Observations of the Micro-Wave Humidity Sounder-II (MWHS-II) onboard FengYun-3C (FY-3C) satellite are crucial for numerical weather prediction. Precise calibration of MWHS-II is fundamental for the accurate application. However, the Moon’s presence in the deep space view (DSV) can corrupt the in-orbit calibration of MWHS-II, and thus it is necessary to quantify the Moon’s influence. This study proposes a method to identify the Moon’s intrusion into the DSV, which improves the accuracy of lunar identification, by taking into account the lunar phases and the detectable radiance change of the sensor. The abrupt jump in the DSV due to the lunar contamination is removed, assuming the radiometric gain of the adjacent scans barely changes. With the corrected DSV counts, the effective lunar brightness temperature (TB) is calculated. It is found that the phase of the Moon observed by the FY-3C MWHS-II is in the range of 95°–140°, and the effective lunar TB is up to 15 K at 89/118.75 GHz and 45-50 K at 150/183.31 GHz. Though the results are subject to the uncertainties of the Moon calculations and the beam pointing, this study offers unique insights into the lunar observations of MWHS-II onboard the polar-orbiting FY-3C.

Since the first microwave humidity sounder (MWHS) launched on May 27, 2008, six MWHSs 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 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.

The short-wave infrared Polarization and Multi-Angle Imager (PMAI) onboard Fengyun-3 precipitation satellite is a new spaceborne imaging polarimeter for clouds and aerosols, with polarization channels of 1030, 1370, and 1640 nm. This study presents a detailed description and assessment of the calibration model of PMAI. For radiometric intensity calibration, multiple parameters in the radiometric model are fitted into a single coefficient to simplify calibration. Results show that the radiometric calibration uncertainty of the full image plane is better than 0.02, and the calibration coefficient increases as field of view increases. The maximal unsaturated incident radiance of all channels is equivalent to 100% albedo, and signal-to-noise ratio at the referenced radiance is greater than 115 and 182 for the polarized and unpolarized channels, respectively. The response of all channels shows high linearity and good uniformity of the full image plane. Based on results of intensity calibration, a polarization calibration model using a fully linear polarized light source is introduced with a polarization measurement matrix established by a simplified method and a calculation method. Assessment of polarization measurement indicates that the uncertainties of the obtained degree of linear polarization (DoLP) and angle of linear polarization (AoLP) based on the two methods are highly consistent. When fully linearly polarized light is incident, the measurement error of DoLP using the simplified polarization measurement matrix is within 0.02 and that of AoLP is less than 1°. Therefore, the simplified radiometric intensity and polarization calibration model meets the measurement accuracy requirements and improves the calibration efficiency.

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-background (O-B). The results 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- and 183-GHz bands.

A re-calibration algorithm of MWRI (Microwave Radiation Imager) onboard FengYun series satellites was developed based on the error budget analysis of MWRI and the main calibration parameters simulation and iteration. SNO (Simultaneous Nadir Overpass) between MWRI and GMI were used to get the brightness temperature of stable radiation target including sea surface and rain forest. Based on those brightness temperature datasets of stable target, DD (double difference) were used with RTTOV simulation result to cancel the effect of different sensor parameters between MWRI and GMI. Finally, emissivity of hot reflector, back lobe of hot reflector, efficiency of hot load, non-linearity parameter of MWRI were simulated and corrected. Using the corrected parameters, re-calibration of MWRI was done and we get the 10 years’ time series dataset. Validation result shows that the dataset has very high accuracy and stability compared with operational dataset.

The second-generation Microwave Humidity Sounder (MWHS-2) onboard Fengyun (FY)-3C has a data quality comparable to that of counterpart microwave moisture sounders. However, channels 13 and 14 have large biases that are negatively correlated with the instrument temperature and prevent the operational assimilation of the data of these two channels. To better understand the biases of channels 13 and 14, the correlation of the observation minus simulation data (O–B bias) of different channels with the instrument temperature and other factors is investigated for FY-3C MWHS-2 in this article. A sensitivity analysis using a gradient boosting decision tree indicates that the instrument temperature and scan position are the two dominant factors, with total bias-contribution scores exceeding 0.8 for both channels 13 and 14. This conclusion is verified through further analysis of the bias distributions for the scan position, scene brightness temperature, and ascending/descending orbits. Using 12-week data recorded in 2016, the correlations of the O–B bias with the scan position and instrument temperature are specifically investigated and a correction algorithm is formulated, with which data for 2016 and 2017 are corrected. The corrected data in both channels have smaller, more stable biases, and are less affected by the instrument temperatures and scan position. The bias contributions associated with these two factors should thus be further studied.

