Intercomparison of S-NPP/NOAA-20/NOAA-21 VIIRS thermal emissive bands long-term on-orbit saturation radiances

The Visible Infrared Imaging Radiometer Suite (VIIRS) is currently onboard the S-NPP, NOAA-20, and NOAA-21 satellites. VIIRS has 7 Thermal Emissive Bands (TEB, I4-I5 and M12-M16). VIIRS I4-I5 Sensor Data Record (SDR) are widely used for active fire, fire radiative power, and thermal anomaly applications. This study evaluates and intercompares S-NPP/NOAA-20/NOAA-21 VIIRS I4 and I5 on-orbit saturation radiance changes over time. A method for estimating TEB half-angle mirror (HAM)-side/detector level saturation radiances using the VIIRS thermal model, prelaunch characterization results, on-orbit degradation factors, instrument temperatures, and telemetry data was proposed and applied to I4 and I5. Overall, S-NPP I4 and I5 exhibit higher saturation radiances than NOAA-20 and NOAA-21. Since March 2024, the saturation radiance of NOAA-20 are ~0.7 K (I4) and ~6.5 K (I5) lower than that of S-NPP, while NOAA-21 are ~1.3 K (I4) and ~8.6 K (I5) lower than that of S-NPP. S-NPP I5 detector level saturation radiances have increased by 2-5 K after 13 years on-orbit due to responsivity degradations. Moreover, the I5 radiance limit used in the NOAA operational processing and reprocessing are suboptimal for S-NPP (especially in recent years) and NOAA-21 during its early mission (launch – March 3, 2023) and can be improved. Validation results using I4 and I5 operational/reprocessed SDRs and lunar observations suggest that the proposed method can estimate I4-I5 saturation radiances reasonably well. The method proposed in this study can also be applied to VIIRS TEB M-bands, except M13 low gain.