Spectral Dependent Degradation of the Solar Diffuser on Suomi-NPP VIIRS Due to Surface Roughness-Induced Rayleigh Scattering

Abstract

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.

Climate Data Records from Meteosat First Generation Part II: Retrieval of the In-Flight Visible Spectral Response

“…Figures A1 and A2 nevertheless demonstrate that Equation (A16) reproduces the ageing characteristics of the solar diffusers equally well or better, in particular in the UV and blue where Mie [49] scattering starts (cf. Figure 6 [19]). Figure A2.…”

“…Unlike the surface-induced Rayleigh scattering model [19] that has been suggested to explain the observed degradation of the VIIRS and MODIS solar diffusers, the concluding Equation (A16) is not physically explanatory in detail. Figures A1 and A2 nevertheless demonstrate that Equation (A16) reproduces the ageing characteristics of the solar diffusers equally well or better, in particular in the UV and blue where Mie [49] scattering starts (cf.…”

“…Identifying the effects that are sources of errors in the evaluation of this term and quantifying the associated uncertainties establishes the metrological traceability to the solution of the inverse problem defined in Equations (19) and (20). The measurement model (or measurement equation) is placed in the centre, and for each parameter within the equation the origin of uncertainties associated with that quantity is considered.…”

“…Multiple studies have presented examples for degradation of instrument performance in Space that is stronger in the UV and blue than in the red and near-infrared (CERES [15], MODIS [12,13], VIIRS [19]). For the VIS channel of MVIRI indirect evidence of an alike kind of chromatic degradation has been brought forth [8].…”

“…The phenomenon of chromatic degradation has been well known from the monitoring and in-flight calibration of numerous optical Earth Observation instruments (e.g., SeaWiFS [14], CERES [15], MERIS [16], VIIRS [17][18][19]) and space-borne astronomical telescopes. Contaminants evaporated from an instrument's interior and condensation or UV-stimulated deposition and polymerisation of contaminant films onto the surface of optical elements have been considered as the main reasons for degradation of optical instruments in Space [20][21][22][23].…”