Mitigating the Saharan PICS natural variability to reduce overlapping sensor intercalibration uncertainties

Abstract

The CEOS recommended Pseudo Invariant Calibration Site (PICS) have 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 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, as well as to radiometrically scale between MODIS, VIIRS, and geostationary imagers to ensure consistent cloud and radiative flux retrievals. The IGCG has recently enhanced their PICS stability assessment by improving their clear-sky identification, BRDF, and atmospheric corrections. The Libya-4 PICS trend stability was similar across all scan angles, spectral bands, Aqua-MODIS and VIIRS imagers. Using the same Libya-4 PICS methodology, the study evaluated the stability of 15 Saharan and Arabian PICS. The optimal PICS would have the least surface natural variability with no long term discernable trends, the most spectrally consistent after atmospheric correction, and have the greatest clear-sky probability.Nine of the 100 km PICS had clear-sky probabilities between 45% to 55%, however the clear-sky probability for Mali, Mauritania-1, and Mauritania-2 was less than 30%. The 1.61µm, 0.65µm, 2.15µm, 0.48µm, and 0.55µm band atmospheric corrections improved the temporal stability by 5-19%, 12-24%, 47-68%, 22-68%, and 14-54%, respectively. The PW atmospheric correction was the most effective across all PICS, whereas the AOD and ozone atmospheric corrections were less effective. Surprisingly, Libya-2 and Libya-4 had 0.48µm and 0.55µm band atmospheric correction reductions less than 22%, whereas Libya-1 had 0.48µm and 0.55µm band atmospheric corrections greater than 46%. However, the AOD and ozone variability was very similar for the Libya PICS. Libya-1 and Libya-2 are the most optimal PICS having temporal fluctuations within 0.5%.