The computation process for the transformation of cloud data from the VIIRS instrument on board the JPSS (Joint Polar Satellite System) operational meterological satellite prototype SNPP, to
To view the retrieval algorithms for these products, visit the following Sentinel/Copernicus page:
https://sentinels.copernicus.eu/documents/247904/2476257/Sentinel-5P-NPP-ATBD-NPP-Clouds

The S5P-NPP Cloud processor is a stand-alone code separate from other TROPOMI-related computations. The retrieval algorithm requires the following input fields:
• Semi-major and semi-minor axis, and eccentricity, of the WGS'84 ellipsoid
• Time window of tolerance to identify relevant NPP files for a given S5P scan line
• Number of VIIRS cloud classifcations, defined S5P FOVs, and VIIRS moderate resolution channels used in the S5P-NPP product
• Maximum and minimum values of transformed coordinates (y & z) which define a specific S5P FOV.
• Instantaneous spatial response function
• Latitude and longitude of S5P pixel corner, centre or sensor, and altitude of sensor
• View zenith angle of S5P pixel and line-of-sight zenith angle of a VIIRS pixel
• Sensing time of S5P scan line and a VIIRS scan line
• Sensing start and stop time of S5P L1 file
• Latitude and longitude of VIIRS pixel
• Sensing start and stop time of NPP L1 file
• Sensing start and stop time of NPP L1 granule
• Sensing start and stop centre-scan latitude and longitude of NPP L1 granule
• VIIRS pixel level geolocation, radiance and cloud mask data quality flags
• VIIRS band averaged spectral radiance
• VIIRS pixel cloud mask
• Ratio of nominal FOV extent to that of L1GPC in across- and along-track directions
• Spatial response function (SRF), including effects of satellite motion

The retrieval is performed in 3 steps:
1) S5P L1 file is ingested, containing the geolocation of each scene for which the S5P-NPP-Cloud information is required.
2) The time difference, ∆tsat, between S5P centre-swath observations and those of spatially co-located VIIRS measurements is calculated to an accuracy of ≈1 minute using metadata in the VIIRS geololcation file.
3) S5P scan lines are looped over: a) VIIRS files which contain information relevant to these scan lines are identified b) The relevant information from these VIIRS files is read and stored in memory c) S5P across-track pixels within the current scan line are looped over.

The NPP-Cloud product is then the re-gridding of the VIIRS L1 and cloud mask products from the SNPP to the resolution acquired by the TROPOMI instrument on the Sentinel 5P satellite.

For more information on the processing algorithm please look at the ATBD (Algorithm Theoretical Baseline Document) on the Sentinel Copernicus webpage, URL linked in the documentation.