Abstract
The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard Suomi National Polar Orbiting Partnership (SNPP) uses a solar diffuser (SD) as its radiometric calibrator for the reflective solar band calibration. The SD is made of Spectralon™ (one type of fluoropolymer) and was chosen because of its controlled reflectance in the Visible/Near-Infrared/Shortwave-Infrared region and its near-Lambertian reflectance property. On-orbit changes in VIIRS SD reflectance as monitored by the Solar Diffuser Stability Monitor showed faster degradation of SD reflectance for 0.4 to 0.6 µm channels than the longer wavelength channels. Analysis of VIIRS SD reflectance data show that the spectral dependent degradation of SD reflectance in short wavelength can be explained with a SD Surface Roughness (length scale << wavelength) based Rayleigh Scattering (SRRS) model due to exposure to solar UV radiation and energetic particles. The characteristic length parameter of the SD surface roughness is derived from the long term reflectance data of the VIIRS SD and it changes at approximately the tens of nanometers level over the operational period of VIIRS. This estimated roughness length scale is consistent with the experimental result from radiation exposure of a fluoropolymer sample and validates the applicability of the Rayleigh scattering-based model. The model is also applicable to explaining the spectral dependent degradation of the SDs on other satellites. This novel approach allows us to better understand the physical processes of the SD degradation, and is complementary to previous mathematics based models.
Highlights
The Suomi-NPP satellite was successfully launched on 28 October 2011
The local fine surface roughness on a solar diffuser (SD) material causes the scattering of light, and the reflectance to be modified as Rm pλq “ R0 pλqr1 ́ ST pλqs [30], where the reduction factor ST pλq represents the fraction of scattered light that is trapped in the material or transmitted through
A physics-based surface roughness Rayleigh scattering (SRRS) model (Equation (8)) is presented in this paper to explain the spectral reflectance degradation of Visible Infrared Imaging Radiometer Suite (VIIRS) SD in terms of Rayleigh scattering from surface roughness caused by space radiation
Summary
The Suomi-NPP satellite was successfully launched on 28 October 2011. VIIRS The VIIRS is a scanning radiometer and has 22 spectral bands covering the spectrum between 0.412 μm and 11.5 μm, including 14 reflective solar bands (RSB), seven thermal emissive bands (TEB), and one day–night band (DNB) It primarily focuses on clouds, Earth surface variables, surface temperature and imagery, and provides moderate-resolution, radiometrically accurate images of the globe once per day for the RSBs and twice daily for the TEBs and DNB. Radiometric calibration of RSB using onboard SD is quite common for modern-day satellite instruments such as MODIS on AQUA and TERRA [21,22], Thermal And Near-infrared Sensor for carbon Observation-Fourier Transform Spectrometer (TANSO-FTS) onboard GoSat [23], Operational. Experimental evidence of spectral dependent degradation of SD material and surface roughness changes are reviewed
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