Vicarious calibration of long-term AVHRR solar reflectance channels using the desert

The Advanced Very High Resolution Radiometers (AVHRR) flown on the NOAA and EUMETSAT's polar satellites have been continuously collecting the Earth surface data since 1978 and will continue to provide the global imagery until 2020. The long-term data record and capabilities for measuring Earth surface properties makes AVHRR indispensable for Earth system science and global change studies. However, the wide spread of AVHRR solar reflectance products is often constrained by calibration uncertainty, largely due to the lack of on-board calibration device. Since mid-1990s, NOAA has been using the Libyan Desert as the reference target to provide the operational calibration coefficients for the AVHRR visible and near-infrared (NIR) channels. The premise of this vicarious calibration is the long-term radiometric stability of the reference target. Yet radiance measured at top-of-atmosphere (TOA) is often affected with short-term variations in the atmospheric components, changes in sun-satellite-viewing geometry, and disrupts caused by the changes in the spectral response functions of the instruments. The objectives of this study thus has three folds, 1) to evaluate the long-term stability of desert surface with the measurement of other satellites with the on-board calibration techniques, 2) to improve the current bidirectional reflectance distribution function (BRDF) for the AVHRR/2 instruments which have the dramatic orbit drifts, and 3) to compensate the changes in the spectral response functions between the AVHRRs using the radiometric transfer model. Results of this research will be presented in the coming meeting.