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The present paper describes the cross calibration of the Remote Sensing Instrument (RSI) onboard Formosat-2 using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) onboard Terra as a reference, which has been continuously calibrated by means of the onboard and vicarious calibration methods. The data integration system for multi-satellite sensor databases on the Global Earth Observation Grid (GEO Grid) is used in order to find pairs of images acquired by both satellite optical sensors over the radiometric calibration sites on the same days. Radiative transfer simulations that consider different imaging conditions using the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) code are performed based on the Moderate-resolution Imaging Spectroradiometer (MODIS) atmosphere and land products, and the optimum methodology for this cross calibration is discussed under the assumption of no ground-based measurements. The derived calibration coefficients are normalized by the values of preflight calibration and are compared with the results of in-flight cross calibration by the Centre National d'Études Spatiales (CNES) and vicarious calibration described in our previous paper. The average degradations of sensor sensitivity since preflight calibration until 2009 are estimated to be 10% (Band2), 11% (Band3), and 8% (Band4). The derived long-term degradation trends are almost consistent with the results of our vicarious calibration within the precision. The cross-calibration methodology used in the present paper facilitates the in-flight radiometric calibration of satellite optical sensors.