This study characterizes the marginally-known angular-dependent polarization difference of the land surface emissivity (LSE), using the microwave humidity sounder (MWHS) onboard China’s sun-synchronous satellite FengYun-3B (FY-3B). The instrument has two quasi-polarization channels at 150 GHz in addition to the three water-vapor absorption channels in the vicinity of 183 GHz, allowing observations at quasi vertical and horizontal polarizations with its cross-track scanning geometry. The Saharan and Taklimakan deserts are taken as examples due to the relatively homogeneous surface conditions. In the two 150 GHz channels of MWHS, the polarization bias is corrected according to the measurements at the Earth incidence angle (EIA) of 0°. The monthly-mean quasi-polarization difference (QPD) at 150 GHz is found to be varying with the EIA and exhibits seasonal variations over deserts. The QPD reaches a maximum value at the EIA of ~35°, while the zero value appears near the EIA of ~53° (equal to the satellite viewing angle of 45°) as expected. In contrast to the relatively stable and low QPD over Sahara, the QPD, quasi polarized emissivity difference and pure polarized emissivity difference are generally larger in the Taklimakan Desert, where the maximum monthly QPD is ~4 K and the polarized emissivity difference increases monotonously to ~0.2 at the EIA of 50°. The complex surface parameters and temporal variabilities impinge on the uncertainties of the land polarization. The increasing soil moisture due to precipitation damps the dichroic ability of the land surface, while the frozen surface due to snowfalls in cold seasons enhances the polarization.

FengYun-3C (FY-3C) is the second-generation polar-orbiting meteorological satellite in China. As one of the most important microwave payloads deployed onboard FY-3C, microwave radiation imager (MWRI) has been continuously observing radiance originating from land and sea surface and supports numerical weather predictions at regional and global scales. With a long-term monitoring of the cold-end reference, MWRI onboard FY-3C was found suffering from discontinuous observations due to its temporary power-OFF/power-ON in the years of 2015 and 2018, resulting in anomalous brightness temperature (TB) jump up to 2-3 K in magnitude at some channels. Analysis of the operating status of FY-3C MWRI indicated that deviation of the receiver system from its optimal operating status affects the nonlinearity characteristics significantly. A correction algorithm, which relates nonlinearity to the autogain control (AGC) operating voltage of the sensor, is thus proposed on the basis of on-ground thermal/vacuum (T/V) test results in the prelaunch phase in order to mitigate the calibration anomalies of FY-3C MWRI. Nonlinearity correction of FY-3C MWRI is further corroborated by comparisons of simultaneously overlapping observations of global precipitation measurement (GPM) microwave imager (GMI). The results demonstrated that after nonlinearity correction the performance of FY-3C MWRI has been improved at all channels except for the 36-V channel where technical failures have been detected since October 2016.

On November 15, 2017, the fourth satellite, Feng Yun-3D (FY-3D), of the new generation of Chinese polar orbit meteorological satellites was successfully launched into orbit. As one of the key payloads, the second-generation Microwave Humidity Sounder (MWHS II) has 15 channels for observing atmospheric temperature and moisture. In the ground data processing system, the calibration is completed for every scanning period, and then, brightness temperature (TB) data are obtained with calibration coefficients. In order to analyze the performance of MWHS II, one year of in-orbit data are used to monitor the trend of internal blackbody target temperature, instrument temperature, calibration counts, and automatic gain control (AGC), and the results indicate whether the instrument is working well. The instrument noise equivalent differential temperature (NEDT) is calculated, and the value satisfies the design requirements. Two methods are used to validate FY-3D MWHS II measurements. One is the simultaneous nadir overpass (SNO) approach: in this method, ATMS is chosen as the reference sensor, and the standard deviations for five humidity sounding channels are all less than 1.3 K. In the alternate approach, a comparison of the observed and simulated TB of MWHS II is carried out between FY-3C and FY-3D. The results also demonstrate a consistent performance between the two instruments.

