星载微波湿度计发射前定标和在轨定标的最大不同是冷源的使用：在轨定标的冷源是宇宙冷空背景温度，通常为2.7K，而发射前定标的冷源通常控制在80-100K之间。这样通过发射前定标得到的非线性系数是否可以用于在轨定标一直是一个非常关心和值得研究的问题。本文通过在FY-3E微波湿度计发射前热真空定标中除了传统需要的冷热、变温定标源之外，另外设计了一个独立的位于180-200K附近的定标源。利用该验证源，本文提出了三点定标公式，并通过外推到冷空亮温的模拟观测计算非线性系数。随后通过比较传统和新型非线性系数推导方法的结果，并将两组系数带入同一组数据中分析两组发射前定标系数用于在轨定标的适用性。

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.