SNPP/JPSS VIIRS reflective solar bands on-orbit calibration bias

The National Oceanic and Atmospheric Administration (NOAA) has continuously operated the Low Earth Orbit (LEO) Advanced Very High-Resolution Radiometer (AVHRR) and Visible Infrared Imaging Radiometer Suite (VIIRS) instruments to monitor the Earth’s lands, oceans, and atmosphere since the late 1970s. This long-term monitoring of the Earth’s ecosystem provides a unique historical time series to model environmental change. The ability to observe small changes in the historical time series requires a consistent and stable radiometric calibration across the entire 40+ year dataset. The AVHRR instruments did not have on-board calibration for their reflective solar bands (RSBs) and relied on vicarious calibration approaches to maintain radiometric performance. Both the Moderate Resolution Imaging Spectroradiometer (MODIS) and VIIRS do have on-orbit calibrators for their RSBs that are National Institute of Standards and Technology (NIST) traceable. Using simultaneous nadir overpasses of the AVHRR, MODIS, and VIIRS instruments, intercomparisons of the instrument’s calibration can be performed and applied to the AVHHR sensor to improve the consistency in calibration across the many instrument builds. However, the intercomparison of MODIS-to-VIIRS and VIIRS-to-VIIRS builds did not always agree, and an investigation into the cause of the inter-satellite biases was performed. The results of the study showed that the characterization of the on-board calibrator for MODIS and Suomi-National Polar-Orbiting Partnership (SNPP) VIIRS had an assumption used during its pre-launch characterization that would bias its absolute calibration accuracy. The NOAA-20 and NOAA-21 VIIRS builds used upgraded pre-launch calibration test equipment that removed the on-board calibration characterization assumption and provided a more accurate calibration source. The findings of the study show that NOAA-20 and -21 provide the best reference for the historical time-series record and should be used as its calibration reference.