The microwave humidity sounder (MWHS) onboard the Fengyun-3D satellite is providing the data for profiling atmospheric temperature and moisture and has become an important data source for improving the weather forecasts. In this article, three data sources are utilized for assessing the MWHS on-orbit performance, including Global Navigation Satellite System Occultation Sounder (GNOS), ECMWF (European Centre for Medium-Range Weather Forecasts) Re-Analysis (ERA)-Interim reanalysis, and Advanced Technology Microwave Sounder (ATMS) data. GNOS-retrieved atmospheric profiles and the reanalysis data are used as inputs to the community radiative transfer model (CRTM) for simulating the MWHS brightness temperatures at the top of the atmosphere in July 2018 for characterizing the instrument performance. Since ATMS is a well-calibrated microwave sounding instrument onboard both Suomi NPP and NOAA-20 satellites, its measurements are also collocated with MWHS data for a consensus analysis using the simultaneous nadir overpasses (SNOs) method. In comparing GNOS simulations, MWHS upper air temperature sounding channels (3–6) have relatively larger biases (less than 2.5 K) than the water vapor sounding channels. However, the standard deviation of the difference between observations and simulations (O-B) is larger for water vapor sounding channels. For ERA simulations, MWHS sounding channels exhibit negative biases similar to GNOS results but the standard deviation of O-B at the water vapor channels is much smaller. When compared with ATMS water vapor channels, MWHS biases are mostly negative and agree with those from ERA simulation. Thus, the large uncertainty in simulating MWHS water vapor sounding channels from GNOS could result from the poor input water vapor profiles and high water vapor variability in the lower troposphere.

TanSat was successfully launched on December 22, 2016, and has been acquiring global measurements of CO2and O2spectral bands in reflected sunlight since early February 2017. The atmospheric carbon dioxide grating spectrometer (ACGS) is a spaceborne three-band grating hyperspectral spectrometer suite onboard TanSat. The ACGS is designed to measure high-spectral-resolution, coboresighted spectra of reflected sunlight within the molecular oxygen (O2) A-band range from 0.758 to 0.778$\mu \text{m}$and the weak and strong absorption bands of carbon dioxide (WCO2and SCO2) ranging from 1.594 to 1.624$\mu \text{m}$and from 2.042 to 2.082$\mu \text{m}$, respectively. The spectral resolving power ($\lambda /\Delta \lambda $) of the ACGS is ~19 000, ~12 800 and ~12 250 in the O2A-band, WCO2band and SCO2band, respectively. The inflight radiometric calibration accuracy is better than 5%, which satisfies the required specification. The wavelength calibration accuracy of the O2A-band is ~0.19 pm, that of the WCO2band is ~0.27 pm, and that of the SCO2band is ~4.75 pm, all of which meet the 0.05 full-width at half-maximum (FWHM) requirement. The spectroscopic performance of the ACGS exceeds the mission requirements by a margin. The ACGS has noise levels that are comparable to or smaller than those observed during prelaunch testing, and the noise has remained stable in the three bands during inflight operations. The signal-to-noise ratio (SNR) levels of the three bands meet the specified requirements. As expected, the ACGS radiometric performance in the O2A, WCO2, and SCO2bands was fairly good during its first 17 months inflight.

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.

Microwave radiation imager (MWRI) is regarded as one of the most important microwave payloads on board China FengYun-3C Meteorological Satellite. The instrument suffers from calibration anomalies and exhibits observation- background (O-B) calibration bias difference between the ascending and descending passes at all channels (hereinafter AD bias). The calibration bias difference of MWRI between ascending and descending orbits hampers data assimilation in the numerical weather predictions and reanalysis systems. This paper proposes a physical-based correction algorithm for MWRI calibration, following a brief introduction to the calibration process of the instrument. The relationship between the observed brightness temperatures and the physical temperature of the hot load reflector is established to mitigate the intrusion of the emissive hot reflector at all channels which was not accurately estimated in the previous calibration process. Before- and after-correction comparisons using one-year observations show that the AD bias is effectively reduced, i.e., from ~2 K before correction to less than 0.2 K after correction, when rectifying the emissivity of the hot reflector in the calibration equation, whereas the change in the mean values of MWRI radiance is negligible.

TanSat is an important satellite in the Chinese Earth Observation Program which is designed to measure global atmospheric CO2concentrations from space. The first Chinese superhigh-resolution grating spectrometer for measuring atmospheric CO2is aboard TanSat. This spectrometer is a suite of three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO2band near 1.61 and$2.06~\mu \text{m}$and in the molecular oxygen A-band (O2A) at$0.76~\mu \text{m}$. Their spectral resolving power ($\lambda /\Delta \lambda$) is ~19 000, ~12 800, and ~12 250 in the O2A, weak absorption band of molecular carbon dioxide band, and strong absorption of carbon dioxide band, respectively. This paper describes the laboratory radiometric calibration of the spectrometer suite, which consists of measurements of the dark current response, gain coefficients, and signal-to-noise ratio (SNR). The SNRs of each channel meet the mission requirements for the O2A and weak CO2band but slightly miss the requirements in a few channels in the strong CO2band. The gain coefficients of the three bands have a negligible random error component and achieve very good stability. Most of the R-squared of gain coefficients model consist of five numbers of nine (e.g., 0.99999) after the decimal point, suggesting that the instrument has significant response linearity. The radiometric calibration results meet the requirements of an absolute calibration uncertainty of less than 5%.

