Determination of the SNPP VIIRS solar diffuser BRDF degradation factor over wavelengths longer than 1μm

Abstract

To radiometrically calibrate its reflective solar bands, the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite observes a sunlit onboard solar diffuser (SD).Once on orbit, the bidirectional reflectance distribution function (BRDF) of the SD degrades over time. The degradation factor is determined by an onboard solar diffuser stability monitor (SDSM). However, the central wavelengths (414 to 929 nm) of the SDSM detectors do not cover the VIIRS short wave infrared (SWIR) bands which have central wavelengths from 1238 to 2257 nm. Although it is known that at wavelengths longer than 1 m the SD BRDF degrades at a much slower rate, the degradation at some of the SWIR bands' central wavelengths may still be non-negligible as Ocean Color products often require a radiometric calibration stability of at least 0.1%. To access the SD BRDF degradation factor in the SWIR bands wavelength region, we investigate a phenomenological model and apply the model to determine the degradation factor in the SWIR bands wavelength region. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/29/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership (SNPP) satellite 1 radiometrically calibrates its solar reflective bands (RSBs) by observing a fully solar illuminated solar diffuser (SD) panel 2-4 . The SD bidirectional reflectance distribution function (BRDF) decreases with time. An onboard solar diffuser stability monitor (SDSM) is employed to determine the degradation factor 5-8 .The SDSM observes the Sun through a screen with pinholes (the SDSM screen) and a fully sunlit SD at almost the same time. When used in a time series, the ratio of the observed detector digital counts is a clear measure of the BRDF degradation. However, the eight detectors of the SDSM, spectrally centered from 414 nm to 929 nm 9 , cover much shorter wavelengths than those of the short wave infrared (SWIR) bands in the RSBs. The central wavelengths for the SWIR bands are from 1238 to 2257 nm. In the literature, the SD BRDF degradations over the VIIRS SWIR band wavelengths are ignored 2-4 . To accurately calibrate the SWIR bands and to satisfy the 0.1% radiometric calibration stability requirement for Ocean Color products 10 , we investigate a phenomenological model and apply the model to determine the SD BRDF degradation in the SWIR band wavelength region. DATA SELECTION FOR ANALYSISTo develop a phenomenological model to determine the SD BRDF degradation in the SWIR band wavelength region, we examine the degradation factors (value is one means no degradation) determined by the SDSM observations from detectors 5 to 8 with band central wavelengths of 675 to 929 nm.The degradation factors are calculated with the SDSM screen relative transmittance and the relative product of the SD screen transmittance times the SD BRDF at the initial time when the BRDF started to degrade. T...

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

“…Recently, there have been several studies [34][35][36][37] to improve the modeling accuracy of the VIIRS SD reflectance degradation and the radiometric calibration accuracy of VIIRS RSB sensor. For example, Lei and Xiong presented a method in [34] to determine the SNPP VIIRS SD BRDF degradation factor over wavelengths longer than 1 µm.…”

“…For example, Lei and Xiong presented a method in [34] to determine the SNPP VIIRS SD BRDF degradation factor over wavelengths longer than 1 µm. They used a phenomenological power law model with fitting function 1´H " α{λ η to simulate the spectral dependent degradation of SD monitored by SDSM.…”

“…However, our model fixed the exponent as 4 since the dominant origin of SD spectral degradation is due to surface roughness-induced Rayleigh scattering in our SRRS model. With two fitting parameters, it is determined in [34] that the best fitting exponent η " 4.07 for SDSM data over four wavelengths: 675, 746, 874 and 929 nm. This fitting exponent is consistent with our SRRS (91{λ 4 )-model.…”

“…This is the origin of the degradation of reflectance in the short wavelength range for SD material after exposure to space radiation. It should be noted that although previous studies have used H " 1´α{λ η equations to mathematically model the SD degradation [34], our study is the first to establish the physical model based on SD Surface Roughness induced Rayleigh scattering on the SD surface and related degradation. To quantify the effect of Rayleigh-type scattering on the spectral dependent degradation of SD material due to surface roughness change, we first introduce parameters to characterize the surface roughness in Section 3.1, and the model for surface roughness-induced degradation is presented in Section 3.2.…”

“…Both facts confirm that the SD surface roughness length is much less than the wavelength and Rayleigh scattering is the dominant mechanism responsible for the observed SD spectral dependent degradation. In [34], it is first empirically determined that the degradation factor is proportional to 07 . 4 / 1 λ , which is consistent with our SRRS theoretical model.…”