A newly designed thermal/vacuum (T/V) prelaunch multiangle calibration test is proposed for Microwave Humidity and Temperature Sounder (MWHTS) onboard the Feng Yun 3D satellite. Compared with other existing prelaunch calibration tests designed at fix scan angle, this test is first proposed to analyze the measurements of MWHTS from multiscan angles. A set of comprehensive calibration method is established and used for calibration data processing. The high-quality data preprocessing methods are performed. Essential calibration coefficients such as cold bias, hot bias, and nonlinearity coefficients, and calibration results such as calibration residual error, sensitivity, and calibration accuracy of MWHTS are all obtained and analyzed in MWHTS's six observing positions. The measured sensitivity value of channel 2 is 1.5 K while the measured sensitivities of the others range from 0.18 to 0.57 K. The calibration accuracy of channel 2 is 1.5 K while the calibration accuracies of the others range from 0.32 to 0.7 K. According to the discrepancy of multiangle calibration test results, the radiation leakage and the response lag for special channels are discovered for the first time in T/V test, and the reasons of these two special phenomena are fully analyzed. Moreover, the measurements of the T/V test with fast changing instrument temperatures are analyzed and the applicability of nonlinearity coefficients obtained in T/V test for in-orbit calibration is verified. In conclusion, MWHTS meets the basic design requirements of sensitivity and calibration accuracy, the changing of the MWHTS observing positions does not impact the calibration coefficients and results, and the nonlinearity coefficients obtained in T/V test are valid when instrument temperature experiences some fast changing.

In this paper, the focus is on precipitation rate retrieval using 118.75 and 183.31-GHz channels from the microwave humidity and temperature sounder (MWHTS) onboard the FY-3C satellite. These channels provide not only atmospheric humidity profile measurements but they also provide temperature profile measurements. These measurements play important roles detecting and retrieving precipitation rates. Data from the MWHTS and the Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis (TMPA) 3B42 are matched in time and space. Matched datasets are used to verify precipitation detection algorithms and precipitation rate retrieval algorithms. To retrieve the precipitation rate, the multiple linear regression algorithm and back propagation (BP) neutral network algorithm are applied in the ten models proposed in this paper. After detailed retrieval analysis between these two algorithms, advanced algorithms are presented this paper to improve the accuracy of the precipitation rate retrieval. In our algorithms, the correlation coefficient of the precipitation rate over the ocean can reach 0.82, the bias is approximately 0.07 mm/hr, and the root mean square error (RMSe) is less than 0.4 mm/hr. Over land, the correlation coefficient is approximately 0.73, the bias is 0.1 mm/hr, and the RMSe is less than 0.5 mm/hr. These results show that our retrieval algorithms improve the retrieval accuracy. Moreover, we study a number of typhoons and find that the FY-3C MWHTS works well to retrieve typhoon precipitation rates. The potential sensitivities of the temperature and humidity channels are analyzed, which can improve the retrieval accuracy and lay the foundation for further research.

The microwave radiation data from the microwave humidity and temperature sounder (MWHTS) onboard FY-3C meteorological satellite was employed in the data assimilation system to estimate the impact of this data on typhoon forecasts. 118.75-GHz and 183.31-GHz channels from the MWHTS provide atmospheric temperature and humidity profile measurements, respectively. These measurements play critical roles forecasting the path of the typhoon and pressure of the typhoon center. In this paper, the data from the MWHTS are implemented in the Weather Research and Forecasting (WRF) model data assimilation system (WRFDA) to forecast the path of the typhoon and pressure of the typhoon center. The results demonstrate that the path prediction of the typhoon has been improved, but the error of the forecast pressure did not decrease significantly. It can be concluded that the data of the 118.75-GHz and 183.31-GHz channels from the MWHTS for predicting the path of the typhoon is effective. In particular, the 118 GHz frequency is the first time applied to polar orbiting meteorological satellites in the world. The application of the 118-GHz channels in data assimilation will deserve further research in the future.

FengYun-3F (FY-3F) is a sun-synchronous satellite launched on August 3, 2023. It crosses the Equator at a local time of 10:15 a.m. and will be a substitute for FY-3C. This study introduces the preflight and in-orbit calibration of microwave humidity sounder-II (MWHS-II) onboard FY-3F. The instrument nonlinearity and radiometric sensitivity are characterized in the ground thermal vacuum chamber (TVAC) tests. The residual of nonlinearity correction is within 0.5 K, depending on viewing scenes and channels. As a substitute for its predecessor FY-3C, the performance of MWHS-II onboard FY-3F is compliant with requirements. The in-orbit radiometric sensitivity of the sensor is consistent with the prediction of the TVAC test results, with margins of 20%-60% relative to the requirements. The in-orbit behavior of the sensor is further evaluated by simultaneous nadir overpass (SNO) with the Advanced Technology Microwave Sounder (ATMS) and the difference between observation and background (O-B). The comparisons confirm that the radiometric performance of MWHS-II onboard FY-3F is improved, compared to FY-3C. With stable and consistent observations, MWHS-II onboard FY-3F will benefit numerical weather predictions by providing accurate temperature and humidity information at the 118 GHz and 183 GHz bands.

Compared to existing polar-orbiting satellites, the small satellite constellation is of low-cost, rapidly deployable and mission-flexible. Meanwhile, a series of challenges are followed in aspects of size, weight, life, stability, detecting accuracy, consistency of multi-satellite and so on. This paper mainly focuses on the characteristic of diode detector for Microsat microwave radiometer. Given the certain observation target, the retrieved brightness temperatures would have changed correspondingly under different system AGC (Automatic Gain control) values. By adjusting the system gain, a little change happened but still in normal workspace. The variation caused by nonlinearity of diode, by using two-point calibration method, cannot be completely eliminated. Setting a wide range of 3-350K, the experimental results demonstrate that there is obvious nonlinearity for brightness temperatures between retrievals minus actual values and observations. So, in the procedure of real-time calibration for Microsat microwave radiometer, the system must be assured working in approximately linear workplace with a certain and reasonable AGC value.

Since the first Microwave Humidity Sounder (MWHS) launched on May 27th, 2008, six MWHS deployed on polar-orbiting FengYun-3 (FY-3) satellites to date have been continuously providing temperature and humidity information for over 15 years. Compared to the predecessors MWHS onboard FY-3A/B/C/D, the central frequency of the window channel is shifted from 150 GHz to 166 GHz for MWHS-II onboard FY-3E/F, and the additional low noise amplifier (LNA) embedded in the Radio Frequency (RF) module optimizes the performance of the receiver system, leading to a significant reduction in the noise temperature of the receivers. The reduced bias and standard deviation of simultaneous Nadir Overpass (SNO) results between MWHS-II and Advanced Technology Microwave Sounder (ATMS) further corroborate that that the instrument performance has been improved and updated. The interchannel interference of FY-3E/F is mitigated, and the channel sensitivities are also enhanced according to the postlaunch test results.