1. A Combined Vicarious Calibration Method for GOES Imager Visible Channel Fangfang Yu 1* and Xiangqian Wu 2 1 ERT, Inc. @ NOAA/NESDIS Center for Satellite Applications and Research (STAR), College Park, MD, 20740, U.S.A. 2 NOAA/NESDIS Center for Satellite Applications and Research (STAR), College Park, MD, 20740, U.S.A. * Fangfang Yu, Fangfang.Yu@noaa.gov, 301-683-3553, postal address: NOAA, Fangfang Yu, NCWCP, 5830 University Research Court (cube #2745), College Park, MD 20740 Evaluation of GOES Imager Visible Channel Calibrations Introduction GOES instruments experience continuous degradation in space. However, due to the lack of onboard calibration systems, vicarious calibration is the only way to provide accurate radiometric data for the solar reflective channels. Even with the inclusion of onbard calibration devices, vicarious calibration will continue to play an important role in verifying and serving as risk reduction for the onboard systems. Over the past decades, various vicarious calibration methods have been developed at National Oceanic and Atmospheric Admission (NOAA)/National Environmental Satellite, Data, and Information Service (NESDIS) to monitor the sensor degradations of the Imager visible channels and to provide accurate post- launch calibration data as well. These methods include satellite observations of deep convective cloud (DCC), desert, and stable extra-terrestial targets such as the Moon and stars, and the inter-calibrations with different reference sensors. Each method has the pros and cons with compariable calibration accuracy. In this study, a combined calibration algorithm is proposed to iimprove the calibration accuracy. Trending In the GSICS solar calibration project, it is required that each vicarious calibration method should be traceable to the common reference - Aqua Moderate Resolution Imaging Spectroradiometer (MODIS). The calibration targets should be characterized with Aqua MODIS Collection 6 (C6) observations. Target Characterization with Aqua MODISTarget Trending Calibration MethodCharacterized with Aqua MODIS? (absolute calibration) Spectral variationSpatial variation BRDFRadiometric Stability ReflectanceBRDFAtmospheric impacts Observation frequency & data accessibility Seasonal Variation Spectral Band Adjusted Factor Other issues DCCYESVery smoothSmall StableHighSmall Dependent on DCC locations and frequency SmallHyperion/GOME/SCI AMARCHY Insufficient DCC pixels for GOES-West Sonoran DesertYESSome variationsMediumStrongLong-term stable, affected with ENSO events MedianStrong Every half an hourStrongHyperionLarge satellite viewing angles; Target radiance/reflectance may be affected with the instrument SRF degradation, if any, due to target spectral variation MoonYESSmooth--Extremely stableRelatively low-NOScheduled monthly moon obs. + a few irregular unscheduled ones -HyperionRequires a public accessible resource to accurately predict the moon irradiance over a large range of lunar phase angles. StarNO---Extremely stable--NODaily, a group of fixed stars-NOSensitive to instrument diurnal/seasonal optical performance variations GOES-MODIS Inter-calibration (Ray-matching over cloud) YESRelative smooth, yet may be affected with surface at partial cloud pixels ---High-Small3-5 times/month-Hyperion/GOME/SCI AMARCHY Spectral variations at different cloud types Combined Method Conclusions (1)Each vicarious calibration method, including the desert-based one, shows very similar degradation pattern. (2)Effort is still ongoing to integrate the target characterization with the trending analyses. (3)The combined method proposed in this study can improve the relative calibration accuracy as low as 0.5%. This uncertainty result is based on long-term data. Further research is needed to evaluate the uncertainty for the near real-time data, especially for the period soon after the satellite operation. (4)Each individual algorithm contributes to the combined method. DCC and star-based methods are two leading contributors. (5)Effort to reduce the uncertainty at each individual algorithm should continue to validate and improve the results of the combined method. (6)Other combined methods should be also explored. Time-series of GOES-13 DCC reflectance, represented with monthly accumulated mode reflectance Time-series of Sonoran desert daily reflectance around noon time; mean of the clear pixels Star signals – normalized to a specific time observationTrending of lunar observations – ratio between the satellite observations to ROLO model results Time-series of GOES-MODIS inter-calibration correction adjustment factors GOES-MODIS Inter-Calibration Lunar Calibration Star Calibration Desert CalibrationDCC Calibration Average the daily degradation rate at monthly scale Combined trending, anchored at one GOES-MODIS inter-calibration data Recursive Filtering to improve the calibration accuracy Total obs #Obs. # used in the combined method ContributionTrending Uncertainty DCC965456.3%1.28% Desert872629.9%2.13% Moon27414.8%2.05% STAR562850.0%0.97% GOES-MODIS Inter- calibration 251144.0%1.13% Combined Method0.48% References Acknowledgements This work is partially funded by NOAA/NESDIS/STAR Calibration/Validation support. We would like to thank the support from I-L. Chang, C. Dean, M. Grotenhuis, H. Qian and many others who had worked at NOAA GOES Imager visible calibrations. The contents of this document are solely the opinions of the authors and do not constitute a statement of policy, decision, or position on behalf of NOAA or the U.S. government. Yu, F., X. Wu, A. Heidinger and D. Doelling, 2012. Evaluation of GOES Imager visible channel calibrations, EUMETSAT, Sept. 2012, Sopot, Poland (oral talk). Dean, C., I-L. Chang, Z. Li, M. Weinreb, X. Wu and F. Yu, 2012. Recent advances in calibration of the GOES Imager visible channel at NOAA, SPIE, 8510, doi:10.1117/12.929036. Wu, X. and F. Sun, 2005. Post-launch calibration of GOES Imager visible channel using MODIS, SPIE, vol. 5882, doi:10.1117/12.615401. Chang, I-L., C. Dean, D. Han, D. Crobsy, M. Weinreb, J. Baucom, P. Baltimore and X. Wu, Improvements in the star-based monitoring of GOES Imager visible-channel responsivities, SPIE, Vol. 5882, doi:10.1117/12.614601. Wu, X., T.C. Stone, F. Yu, and D. Han, 2006. Vicarious calibration of GOES Imager visible channel using the Moon, SPIE, vol. 6296., doi:10.1117/12.681591. Rao, C.R.N, J. Chan, J. Sullivan and N. Zhang, 1999. Post-launch calibration of meteorological satellite sensors, Adv. Space Res. Vol. 23(8), 1357-1365. Table. Contributions of each individual vicarious calibration to the combined method