The Medium Resolution Spectral Imager-III (MERSI-III), the third generation of the MERSI series, began observations from the Feng Yun-3F (FY-3F) in a sun-synchronous morning orbit on August 3, 2023. MERSI-III collects data from 25 spectral bands, including 19 reflective solar bands (RSBs) and 6 thermal emissive bands with wavelengths ranging from 0.4 to 2.15 μm and 3.7 to 12.5 μm, respectively. Compared with its predecessors, MERSI-III features an enhanced on-orbit calibration capability for RSBs thanks to the incorporation of a newly designed visible on-board calibrator (VOC). This VOC is a solar diffuser (SD)-based on-board calibration system consisting of an SD panel, a solar attenuation plate positioned in front of the SD panel, and an on-board SD degradation monitor. We provide an overview of the MERSI-III instrument, its VOC functions, and the corresponding on-orbit calibration methodologies. The performance of the on-orbit calibration is also assessed using a wide dynamic-integrated vicarious calibration (VC) technique, which combines multiple VC methods, including inter-comparison using invariant pixels, absolute calibration with pseudo-invariant calibration sites, and Rayleigh scattering calibration. Based on three months of validation results, the on-orbit calibration demonstrates strong performance in both absolute accuracy and radiometric stability. Among the 14 validated RSBs, 13 bands meet the basic calibration accuracy requirement of a calibration bias of less than 5%, while 7 bands meet the desired requirement of a calibration bias of less than 3%.

In order to ensure the accuracy and reliability of the observational data from the remote sensor during its on-orbit operation, a sampling correction approach with the interpolation sliding window (ISWSCA) is proposed on the basis of the quadratic fitting and interpolation correction. The ISWSCA mitigates the effects of the inhomogeneous distribution of the reflective properties on the lunar surface, significantly enhancing the sampling correction accuracy of the lunar observation data by incorporating a subpixel correction term. Based on the lunar observation data from the Fengyun-3D (FY-3D)/Medium Resolution Spectral Imager (MERSI)-II collected between 2018 and 2023, the feasibility of the ISWSCA is verified, and the ISWSCA improves the sampling correction accuracy by 2.13% and 5.25% during low and high lunar phases in comparison to the scaling method of the lunar full-disk irradiance (LFDISM), together with the correction accuracy improved by 1.37% and 6% in low and high spatial resolutions. The ISWSCA gives the long time series of the normalized calibration coefficient, together with the calibration uncertainty of the effective data being analyzed, and the results show that the on-orbit stability of the FY-3D satellite is excellent in the visible (VIS) and near-infrared (NIF) bands, with the attenuation rate below 1.29% and the calibration uncertainty within 2.11%. This study has an important significance for the on-orbit radiation calibration of the spatial remote sensor.

The Hyperspectral Infrared Atmospheric Sounder (HIRAS) carried on the FengYun(FY)-3D satellite is a Fourier transform infrared spectrometer, which can improve the vertical detection capability of the atmosphere. The radiometric calibration accuracy of HIRAS depends on accurate spectral calibration. During the operation of the satellite, the spectral calibration accuracy of HIRAS will change continuously due to the mirror misalignment and the volatile silica gel gas appearing in the path of the interferometer optical system. Here the long-term absolute accuracy of the HIRAS spectral calibration is assessed by a forward radiative transfer model through a cross-correlation method. Subsequently, A two-step method is developed to determine the final configurations of the four detectors on each of the three HIRAS focal planes. The off-axis apodization correction is then performed using the dynamically updated off-axis parameters for all the detectors, achieving a spectral calibration accuracy superior to 2 ppm.

