Suomi NPP VIIRS DNB and RSB M bands detector-to-detector and HAM side calibration differences assessment using a homogenous ground target

Near-nadir observations of the Libya 4 site from the S-NPP VIIRS Day-Night Band (DNB) and Moderate resolution Bands (M bands) are used to assess the detector calibration stability and half-angle mirror (HAM) side differences. Almost seven years of Sensor Data Records products are extracted from the Libya 4 site center over an area of 32 x32 pixels. The mean values of the radiance from individual detectors per HAM side are computed separately. The comparison of the normalized radiance between detectors indicates that the detector calibration differences are wavelength dependent and the differences have been slowly increasing with time for short wavelength bands, especially for M1-M4. The maximum annual average differences between DNB detectors are 0.77% in 2017 at HAM-A. For the M bands, the maximum detector differences in 2017 are 1.7% for Ml, 1.8% for M2, 1.3% for M3, 1.2% for M4, 0.67% for M5, 0.75% for M7, 0.57% for M8, 13% for M9, 0.63% for M10, and 0.66% for M11. The average HAM side A to B difference in 2017 are 0.00% for DNB, 0.22% for Ml, 0.17% for M2, 0.15% for M3, 0.09% for M4, -0.07% for M5, 0.02% for M7, 0.01% for M8, 1.4% for M9, 0.01% for M10, and 0.03% for M11. Results for M6 are not available due to the signal saturation and M9 results are not accurate because of the low reflectance from the desert site and the strong atmospheric absorption in this channel. The results in this study help scientists better understand each detector's performance and HAM side characteristics. Additionally, they provide evidence and motivation for future VIIRS calibration improvements.