Effect of Solar Irradiance Fluctuations on S-NPP Reflective Band Calibration

The Visible-Infrared Imaging Radiometer Suite (VIIRS) was launched on-board the Suomi National Polar-Orbiting Partnership (S-NPP) spacecraft on October 28, 2011. The instrument has 22 spectral bands: 14 reflective solar bands (RSB), 7 thermal emissive bands (TEB), and a Day Night Band (DNB). The RSBs span wavelengths from 412 nm to 2250 nm, which are calibrated by using solar radiance reflected from a Solar Diffuser (SD). The SD reflectance degrades over time, and a Solar Diffuser Stability Monitor (SDSM) is used to track the changes. The ratio between the calculated solar radiance reflected from the SD and the VIIRS measurement of this radiance using the pre-launch calibration coefficients is known as the “F Factor”. This paper will discuss F Factor fluctuations and investigate their cause. Specifically, the F Factors of several visible bands showed cyclic fluctuations approximately in the range of 1.5 to 2 weeks. One hypothesis considers the fact that fluctuations in solar irradiance are not being compensated for in the VIIRS irradiance measurements. Two approaches are used to analyze the fluctuations in solar irradiance and identify the impact on the F Factors. The first is based on Total Solar Irradiance (TSI) measurements and the other uses Spectral Solar Irradiance (SSI) measurements. The measurements are collected from other on-orbit solar irradiance sensors.