Fengyun-3D Research Articles

Intercalibration of FY-3D MWTS Against S-NPP ATMS Based on Microwave Radiative Transfer Model

Accurate and stable in-orbit radiometric calibration of a satellite instrument is fundamental to Earth geophysical parameter estimation. This article addresses the intercalibration of the microwave temperature sounder (MWTS) on the Chinese second-generation polar-orbiting meteorological satellite, Fengyun 3D (FY-3D), against the advanced technology microwave sounder (ATMS) aboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite. First, ocean and land microwave radiative transfer models (RTM) are constructed by combining the sea and land surface emissivity models and atmospheric absorption model, as well as the intercalibration equations. Then, the MWTS and ATMS observations are resampled into a 1° × 1° regular grid space, and the matching brightness temperatures (TBs) under clear-sky/near clear-sky conditions are collected. Next, the TBs at top-of-atmosphere are simulated using the RTM and the fifth generation of European Centre for Medium-Range Weather Forecast atmospheric reanalysis (ERA5) data. After that, the double differences between FY-3D MWTS and S-NPP ATMS and the theoretical observations in FY-3D MWTS channels are calculated. Finally, the radiometric calibration coefficients of FY-3D MWTS are successfully derived from the observations of S-NPP ATMS by linear fits on the matching TBs. In contrast to the ATMS measurements, FY-3D MWTS observations are generally overestimated, and the in-orbit radiometric calibration errors (mean ± standard deviation at the mean) are 1.83 ± 1.45, 0.45 ± 0.94, 1.87 ± 0.60, −0.20 ± 0.36, −0.02 ± 0.37, 0.19 ± 0.24, 1.69 ± 0.28, 2.25 ± 0.29, 1.97 ± 0.33, 1.74 ± 0.42, 2.84 ± 0.42, 0.07 ± 0.65, and 0.32 ± 1.18 K in FY-3D MWTS channels 1–13, respectively. The results with Hewison's semi-empirical land surface emissivity (LSE) model and the results with LSEs derived from the coincident ATMS observations at 50.3 GHz are consistent. Moreover, the intercalibration results obtained by the RTM in this work also agree well with the results obtained by the community radiative transfer model.

Wide-field aurora imager onboard Fengyun satellite: Data products and validation

New observations of auroras based on the wide-field aurora imager (WAI) onboard Fengyun-3D (FY-3D) satellite are exhibited in this paper. Validity of the WAI data is analyzed by comparing auroral boundaries derived from WAI observations with results obtained from data collected by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) aboard the Defense Meteorological Satellite Program (DMSP F18). Dynamic variations of the aurora with the solar wind, interplanetary magnetic field (IMF) parameters, and the SYM-H index are also investigated. The comparison of auroral boundaries indicates that the WAI data are morphologically valid and suitable to the study of auroral dynamics. Effective responses to solar wind parameters indicate that the WAI data can be useful to monitor and predict the Earth’s space weather. Since the configuration of aurora is a good indicator of the solar wind–magnetosphere–ionosphere (SW-M-I) coupling system, and can reflect the disturbance of the space environment, the WAI will provide important data to help us to study the physical processes in space.

Vicarious Radiometric Calibration of Ocean Color Bands for FY-3D/MERSI-II at Lake Qinghai, China.

To calibrate the low signal response of the ocean color (OC) bands and test the stability of the Fengyun-3D (FY-3D)/Medium Resolution Spectral Imager II (MERSI-II), an absolute radiometric calibration field test of FY-3D/MERSI-II at the Lake Qinghai Radiometric Calibration Site (RCS) was carried out in August 2018. The lake surface and atmospheric parameters were mainly measured by advanced observation instruments, and the MODerate spectral resolution atmospheric TRANsmittance algorithm and computer model (MODTRAN4.0) was used to simulate the multiple scattering radiance value at the altitude of the sensor. The results showed that the relative deviations between bands 9 and 12 are within 5.0%, while the relative deviations of bands 8, and 13 are 17.1%, and 12.0%, respectively. The precision of the calibration method was verified by calibrating the Aqua/Moderate-resolution Imaging Spectroradiometer (MODIS) and National Polar-orbiting Partnership (NPP)/Visible Infrared Imaging Radiometer (VIIRS), and the deviation of the calibration results was evaluated with the results of the Dunhuang RCS calibration and lunar calibration. The results showed that the relative deviations of NPP/VIIRS were within 7.0%, and the relative deviations of Aqua/MODIS were within 4.1% from 400 nm to 600 nm. The comparisons of three on-orbit calibration methods indicated that band 8 exhibited a large attenuation after launch and the calibration results had good consistency at the other bands except for band 13. The uncertainty value of the whole calibration system was approximately 6.3%, and the uncertainty brought by the field surface measurement reached 5.4%, which might be the main reason for the relatively large deviation of band 13. This study verifies the feasibility of the vicarious calibration method at the Lake Qinghai RCS and provides the basis and reference for the subsequent on-orbit calibration of FY-3D/MERSI-II.

