?

Thank you
Andrei

[OB.DAAC - SeanBailey]

The POLYMER “L1C” is really just a reformatted L1B. There are zero corrections made to the input L1 when l2gen outputs TOA reflectance (rhot) other than conversion to reflectance, and no changes in the various releases of l2gen would impart any. What you are likely seeing are impacts of the calibration updates (at least for the MODIS and SNPP VIIRS instruments - we are still using a static LUT for JPSS1). We have released several “mini” reprocessing over the last few years for the MODIS and SNPP_VIIRS data to address issues with the long-term on-orbit calibration of these aging instruments. We also release “refined” processings on a roughly monthly cadence to try to keep the calibration consistent in the forward stream.

Regards,
Sean
(sorry for the long delay in responding)