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This paper describes radiometric performance and prelaunch radiometric and spectral calibrations of the GHz component of the Microwave Limb Sounder (MLS) experiment on NASA's Aura spacecraft. Estimated systematic scaling uncertainties (3σ) on limb port radiances are ∼0.5% from radiometric calibration and ∼0.5% to ∼1% from spectral calibrations. Operational noise performance is consistent with prelaunch expectations, and in-orbit measurements to date indicate no changes in noise characteristics, and no observable calibration drifts. Spectral baseline has remained stable to ∼20 mK since launch. Refinements to calibrations based on in-flight data are discussed, and radiometric calibration algorithms are described.