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The CEOS recommended Libya-4 Pseudo Invariant Calibration Site (PICS), located at 28.55° N and 23.39° E, has been extensively used for post-launch radiometric calibration and stability assessment of high, medium, and low-resolution satellite imagers, including MODIS and VIIRS. The NASA LaRC CERES Imager and Geostationary Calibration Group (IGCG) utilizes Libya-4 to perform an independent assessment of the radiometric stability of the MODIS and VIIRS L1B products, which are used in scene identification to convert CERES broadband radiances into fluxes. The site is also used for absolute radiometric scaling between MODIS, VIIRS, and geostationary imagers to ensure consistent cloud and radiative flux retrievals. The Libya-4 clear-sky TOA observed reflectances from sun-synchronous sensors are modeled as a function of solar angle. The model provides observed relative reflectances within 1% for bands 1-6 and 1.8% for band 7 (2.1-μm). The Terra-MODIS, Aqua-MODIS, and NPP-VIIRS Libya-4 TOA observed relative reflectances are shown to fluctuate in tandem. The residual reflectance variability is associated with cloudy and dust storm events as well as seasonal variations of atmospheric parameters, such as precipitable water (PW) and ozone. By correlating the Aqua-MODIS TOA reflectances with PW and solar angle, the 0.64-μm, 0.86-μm, 1.24-μm, and 2.1-μm relative reflectance variability is reduced by 10%, 25%, 20%, and 50%, respectively. The relative reflectance dependency with ozone was minimal. Bright reflectance outliers were associated with large AOD events, whereas darker outliers were related to cloud events. The improved Libya-4 approach provides Aqua-MODIS C6.1 channel stability assessments that range between 0.5% for the 1.6-μm band and 0.9% for the 0.46-μm band.
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