Water Vapor Retrievals from Near-infrared Channels of the Advanced Medium Resolution Spectral Imager Instrument onboard the Fengyun-3D Satellite

Water vapor plays a key role in weather, climate and environmental research on local and global scales. Knowledge about atmospheric water vapor and its spatiotemporal variability is essential for climate and weather research. Because of the advantage of a unique temporal and spatial resolution, satellite observations provide global or regional water vapor distributions. The advanced Medium Resolution Spectral Imager (MERSI) instrument—that is, MERSI-II—onboard the Fengyun-3D (FY-3D) meteorological satellite, has been one of the major satellite sensors routinely providing precipitable water vapor (PWV) products to the community using near-infrared (NIR) measurements since June 2018. In this paper, the major updates related to the production of the NIR PWV products of MERSI-II are discussed for the first time. In addition, the water vapor retrieval algorithm based on the MERSI-II NIR channels is introduced and derivations are made over clear land areas, clouds, and sun-glint areas over the ocean. Finally, the status and samples of the MERSI-II PWV products are presented. The accuracy of MERSI-II PWV products is validated using ground-based GPS measurements. The results show that the accuracies of the water vapor products based on the updated MERSI-II instrument are significantly improved compared with those of MERSI, because MERSI-II provides a better channel setting and new calibration method. The root-mean-square error and relative bias of MERSI-II PWV products are typically 1.8–5.5 mm and −3.0% to −14.3%, respectively, and thus comparable with those of other global remote sensing products of the same type.

Development of CO2 Band-Based Cloud Emission and Scattering Indices and Their Applications to FY-3D Hyperspectral Infrared Atmospheric Sounder

The Hyperspectral Infrared Atmospheric Sounder (HIRAS) onboard the Feng Yun-3D (FY-3D) satellite is the first Chinese hyperspectral infrared instrument. In this study, an improved cloud detection scheme using brightness temperature observations from paired HIRAS long-wave infrared (LWIR) and short-wave infrared (SWIR) channels at CO2 absorption bands (15-μm and 4.3-μm) is developed. The weighting function broadness and a set of height-dependent thresholds of cloud-sensitive-level differences are incorporated into pairing LWIR and SWIR channels. HIRAS brightness temperature observations made under clear-sky conditions during a training period are used to develop a set of linear regression equations between paired LWIR and SWIR channels. Moderate-resolution Imaging Spectroradiometer (MODIS) cloud mask data are used for selecting HIRAS clear-sky observations. Cloud Emission and Scattering Indices (CESIs) are defined as the differences in SWIR channels between HIRAS observations and regression simulations from LWIR observations. The cloud retrieval products of ice cloud optical depth and cloud-top pressure from the Atmospheric Infrared Sounder (AIRS) are used to illustrate the effectiveness of the proposed cloud detection scheme for FY-3D HIRAS observations. Results show that the distributions of modified CESIs at different altitudes can capture features in the distributions of AIRS-retrieved ice cloud optical depth and cloud-top pressure better than the CESIs obtained by the original method.

