Cross-calibration of NOAA-20 and SNPP VIIRS moderate-resolution reflective solar bands over water scenes

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the joint NASA/NOAA Suomi NPP (SNPP) satellite and NOAAs new generation of operational polar-orbiting weather platforms (NOAA-20) record Earth Science observations and continue the data record produced from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) sensors. However, long-term data records require the reflected solar bands to be cross calibrated to remove biases in each sensors data products. Pervious work reported by our group developed a cross-calibration of SNPP VIIRS and Aqua MODIS to create a continuous AOD data set over the life of both sensors. This study presents a cross-calibration of VIIRS bands of NOAA-20 against SNPP for use over water scenes. Since the spectral response functions of NOAA-20 and SNPP VIIRS are comparable, the ratio of top of atmosphere (TOA) reflectance is directly used to calculate the cross-calibration. However, maintaining strict temporal and view geometry thresholds would not be possible when directly comparing NOAA-20 to SNPP. Since both sensors co-locate temporally and spatially with Aqua MODIS, we used the MODIS/VIIRS matchup product created by the Science Investigator-led Processing Systems (SIPS) at the University of Wisconsin as an intermediate step. The ratio of VIIRS and MODIS TOA reflectance was taken with a 10-minute time difference threshold and 3° view zenith and scattering angle threshold. The data was aggregated at a monthly interval and linear trends were calculated for each band. Therefore, the double ratio of the SNPP and NOAA-20 trends yielded the final NOAA-20/SNPP VIIRS cross-calibration. This double ratio calibration was calculated over dark ocean, desert calibration sites, and the Dome C calibration site. The Dome C site yielded the least noisy results and was chosen for the final calibration. It was found that the corrections to TOA reflectance bands which brought the two sensors AOD data product into agreement were approximately between -1 to -4%.