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The main different between prelaunch and on-board calibration of a microwave radiometer is the different of the cold calibration target used in two-point calibration. On board calibration usually uses cold sky which is called cosmic background temperature of 2.7 K, as the lower end in the response of a microwave radiometer, while in the prelaunch calibration, a blackbody at the temperature between 80 K to 100 K is often employed in the calibration practice. Therefore, the applicability of the nonlinear coefficients derived from the prelaunch calibration test for on-board calibration is a question which has long been concerned and never been validated. In this paper, a special program of prelaunch calibration was designed, which adds a validation blackbody target which works at ambient temperature of 180~200 K in the thermal vacuum chamber, in addition of a traditional cold blackbody target, a warm blackbody target, and a varied-temperature blackbody target. This validation target provides an opportunity of to compute nonlinear coefficients directly through three-point calibration through extrapolating the measurements to the cold sky temperature. Through comparing the results from the traditional and the novel methods of deriving nonlinear coefficients and applying the two groups of nonlinearities to the same data sets, the errors in applying the nonlinear coefficients by traditional calibration method can be evaluated.