SNPP VIIRS Solar Diffuser BRDF Degradation Trend Changes in Early 2014

The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar Orbiting Partnership (SNPP) satellite collects radiometric and imagery data in 22 spectral bands within the visible and infrared region ranging from 0.4 to 12.5 um. These data are calibrated and geolocated in ground processing to generate Sensor Data Records (SDRs). VIIRS includes a separate instrument, the Solar Diffuser Stability Monitor (SDSM), which observes the sun through an attenuation screen and the VIIRS solar diffuser (SD) directly when the SD is illuminated by the sun. The illuminated VIIRS SD is the on-board source for VIIRS reflective solar band (RSB) calibration, and data from this source are used to derive a calibration scale factor (F factor) that compensates for on-orbit changes in instrument responsivity. By combining the data from the SDSM solar and SD views, a scale factor (H factor) representing the degradation of the SD bidirectional reflectance distribution function (BRDF) is derived and used to maintain the RSB calibration. In early February of 2014 the H factor trend changed distinctly from its typical monotonically downward trend caused by solar UV degradation of the SD surface. The trends in the shorter wavelength SDSM detectors became slightly increasing over time. Then in late May of 2014 the H factor trends changed distinctly again, returning to a more typical downward direction. This presentation describes the analysis performed to assess whether the H factor trend changes were artifacts of the SDSM behavior or caused by an actual change in the typical rate of SDSM BRDF degradation. The analysis indicates that the SDSM BRDF behavior over time did in fact change. The analysis also addressed whether or not there might have been a change in VIIRS response to the illuminated solar diffuser during the time of the H factor trend change, but uncertainties are greater regarding this issue.