Comparing the Thermal Structures of Tropical Cyclones Derived From Suomi NPP ATMS and FY-3D Microwave Sounders

Accurate information on the thermal structures of tropical cyclones (TCs) is essential for monitoring and forecasting their intensity and location. In this study, a scene-dependent 1-D variation (SD1DVAR) algorithm is developed to retrieve atmospheric temperature and moisture profiles under all-weather conditions. In SD1DVAR, the background and observation error matrix varies according to the scattering intensity. Especially, the observation error matrix increases in precipitating atmospheres due to a larger uncertainty in the forward operator. With the data from the Advanced Technology Microwave Sounder (ATMS) onboard Suomi National Polar-orbiting Partnership (NPP) satellite, SD1DVAR can retrieve better thermal structures in the storm life cycle than NOAA Microwave Integrated Retrieval System (MIRS). Comparing with the aircraft dropsonde observations, the temperature and humidity errors from SD1DVAR are about 3 K and 20%, respectively, whereas those from MIRS are around 4 K–5 K and 30%, respectively. SD1DVAR is also applied for Microwave Temperature Sounder (MWTS) and Microwave Humidity Sounder (MWHS) onboard FengYun-3D (FY-3D) satellite. The MWTS and MWHS data sets are first combined into a single Comprehensive MicroWave Suite (CMWS) data stream and then used to retrieve the hurricane thermal structures. It is shown that the hurricane structure from CMWS is very similar to that from ATMS. However, due to the availability of 118-GHz measurements from the CMWS, the hurricane temperature vertical structure is better resolved, and the humidity error is also reduced by about 5%.

An Algorithm to Retrieve Total Precipitable Water Vapor in the Atmosphere from FengYun 3D Medium Resolution Spectral Imager 2 (FY-3D MERSI-2) Data

The atmosphere has substantial effects on optical remote sensing imagery of the Earth’s surface from space. These effects come through the functioning of atmospheric particles on the radiometric transfer from the Earth’s surface through the atmosphere to the sensor in space. Precipitable water vapor (PWV), CO2, ozone, and aerosol in the atmosphere are very important among the particles through their functioning. This study presented an algorithm to retrieve total PWV from the Chinese second-generation polar-orbiting meteorological satellite FengYun 3D Medium Resolution Spectral Imager 2 (FY-3D MERSI-2) data, which have three near-infrared (NIR) water vapor absorbing channels, i.e., channel 16, 17, and 18. The algorithm was improved from the radiance ratio technique initially developed for Moderate-Resolution Imaging Spectroradiometer (MODIS) data. MODTRAN 5 was used to simulate the process of radiant transfer from the ground surfaces to the sensor at various atmospheric conditions for estimation of the coefficients of ratio technique, which was achieved through statistical regression analysis between the simulated radiance and transmittance values for FY-3D MERSI-2 NIR channels. The algorithm was then constructed as a linear combination of the three-water vapor absorbing channels of FY-3D MERSI-2. Measurements from two ground-based reference datasets were used to validate the algorithm: the sun photometer measurements of Aerosol Robotic Network (AERONET) and the microwave radiometer measurements of Energy’s Atmospheric Radiation Measurement Program (ARMP). The validation results showed that the algorithm performs very well when compared with the ground-based reference datasets. The estimated PWV values come with root mean square error (RMSE) of 0.28 g/cm2 for the ARMP and 0.26 g/cm2 for the AERONET datasets, with bias of 0.072 g/cm2 and 0.096 g/cm2 for the two reference datasets, respectively. The accuracy of the proposed algorithm revealed a better consistency with ground-based reference datasets. Thus, the proposed algorithm could be used as an alternative to retrieve PWV from FY-3D MERSI-2 data for various remote sensing applications such as agricultural monitoring, climate change, hydrologic cycle, and so on at various regional and global scales.

