Assessment of Radiometric Degradation of FY-3A MERSI Reflective Solar Bands Using TOA Reflectance of Pseudoinvariant Calibration Sites

Radiometric sensor calibration is critical for quantitative use of the data obtained from the FeungYun-3A MEdium-Resolution Spectral Imager (MERSI) sensor. To meet the required calibration criteria, several vicarious calibration (VC) techniques are being employed in the current operational calibration of MERSI. This letter presents the independent results of sensor degradation assessment which were obtained directly from sequences of top-of-atmosphere (TOA) reflectance from calibration sites. The absolute calibration approaches and the desert tracking approach used in the current operational process involve a radiative transfer model (RTM) when simulating the reference TOA reflectance. However, collecting synchronous in situ measurements for an RTM is usually expensive and time consuming, thus greatly limiting the frequency of such calibration processes. The direct approach developed in this study is an efficient and complementary VC technique that can increase the reliability of VC results in concert with other techniques. TOA reflectance measurements from three pseudoinvariant calibration sites are used to estimate sensor degradation rates. The results are compared with those obtained by other operational methods such as the multisite and the deep convective cloud tracking methods. The direct approach is consistent across the three calibration sites, producing degradation estimates comparable to those of the two other operational methods, suggesting the effectiveness of the direct approach for the MERSI radiometric sensor calibration.