Radiometric Stability and Consistency of Suomi NPP, NOAA-20 and NOAA-21 VIIRS Global Area Coverage (VGAC) Reflective Solar Band Data over Calibration Sites

The Visible Infrared Imaging Radiometer Suite (VIIRS) instruments onboard the Joint Polar Satellite System (JPSS) missions have been producing critical Earth observations for over a decade. These observations are essential for weather forecasting, environmental monitoring, disaster response, and climate change studies. The VIIRS Global Area Coverage (VGAC) product is derived from VIIRS sensor data record (SDR) with approximately 30 times reduction in data volume at low spatial resolution, ensuring continuity of the decades of heritage products from AVHRR, and enhancing accessibility and usability compared to standard VIIRS SDRs at full spatial resolution.

Two types of VGAC data are being produced: (1) one derived from recalibrated VIIRS SDR data for improved accuracy and stability, and (2) another processed from operational products for timely access to VGAC data. VGAC data derived from recalibrated VIIRS SDRs are particularly suitable for developing long-term environmental data records (EDRs), such as cloud climatology, aerosol, vegetation, and sea surface temperature, supporting the continuation of AVHRR Global Area Coverage (GAC) data records for climate studies. This study analyzed the long-term VGAC data from the recalibrated Suomi NPP (2012 to 2020) and NOAA-20 (2018 to 2022) reflective solar bands to extract reflectance time series over calibration sites to assess their stability and consistency. Temporal radiometric stability was characterized using reflectance time series over pseudo-invariant calibration sites (PICS) such as Libya 1, Libya 4 Saharan desert sites, and Antarctica Dome C. The cloud-free near-nadir top of atmosphere (TOA) reflectance observed by Suomi NPP and NOAA-20 VIIRS over these calibration sites was trended over time to quantify sensor stability. It is found that the Suomi NPP VGAC based on recalibrated VIIRS dataset is extremely stable, with degradation better than 0.1% per year for all moderate resolution bands over the calibration sites.

To support the automated processing of the operational VIIRS SDRs into VGAC, software modules were developed to process VIIRS SDRs from the Cloud-based NOAA Open Data Dissemination (NODD) system into preliminary VGAC for NOAA-20 and NOAA-21. Testing of these processing modules for NOAA-20 and NOAA-21 VGAC began in November 2023. This study also evaluates the radiometric consistency of the preliminary NOAA-20 and NOAA-21 VGAC data using the time series of reflectance data over calibration sites. Since the VGAC data is derived from VIIRS SDRs, this work also helps assess the radiometric performance, i.e., stability and consistency, of recalibrated Suomi NPP and NOAA-20 VIIRS SDRs, as well as the consistency of operational SDRs of NOAA-20 and NOAA-21 VIIRS. The results from evaluating the stability of decade-long VGAC data derived from consistently recalibrated/reprocessed SDR data also demonstrate its suitability for a wide range of long-term scientific applications that require stringent climate-quality calibration.