INTER-COMPARISONS AMONG PASSIVE MICROWAVE SEA ICE CONCENTRATION PRODUCTS FROM FY-3D MWRI AND AMSR2

Abstract. Passive microwave (PM) sensors on satellite can monitor sea ice distribution with their strengths of daylight- and weather-independent observations. Microwave Radiation Imager (MWRI) sensor aboard on the Chinese FengYun-3D (FY-3D) satellites was launched in 2017 and provides continuous observation for Arctic sea ice since then. In this study, sea ice concentration (SIC) product is derived from brightness temperature (TB) data of MWRI, based on an Arctic Radiation and Turbulence Interaction Study Sea Ice (ASI) dynamic tie points algorithm. Our product is inter-compared with a published MWRI SIC product by the Enhanced NASA Team (NT2) algorithm, and three Advanced Microwave Scanning Radiometer 2 (AMSR2) SIC products by the ASI, Bootstrap (BST) and NT2 algorithm. Results show that MWRI SIC are generally higher than AMSR2 SIC and the median of monthly SIC differences are larger in summer. Regional analysis indicates that the smaller differences between AMSR2 SIC and MWRI-ASI SIC occur in the higher SIC areas, and the biases are within ±5% in the Beaufort Sea, Chukchi Sea, East Siberian Sea, Canadian Archipelago Sea and Central Arctic Sea. There is the smallest SIC difference in the Central Arctic Sea with the biases of −0.77%, −0.60%, and 0.19% for AMSR2-ASI, AMSR2-BST and AMSR2-NT2, respectively. The trends of MWRI and AMSR2 sea ice extent and sea ice area are consistent with correlation coefficients all greater than 0.997. Besides, mean SIC, sea ice extent and sea ice area of MWRI-ASI are closer to those of AMSR2 than those of MWRI-NT2.

Observation of thermosphere and ionosphere using the ionosphere PhotoMeter (IPM) on the Chinese meteorological satellite FY-3D

The Ionosphere PhotoMeter (IPM) is a far ultraviolet nadir-viewing photometer that flew aboard the second-generation, polar-orbiting Chinese meteorological satellite Feng-Yun 3D (FY-3D), which was launched on November 25th, 2017. By observing far ultraviolet night-time OI 135.6 nm emissions, which are produced by the recombination of O+ ions and electrons, the F2 layer peak electron density (NmF2) can be deduced. During daytime IPM was designed to measure the OI 135.6 nm and N2 Lyman-Birge-Hopfield (LBH) band emissions, which would be used to infer the O/N2 ratio, a key diagnostic of neutral dynamics in the daytime. The paper shows the measurement results from IPM taken during its initial months of Operation. A quantitative comparison showing NmF2 from IPM and ground-based ionosondes is presented, and the correlation value R between the ground-based ionosondes and the satellite NmF2 is 0.91. The global O/N2 ratio distribution during geomagnetic storm on 25 through 30 August is presented and illustrated, which could show the latitudes of the ionospheric positive/negative storm in different longitude ranges. The O/N2 ratio significantly decreases during the main and recovery phase of the storm as expected. The correlation value R between the O/N2 ratio retrieved from IPM data and the ionospheric electron density provided by ground-based ionosondes during storms in August 2018 is about 0.78.

Impact of FY-3D MWRI Radiance Assimilation in GRAPES 4DVar on Forecasts of Typhoon Shanshan

In this study, Fengyun-3D (FY-3D) MicroWave Radiation Imager (MWRI) radiance data were directly assimilated into the Global/Regional Assimilation and PrEdiction System (GRAPES) four-dimensional variational (4DVar) system. Quality control procedures were developed for MWRI applications by using algorithms from similar microwave instruments. Compared with the FY-3C MWRI, the bias of FY-3D MWRI observations did not show a clear node-dependent difference from the numerical weather prediction background simulation. A conventional bias correction approach can therefore be used to remove systematic biases before the assimilation of data. After assimilating the MWRI radiance data into GRAPES, the geopotential height and humidity analysis fields were improved relative to the control experiment. There was a positive impact on the location of the subtropical high, which led to improvements in forecasts of the track of Typhoon Shanshan.

A test of a sun glint correction method for the near-3.9 μm channels of the FengYun-3D Hyperspectral InfraRed Atmospheric Sounder (HIRAS)

ABSTRACT The Hyperspectral InfraRed Atmospheric Sounder (HIRAS), the first Chinese sun-synchronous hyperspectral infrared sounder onboard FengYun 3D (FY-3D), was launched on 15 November 2017. It will play a significant role in improving forecast accuracy of numerical weather prediction (NWP) systems. However, sun glint effects on the FY-3D HIRAS 3.9 μm channels cannot be currently estimated by radiative transfer models. In this study, a new method for quantifying sun glint effects has been developed through a cubic fitting relationship between sun glint angles and the observed brightness temperature minus simulated brightness temperature (O − B) after bias correction. The results show that the brightness temperatures of channel 2275 (2550 cm−1) are higher over the ocean between 40°S and 40°N within areas of sun glint angle less than 40°; in these regions, O − B values can reach 10 K during the daytime. Daytime global mean clear-sky O − B biases from channel 2035 (2400 cm−1) to channel 2275 (2550 cm−1) increase gradually from 0 to 2.5 K. In contrast, daytime O − B biases after sun glint correction are within ±0.2 K, indicating that the corrected radiances are suitable to be assimilated into NWP systems.

