The Radiometric Characteristics of Sounding Channels for FY-3A/MWHS

The microwave humidity sounder (MWHS) is installed on new generation of Fengyun polar orbiters, FY 3A satellite, which is launched on 27 May 2008, for global all weather humidity sounding. The sounder produced by Center for Space Science and Applied Research Chinese Academy of Sciences has 5 channels in the range of 150—191 GHz. Around the center frequency of 183.31 GHz water vapor line, there are 3 sounding channels at 183.31±1, 183.31±3, and 183.31±7 GHz. All these three channels have two pass bands, and are only sensitive to vertical polarization at nadir. At 150 GHz there are 2 subsidiary window channels of V/H polarization.Every 2.667 s, MWHS scans through 98 earth views, 3 space views, and 3 internal black body target views. Each pixel has a nominal field of 1.1° (15 km on surface at nadir). One scan line could cover±53.35° from sub satellite point. The instruments are comprehensively tested to determine their actual antenna, spectral, thermal and radiometric characteristics.In order to determine the radiometric performance and verify the method for calibration of MWHS, an extensive test is performed on the flight models. The instruments are placed in a 3 m thermal-vacuum chamber where the temperature could be controlled simulating the conditions in orbit and they are used to observe an earth target and a space one. The thermal-vacuum facility is used to modify temperature circularly to test the instrument in vacuum before launch. The temperature of MWHS is controlled at 273 K, 283 K, 293 K and 303 K except in the earth and space directions where the shroud is maintained at any desired temperature between 100 K and 350 K for an earth target, and 90 K for space target. The whole targets temperature is measured to an absolute accuracy of better than 0.1 K by regularly calibrating the PRT with a standard one traceable to a national standard.The T/V test data are quantitatively analyzed, and the T_NE of MWHS three channels on 183.31 GHz water vapor line at 183.31±1, 183.31±3 and 183.31±7 GHz are calculated. All three channel T_NEare less than 0.90 K. MWHS is designed to allow a 2 point calibration, for each scan line, from the internal target and space views. The instrument response is assumed to be linear between these two calibration points. After correcting all the biases, the calibration accuracy is well below 1.05 K, and the changes of calibration results with instrument temperature are relatively small. Therefore, it's expected that temperature variations in orbit will not affect measuring brightness temperatures much.The radiometric characteristics analysis of sounding channels for FY-3A/MWHS will be the base for quantitative application in orbit. And the T/V test results will be directly used in orbit calibrating. Once MWHS is in orbit, the T_NE will not be measured directly but the variance of the internal target will provide a measurement of the noise for each channel, and the in orbit radiometric sensitivities can be monitored. The MWHS calibration test can be validated using NWP results and intensive field campaigns. The lab test results will be valuable for interpretation of the in orbit data.