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The Orbiting Carbon Observatory (OCO) by NASA's Jet Propulsion Laboratory (JPL) and Greenhouse gases Observing SATellite (GOSAT) by JAXA were built to provide independent measures of the global distributions of carbon dioxide (CO2) from space. GOSAT achieved a successful orbit on January 23, 2009, and OCO failed its launch attempt on February 24, 2009. Both sensors detect absorptions at the 0.76-??m oxygen band and at the weak and strong CO2 bands at 1.6 and 2.0 ??m, respectively. In order to establish the uncertainties and biases between the respective data products, the OCO and GOSAT teams have planned a number of cross-comparison studies. The first of these, discussed here, is the validation of the prelaunch absolute radiometric calibrations, specified at ??5%. The cross-comparison campaign to validate this OCO approach was performed at NASA's JPL in April 2008. In this paper, the OCO reference detectors and three GOSAT radiometers viewed the OCO sphere and radiometric standard. The overall agreements between the OCO calibration and GOSAT measurement of the OCO integrating sphere were 1.5% at 0.76 ??m, 2.7% ?? 1.1% at 1.6 ??m, and 0.2% ?? 4.1% at 2.0 ??m. To validate the GOSAT preflight calibration, the cross-calibration experiment continued at JAXA's Tsukuba Space Center in December 2008, where the same radiometers measured the two GOSAT spheres. Agreements are better than 1.8% at 0.76 ??m, 1.6% at 1.6 ??m, and 1.4% at 2 ??m. These studies give confirmation that the flight instruments have been calibrated to within their uncertainty requirements.