High Spectral Infrared Atmospheric Sounder (HIRAS): System Overview and On-Orbit Performance Assessment

The High Spectral Infrared Atmospheric Sounder (HIRAS) is the first Chinese Fourier Transform Michelson interferometer onboard the FengYun 3D (FY-3D) polar-orbiting meteorological satellite launched on November 15, 2017. The FY-3D HIRAS provides infrared (IR) radiance spectra measurements in three spectral bands: the long-wave IR (LWIR) band from 650 to 1135 cm−1, middle-wave IR (MWIR) band from 1210 to 1750 cm−1, and short-wave IR (SWIR) band from 2155 to 2550 cm−1. The ground system processes the interferogram measurements into calibrated radiance spectra. In each cross-track scan, there are 29 observations, each with a field-of-regard (FOR) comprising an array of $2\times 2$ field of views. In a six-month intensive campaign period, the HIRAS system was tuned, characterized, and validated. For the operational Level 1 product, the radiance noise levels meet the specifications. The spectral frequency accuracy was improved by maximizing the spectral correlation between the measured and simulated spectra by tuning the instrument-line-shape parameters. The absolute spectral frequency biases are less than 3 part per million (ppm) for all the three bands, and spectral bias standard deviations are less than 3 ppm in the LWIR and MWIR bands, and are about 3–5 ppm in the SWIR band. The radiometric calibration uncertainties were assessed by the comparisons of the radiance spectra between HIRAS and other IR hyperspectral sensors on different satellites. The radiance differences of the cross-sensor comparisons are in general less than 0.3, 0.7, and 1.0 K in the LWIR, MWIR, and SWIR bands, respectively. The HIRAS spectra were also compared with the spectra simulated with a fast radiative transfer model. Some remaining issues for the FY-3D HIRAS are also discussed.

Evaluation of the in-orbit performance of the microwave temperature sounder onboard the FY-3D satellite using different radiative transfer models

The in-orbit performance of the Microwave Temperature Sounder (MWTS) onboard the Fengyun-3D (FY-3D) satellite is evaluated using the Community Radiative Transfer Model (CRTM) and the Radiative Transfer for the Television Infrared Observation Satellite Operational Vertical Sounder (RTTOV) model. Both radio-occultation data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Data Analysis and Archive Center and ERA-Interim analysis data from the European centre for Medium-Range Weather Forecasts are taken as inputs to the two fast radiative transfer models. The radio-occultation data are quality controlled and the collocation criterion between the radio-occultation data and the MWTS measurements is defined such that the spatial and temporal difference is <50 km and <3 h, respectively. The biases of MWTS channels 5–10 produced by both CRTM and RTTOV agree well over the oceans under clear sky conditions between 60° S and 60° N from July to December 2018. The mean biases simulated by the radio-occultation and ERA-Interim data are negative and the absolute values of the biases are <0.6 and <1.5 K for channels 5–10 of the MWTS, respectively. The biases at channels 4 and 11 between the CRTM and RTTOV simulations are inconsistent and require further investigation of the transmittance difference between the fast models. The standard deviation of the biases from the radio-occultation and ERA-Interim data are <0.5 and <1.8 K for the limited amount of collocated radio-occultation data. Asymmetrical patterns are detected for the MWTS through the scan angle-dependent bias. The long-term mean MWTS bias shows only a weak dependence on latitude, which suggests that biases do not vary systematically with brightness temperature.