The Microwave Humidity Sounder (MWHS) is a five-channel microwave radiometer in the range of 150-191 GHz onboard FengYun 3 (FY-3). Before the launch of FY-3A, the intensive thermal vacuum (TVAC) tests for MWHS had been carried out in a 2-m TVAC chamber, and the basic parameters such as receiver nonlinearity were obtained. Four-year operation shows that the performance of FY-3A MWHS remains stabile. The variations among space views and warm target views in the interval of scan lines were generally within 20 counts. In addition, the temperature fluctuation of the warm target in a single pass is within 0.5 K. Primarily, due to the nonlinearity correction obtained from the TVAC test, radiance measurements of MWHS agree well with NOAA-17 AMSU-B and NOAA-18 Microwave Humidity Sounder (MHS) data. Averaged brightness temperature differences between FY-3 MWHS and NOAA-17 AMSU-B, at simultaneous cross-overpass points, are less than 1.08 K for the channels at 183 ± 1 GHz. In the observation-minus-background comparison, the observed and simulated brightness temperature biases of FY-3 MWHS and NOAA-18 MHS exhibit similar performance in the 183.31 ± 1 GHz channel; but in the 183.31 ± 7 GHz channel, MWHS agrees better with background. It is anticipated that FY-3 MWHS data will contribute to numerical weather prediction.

The Chinese FengYun-3 (FY-3)A satellite was successfully launched on May 27, 2008, with a Microwave Temperature Sounder (MWTS) onboard. MWTS has four channels with frequencies of 50.3, 53.596, 54.94, and 57.29 GHz, respectively. The MWTS measurements are primarily used for profiling atmospheric temperatures from surface to lower stratosphere. MWTS is a cross-track scanning instrument, and its Earth-view measurements are calibrated through the warm target and cold space measurements during every scan cycle. In this paper, the FY-3A MWTS and its channel characteristics are first introduced. The calibration process and the postlaunch instrument performance are then presented, including the long-term trends of noise equivalent differential temperature (NEDT), calibration counts from cold space and warm targets, instrument telemetry, and channel gains. The observed and simulated brightness temperature (BT) differences of MWTS and Advanced Microwave Sounding Unit-A (AMSU-A) are compared. It is shown that the MWTS NEDT values at all channels are much better than its specification. The BT biases of MWTS channels 1 and 3 with respect to the simulations are similar in magnitude to those from National Oceanic and Atmospheric Administration-18 AMSU-A. The MWTS biases at channels 2 and 4 are larger than AMSU-A.

On 27 May 2008, China has successfully launched the FY-3A meteorological satellite as a research and development (R&D) satellite, with 11 payloads mounted, and the three instruments among the 11 payloads are vertical atmospheric sounding instruments similar to ATOVS: MicroWave Temperature Sounder (MWTS), MicroWave Humidity Sounder and InfraRed Atmospheric Sounder, generally called Vertical Atmospheric Sounder System, i.e., VASS, which will help to improve NWP forecast skill. Chinese Meteorological Administration has developed a three dimension variational data assimilation system (Global/Regional Assimilation and Prediction System, shorten as Grapes 3Dvar), and with the ATOVS radiance data directly assimilated by RTTOV as observation operator. In this study some challenging research works related to assimilating the FY-3A VASS data into Chinese 3Dvar assimilation system Grapes 3Dvar are performed, such as generation of the RTTOV coefficients for FY-3A VASS three instruments, data quality control, channel selection and bias correction, etc. To investigate the effects of the FY-3A VASS data in NWP model, 2 sets of assimilation experiments conducted: one is single instrument data assimilation experiment and the other is combinatorial data assimilation experiments. The preliminary results indicate that: the forecast skills are improved greater after FY-3A VASS data assimilation than before, and the different instrument has different effects on NWP forecast; the microwave sounders have more contribution than infrared sounder; the combinatorial data assimilation experiments get more positive effect than single instrument data assimilation experiment; the positive impact on the South Hemisphere is more clear than on North Hemisphere.