A gradual, but persistent, decrease in the optical throughput was detected during the early commissioning phase for the
Suomi National Polar-Orbiting Partnership (SNPP) Visible Infrared Imager Radiometer Suite (VIIRS) Near Infrared (NIR)
bands. Its initial rate and unknown cause were coincidently coupled with a decrease in sensitivity in the same spectral
wavelength of the Solar Diffuser Stability Monitor (SDSM) raising concerns about contamination or the possibility of a
system-level satellite problem.
An anomaly team was formed to investigate and provide recommendations before commissioning could resume. With few
hard facts in hand, there was much speculation about possible causes and consequences of the degradation. Two different
causes were determined as will be explained in this paper. This paper will describe the build and test history of VIIRS, why
there were no indicators, even with hindsight, of an on-orbit problem, the appearance of the on-orbit anomaly, the initial
work attempting to understand and determine the cause, the discovery of the root cause and what Test-As-You-Fly (TAYF)
activities, can be done in the future to greatly reduce the likelihood of similar optical anomalies. These TAYF activities are
captured in the “lessons learned” section of this paper.