VIIRS VisNIR/SMWIR end of life sensitivity predictions
2012
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting
Partnership (NPP) satellite launched on October 28, 2011 into a polar orbit of 824 km nominal altitude. VIIRS collects
radiometric and imagery data of the Earth’s atmosphere, oceans, and land surfaces in 22 spectral bands spanning the
visible and infrared spectrum from 0.4 to 12.5 μm. The radiometric response for VIIRS spectral bands in the 600 – 2300
nm wavelength range (I1, M5, M6, M7 / I2, M8, M9, M10 / I3, M11), which started with significant signal to noise ratio
margin at beginning of life, has shown some degradation on orbit. This degradation has been correlated with UV
exposure of the VIIRS optics. UV exposure of witness samples from the Rotating Telescope Assembly (RTA) mirrors
by Aerospace Corporation showed reflectance loss with the same spectral signature as the response degradation observed
for VIIRS. The investigation and root cause determination for the VIIRS response degradation are discussed in separate
papers.
A model of VIIRS throughput degradation has been developed from witness sample UV exposure test results made by
Aerospace. A direct relationship is assumed between UV dose (fluence) and the reflectance degradation of the RTA
mirrors. The UV dose on orbit for the primary mirror is proportional to the incident earthshine and its solid angle of
view. For subsequent mirrors the UV dose is weighted by solid angle and preceding mirror UV reflectance. UV dose is
converted to reflectance change based on witness sample exposure measurements. The change in VIIRS throughput is
calculated by multiplying the reflectance of the four RTA mirrors and agrees with the on-orbit measured response
changes as a function of UV exposure time. Model predictions of the radiometric sensitivity for the affected VIIRS
bands show positive margin at end of life for all affected bands.
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