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Since launch, Terra and Aqua MODIS have performed more than 12 and 10 years of scientific measurements of the Earth's surface. MODIS has 36 spectral bands, among which 20 are Reflective Solar Bands (RSB), covering a spectral range from 0.41 mu m to 2.1 mu m. MODIS was developed with stringent requirements for calibration and uncertainty and is equipped with a set of on-board calibrators (OBC) that facilitate a constant monitoring and update of its on-orbit calibration coefficients. The RSB are calibrated on-orbit using a Solar Diffuser (SD) and a Solar Diffuser Stability Monitor (SDSM), with help from the lunar observations via a Space View (SV) port and an onboard Spectroradiometric Calibration Assembly (SRCA). The algorithms to accurately characterize the sensor's gain change and the on-orbit change in the response versus scan-angle (RVS) have been applied to improve the quality of the Earth-view measurements. Various improvements to the calibration algorithms have been incorporated since launch and the following paper will discuss the calibration algorithms and enhancements developed for MODIS Collection 6 (C6) processing. In addition, to supplement the measurements from the on-board calibrators, pseudo-invariant desert targets are also used to track the on-orbit response change for selective RSB. Discussions of the on-orbit calibration uncertainty and the Level 1B (L1B) Uncertainty index (UI) product are also included. A comprehensive assessment of the impact on the L1B product in comparison to Collection 5 (C5) is also discussed. Significant improvements are observed in the case of VIS bands wherein the long-term bias observed in C5 products is eliminated to provide a more accurate radiometric product.