LEO–BDS–GPS integrated precise orbit modeling using FengYun-3D, FengYun-3C onboard and ground observations

The new FengYun-3 (FY-3) meteorological satellite, FengYun-3D (FY-3D), carries an enhanced version of the GNSS Occultation Sounder (GNOS) instrument with increased BDS and GPS tracking channels. These high-quality onboard BDS observations together with FengYun-3C (FY-3C) data can serve as an effective supplement to overcome the weakness in BDS tracking geometry. To assess the scope of the low earth orbit (LEO)-induced improvements on the BDS satellite orbits, we processed the ground network and two FY-3 satellites (FY-3C and FY-3D) in a common least-squares adjustment. Three integrated precise orbit determination (POD) schemes with the individual LEO and a combination of two LEOs are designed to investigate the contribution of the new FY-3D satellite. The performance of FY-3D POD is discussed first. Due to the increase in observation redundancy, the FY-3D orbit presents smaller overlap differences than FY-3C. The corresponding precision improvement can reach 72% for the BDS-only POD, 13% for the GPS-only POD and 25% for the GPS and BDS POD. The overlap result of integrated POD indicates that FY-3D contributes a stronger enhancement to GPS and BDS orbits than FY-3C because of its noticeable increase in onboard observations. The most pronounced benefit can be observed in BDS GEO orbits, which is improved by 44% for the regional solution and 41% for the global solution compared to the FY-3C solution. As expected, a further reduction in the overlap differences can be noted when adding FY-3C to the FY-3D global solution. Compared with the FY-3D solution, the orbit precision of BDS GEO, IGSO and MEO for the 2-LEO solution is slightly improved by 3%, 3%, and 1%, respectively, which is mainly limited by the few observations contributed by FY-3C.

Impacts of cloud scattering properties on FY-3D HIRAS simulations

Cloudy sky spectral radiance at the top of the atmosphere has always been an important while difficult variable to simulate for fast radiative transfer models. In this paper, we focus on examining the impacts of cloud scattering properties on the spectral radiance signature of the High-spectral Resolution Infrared Atmospheric Sounder (HIRAS) onboard the Fengyun-3D (FY-3D) satellite by using the Advanced Radiative transfer Modeling System (ARMS) and the Community Radiative Transfer Model (CRTM). Cloud scattering properties used in the radiative transfer models are critical for modeling the spectral radiance under cloudy sky, which involves choices of appropriate cloud particle models and particle size distributions, etc. Multiple FY-3D HIRAS observations over Southern China and Southeast Asia with ice or liquid water cloud cover on 6 May 2018 are examined, respectively. Vertical atmospheric profiles are derived from the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis product. Cloud property retrievals from the Moderate Resolution Imaging Spectroradiometer are used. Cloud scattering property parameterization schemes based on spherical and nonspherical cloud particle shapes are implemented for liquid water and ice clouds in ARMS and CRTM, respectively. Results show that both ARMS and CRTM can well simulate the radiance at the HIRAS spectral ranges under liquid water cloud condition as compared with the HIRAS observation with mean absolute error (MAE) of brightness temperature of less than 1 K. However, for ice cloud conditions, ARMS model using assumed spherical ice properties exhibits large biases between simulation and observation. CRTM with nonspherical ice properties using 16-stream approximation shows MAE less than 1 K and MAE of about 1 K using 2-stream approximation.

FY-3D HIRAS Radiometric Calibration and Accuracy Assessment

The High-Spectral Infrared Atmospheric Sounder (HIRAS) is a Fourier transform spectrometer onboard the fourth polar-orbiting FengYun 3D satellite (FY-3D). The FY-3D HIRAS provides interferogram measurements of Earth view radiance spectra in three infrared spectral bands at 29 cross-track positions, each with a $2 \times 2$ array of field of views (FOVs). The HIRAS level 1 radiance data cover the spectral bands from 650 to 1135 cm−1 [long-wave (LW) band], 1210 to 1750 cm−1 [mid-wave (MW) band], and 2155 to 2550 cm−1 [short-wave (SW) band] with a spectral resolution of 0.625 cm−1. The radiometric calibration algorithm and the methods of refining the nonlinearity (NL) and the polarization correction coefficients on orbit are summarized in this article. The NL correction coefficients are derived by minimizing the spread of the responsivity functions derived from the measurements of the internal calibration target with varying temperatures. The polarization correction coefficients are derived from the cold space observations and the routine Earth scene measurements. The radiometric accuracy is assessed by comparing the HIRAS measurements to the collocated Cross-track Infrared Sounder (CrIS) observations and radiance simulations. The results show that, compared to CrIS, the radiometric differences are about 0.3 and 0.7 K for the LW and MW bands, respectively, and 0.5 K for the CO absorption and window regions in the SW band. The consistency of the radiometric calibration among the four FOVs is estimated to be within 0.2 K for most of the spectral domain. Some remaining issues for the FY-3D HIRAS are also discussed.

