An update of NOAA-21 VIIRS on-orbit calibration and performance

The NOAA-21 VIIRS has successfully operated since its launch in November 2022 in an orbital constellation that includes the S-NPP and NOAA-20 launched in 2011 and 2017, respectively. Each VIIRS instrument makes daily global observations with 22 spectral bands, covering wavelengths from 0.41 to 12μm. It includes a day-night band (DNB) with a nominal spectral bandwidth of 0.5-0.9μm. The VIIRS instrument is regularly calibrated on orbit by a set of on-board calibrators (OBC), which include a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody (BB), and a space view (SV) port. On-orbit calibration activities and strategies also include near-monthly lunar observations performed via spacecraft roll maneuvers. The lunar observations are often used along with SD measurements to help improve on-orbit calibration of the reflective solar bands. In this paper, we provide an update of NOAA-21 VIIRS on-orbit calibration and performance since launch, and a comprehensive assessment of on-orbit changes of its spectral band responses before and after the second mid-mission outgas (MMOG) performed in February 2024. In addition to detector noise characterization, the OBC performance of NOAA-21 VIIRS is illustrated and compared with that of S-NPP and NOAA-20 over the same operating period. Results show that the NOAA-21 VIIRS overall performance is comparable to or better than its predecessors.