Assessment of MODIS and VIIRS solar diffuser on-orbit degradation

Abstract

Both MODIS and VIIRS instruments use a solar diffuser (SD) for their reflective solar bands (RSB) on-orbit calibration. On-orbit changes in SD bi-directional reflectance factor (BRF) are tracked by a solar diffuser stability monitor (SDSM) using its alternate measurements of the sunlight reflected off the SD panel and direct sunlight through a fixed attenuation screen. The SDSM calibration data are collected by a number of filtered detectors, covering wavelengths from 0.41 to 0.94 m. In this paper we describe briefly the Terra and Aqua MODIS and S-NPP VIIRS SDSM on-orbit operation and calibration activities and strategies, provide an overall assessment of their SDSM on-orbit performance, including wavelength-dependent changes in the SDSM detector responses and changes in their SD BRF, and discuss remaining challenging issues and their potential impact on RSB calibration quality. Due to different launch dates, operating configurations, and calibration frequencies, the Terra and Aqua MODIS and S-NPP VIIRS SD have experienced different amount of SD degradation. However, in general the shorter the wavelength, the larger is the SD on-orbit degradation. On the other hand, the larger changes in SDSM detector responses are observed at longer wavelengths in the near infrared (NIR).

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

“…Our model shows deviation of SDSM data from the model for short wavelength ( Figure 6) when only one fitting parameter, i.e., σ s l, is used to fit for all of the M bands. The deviation can be due to the impacts of SDSM detector relative spectral responses [35]. The SDSM detector response degradation is wavelength-dependent with larger changes at longer (NIR) wavelengths.…”

“…This type of degradation is most likely due to displacement damage to the SDSM detector system by the high-energy protons in the space operation environment. Since the SDSM is essentially a ratioing radiometer, its degradation has minimal impact on the determination of the SD reflectance degradation [35]. While using high order fitting parameters improves the modeling accuracy, our work focused on revealing the dominant physical origin of the SD BRDF degradation, which suggests that the degradation process is dominated by 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.…”

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

“…The difference between grey and chromatic degradation is most clearly noticeable for broad band rather than narrow band optical devices. Figure 2 illustrates an example of chromatic degradation picked-up from the monitoring of the MODIS solar diffusers [12,13], which are broad band optical devices and though not in the optical path for Earth observations are nevertheless an example of degradation due to contamination of reflective optical elements in Space. Illustrative example: chromatic degradation of the Aqua-MODIS solar diffuser.…”

“…Illustrative example: chromatic degradation of the Aqua-MODIS solar diffuser. The coloured surface interpolates between data measured in nine narrow-band channels (data courteously provided by Xiong et al [13]).…”