Radiometric Characteristics of FY-3C Microwave Humidity and Temperature Sounder

The microwave humidity sounder (MWHS) is a five channel microwave radiometer in the range of 150-191 GHz onboard FY-3A and FY-3B. FY-3A and FY-3B are successfully launched in 2008 and 2010, respectively. The next generation of MWHS is a microwave humidity and temperature sounder. This sensor is developed to fly on the third satellite of new generation polar orbit meteorological satellite of China (FY-3C) is launched in September 2013.The microwave humidity and temperature sounder has 15 channels in the range of 89-191 GHz. Eight temperature sounding channels with central frequency of 118.75 GHz oxygen gas line and five humidity sounding channels with central frequency of 183.31 GHz water vapor line. Two window channels center at 89 GHz and 150 GHz. 118 GHz channel is first used to detect atmosphere on current operational satellite. Channels in the oxygen band are at around 54 GHz used by AMSU-A (advanced microwave sounding unit-A) and ATMS (advanced technology microwave sounder). Channels in the next oxygen absorption band are at around 118.75 GHz, which can well detect atmosphere temperature in the lower troposphere. The temperature sounding channels around 118.75 GHz detect the atmosphere temperature from 900 hPa to 25 hPa. The microwave humidity and temperature sounder adds two humidity sounding channels compared with MWHS that can obtain fine vertical distribution structure of atmosphere humidity.In order to determine the radiometric performance and the on-orbit use of the microwave humidity and temperature sounder, an extensive test is performed before launch. The microwave humidity and temperature sounder is placed in a thermal-vacuum chamber where the cold and earth targets are installed at fixed position. The instrument temperature is controlled at 5℃, 15℃ and 25℃ which is expected in orbit. The temperature of earth target maintains from 95 K to 330 K and space target is controlled at 95 K. Temperatures of these whole targets are measured by PRT (platinum resistance thermometer) and the temperature measurement accuracy is better than 0.1 K. The test database include counts of internal blackbody, earth and cold targets are obtained by the new microwave radiometer and the temperature measured by PRT.The sounder is calibrated with the thermal-vacuum chamber test method, and test data are quantitatively analyzed. Results for noise equivalent differential temperatures of fifteen channels show that all fifteen channel measured sensitivities meet requirements of indicators. Noise equivalent differential temperatures of humidity channels are all below 0.5 K which are also at the same level of indicators from ATMS. The channels around 118.75 GHz except channel 2 are all below 1 K, and that means observations from these channels used for temperature retrieval are well. Because the narrow bandwidth of channel 2, the noise equivalent differential temperature of this channel is about 1.7 K that maybe affects retrieval precision. Correlations between all channels are independent. After correcting all biases, the calibration accuracy is well below 1.12 K. Calibration results of microwave humidity and temperature sounder are stability for each channel. The radiometric characteristic analysis of all channels provide useful reference for in-orbit application of the new microwave radiometer sounder on FY-3C.