Prelaunch Radiometric Characterization and Reflector Emissivity Measurement of the Second-Generation Microwave Radiation Imager Onboard Fengyun-3 Satellite

The second-generation Microwave Radiation Imager (MWRI-II) is a major passive microwave radiometer carried on the Fengyun-3 Polar-Orbiting Satellite Series. Its observations can enhance numerical weather forecasting, climate change monitoring, and disaster mitigation. This study introduced an expanded thermal-vacuum (TVAC) campaign for MWRI-II, focusing on radiometric performance characterization and reflector emissivity measurement. To mitigate frequent in-flight automatic gain control (AGC) adjustments, optimal AGC voltages were determined for three instrument temperature windows: AGC-1 (5–15 °C), AGC-2 (10–20 °C), and AGC-3 (15–25 °C). Under these optimal operational states, the instrument’s radiometric nonlinearity and sensitivity were thoroughly assessed. Results show that peak nonlinearities (at 190 K) are below 1.0 K for almost all channels, with channel-dependent variations. After nonlinear calibration, the residual uncertainty is only 0.1 K across all channels. Radiometric sensitivity, evaluated using both standard deviation and Allan deviation methods, meets design specifications. To isolate the reflector’s thermal emission from the total received radiation, its emissivity was characterized in the TVAC chamber supplemented by a cold reflector and a larger cold source. By modulating the reflector’s physical temperature and monitoring the corresponding changes in radiometric response, the equivalent emissivity was derived. The emissivity measurements are around 0.004 for most channels. Compared with conventional TVAC testing, this campaign provides refined estimates of reflector emissivity and nonlinear coefficients under the instrument’s optimal operational state.