Simulating Radiometric Resolution of Microwave Humidity and Temperature Sounder Onboard the FY-3D Satellite

Microwave humidity and temperature sounder (MWHTS) is an important payload of FENGYUN-3D (FY-3D) satellite. MWHTS is designed as a total power radiometer because of its high radiometric resolution (NEDT) and simple configuration. NEDT is one of the most important indicators for MWHTS. However, the traditional NEDT analysis results have a large error compared with the measured results. To estimate the overall radiometer performance, this article presents a numerical system simulation model that can be used to estimate NEDT. The model considers two of three main components that degrade the NEDT: thermal noise and calibration error. The third component $1/f$ noise is calculated by analyzing the power spectrum of the actual output voltage. Based on digital signal processing, this article presents a method for accurate system modeling, including radiation source modeling, frontend modeling, and backend modeling. The simulation model is validated by comparing the dynamic range, linearity, and standard deviation in output signals of the simulation system and actual instrument. In this article, the model has been applied to NEDT estimate of 15 MWHTS channels operating from 89 to 183 GHz. The comparison of the estimate results between the simulation model and actual instrument shows they are in good agreement and the maximum error is no more than 18%.

Calibration and Validation of Feng Yun-3-D Microwave Humidity Sounder II

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.

Nonlinear characteristics of FY-3D microwave radiation imager and an optimal calculation method

The Earth’s radiation was observed by a microwave radiation imager(MWRI) on board FengYun-3D(FY-3D) satellite at 10.65 GHz, 18.7 GHz, 23.8 GHz, 36.5 GHz, and 89.0 GHz with dual polarization. The nonlinearity of this payload, as an important parameter in calibration algorithms, is represented by the nonlinear parameter u from vacuum calibration ground tests. Therefore, an accurate knowledge on the nonlinear characteristics of MWRI is required to achieve precise remote sensing.The nonlinear parameter applied in the calibration algorithm is usually averaged over a set of u, which is calculated at corresponding observed brightness temperatures in the range between 95 K and 298 K. A t-distribution test method is proposed to screen u and further optimize the nonlinearity calculations in this study instead of conventional empirically filtering of u before averaging. The t-method examines the validity of u values at each observed brightness temperatures, and its effectivity is proven in this study.Nonlinear fitting brightness temperatures, as well as means and standard deviation of residual error, were calculated using empirically filtering method and the t-method to demonstrate the results in the nonlinearity calculation. The nonlinear parameter of MWRI is a physical parameter of the instrument, and the nonlinearity of up to 2 K at 10 GHz is mainly introduced by higher-order harmonic during detection.The nonlinear brightness temperature calculated using the t-test distribution method is improved by approximately 0.04 K with better fitting results than the ground tests, especially at 10 GHz.According to the results presented in this study, the nonlinear parameter is independent of observed brightness temperatures, and its nonlinearity is correlated to the instrument’s working condition. The proposed algorithm using the t-test distribution method can improve the nonlinearity fitting results and calibration precision. This method provides accurate nonlinear parameter for on-orbit calibration and can play a role in the total life cycle of MWRI after being launched.

Fengyun-3D MERSI True Color Imagery Developed for Environmental Applications

Many techniques were developed for creating true color images from satellite solar reflective bands, and the so-derived images have been widely used for environmental monitoring. For the newly launched Fengyun-3D (FY-3D) satellite, the same capability is required for its Medium Resolution Spectrum Imager-II (MERSI-II). In processing the MERSI-II true color image, a more comprehensive processing technique is developed, including the atmospheric correction, nonlinear enhancement, and image splicing. The effect of atmospheric molecular scattering on the total reflectance is corrected by using a parameterized radiative transfer model. A nonlinear stretching of the solar band reflectance is applied for increasing the image contrast. The discontinuity in composing images from multiple orbits and different granules is eliminated through the distance weighted pixel blending (DWPB) method. Through these processing steps, the MERSI-II true color imagery can vividly detect many natural events such as sand and dust storms, snow, algal bloom, fire, and typhoon. Through a comprehensive analysis of the true color imagery, the specific natural disaster events and their magnitudes can be quantified much easily, compared to using the individual channel data.