A fast piecewise-defined neural network (PDNN) method is presented to produce accurate atmospheric temperature and humidity profiles from satellite hyperspectral infrared (IR) and microwave (MW) observations in all-sky conditions. The PDNN method relies on a novel classification approach and a principal component-based neural network (NN) function to better capture the nonlinear relationship between the spectral radiances and atmospheric state vectors. The algorithm was designed to only rely on satellite measurements. Large datasets were used for network training to make the retrieval robust to random errors in the reference data. For each retrieved profile, an effective quality indicator (QI) was obtained by training against the absolute value of the retrieval error. In addition, a reliable rain cloud flag was generated based on the scattering difference of cloud particles between the 50 and 118 GHz channels. Besides the independent reanalysis of field data, the algorithm’s performance was also evaluated using radiosonde measurements. Preliminary validation shows that the best temperature retrieval occurred in the mid-troposphere (around 1.0 K). Depending on the reference data, errors in the boundary layer typically ranged from 1.8 to 2.5 K. For water vapor, the retrieval error was less than 22% in the low-troposphere and less than 35% in the mid- and upper-troposphere when validated against the reanalysis field. The humidity error was approximately 10% lower when compared to radiosondes. The PDNN is used operationally to produce atmospheric temperature and moisture soundings from the Vertical Atmospheric Sounding System (VASS) of the FengYun-3E (FY-3E) satellite, an early-morning-orbit meteorological satellite launched in 2021.

In order to further improve the accuracy of the lunar radiation correction, an oversampling correction method based on the lunar phase function was firstly proposed in this paper. This method avoided the dependence of the MODIS algorithm on the relative space position and velocity accuracy, and overcame the limitation of the classical SeaWIFS algorithm only applicable to low lunar phase angles, in order to realize the full lunar phase observation of the sky, earth and space. Based on the least square fitting of the actual image data together with the analytic solution of the lunar phase function, the exact expression of the oversampling correction coefficient was given, and the spatial resolution of the remote sensor was improved to the sub-pixel level by the numerical difference calculation. The effectiveness of the novel method was validated through the Terra MODIS freemoon data with the lunar phase angle range of [55°,80°], and the lunar phase fitting function was highly consistent with the lunar phase shape observed by the remote sensor, together with the normalized calibration coefficient calculated by the novel method coinciding basically with the results of the MODIS maneuver calibration data with the lunar phase angle range of about 55°, which demonstrated the competitive efficiency and accuracy of the novel method. To evaluate the on-orbit stability of the FY3D/MERSI satellite, the long time series of the normalized calibration coefficient was obtained by the oversampling correction method, and the calibration uncertainty of the effective data was analyzed, showing no significant change for the gain of the FY3D/MERSI in most bands. This research had significance for the lunar observation together with the on-orbit calibration and deep space detection.

The third generation of MicroWave Temperature Sounder (MWTS-III) is a major payload of FengYun-3F (FY-3F) satellite, which was launched on August 3, 2023. It can provide valuable atmospheric temperature profiles in morning time for numerical weather prediction (NWP) applications. The prelaunch thermal vacuum (TVAC) calibration test was performed to examine the instrument quality. This paper briefly describes of the instrument design of MWTS-III and details the TVAC tests calibration methodology and radiometric performance, including sensitivity, striping, nonlinearity, and calibration accuracy. Channels 1 and 2, which are received by direct-detection receivers, have a noise equivalent delta temperature (NEDT) of less than 0.2K, weak correlations with other channels, striping indexes (SI) of 1.0 and 1.2, small peak nonlinearity and calibration residual errors within ±0.03K. The NEDT for Channels 3-17 is 0.16 K-1.38 K. The inter-channel correlation and striping phenomena are more significant for these 15 channels. Since the linear calibration deviations were well mitigated by the nonlinear calibration, the calibration residual errors for channels 3-11 are within ±0.05 K and for channels 12-17 are within ±0.17 K. The predicted on-orbit calibration accuracy for all channels are between 0.37 K and 1.48K. In summary, all channels of FY-3F MWTS-III meet the specifications, and the instrument has great application potential.

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 2×2 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.

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 × 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.

Data assimilation of satellite microwave sounders are very important for numerical weather prediction. Fengyun-3 (FY-3) polar satellites carry two such sounders: MicroWave Temperature Sounder (MWTS) and MicroWave Humidity Sounder (MWHS). The microwave observations should be quality-controlled before assimilation and rain check is one of the crucial steps. Microwave Radiation Imager (MWRI), on board FY-3 too, can provide useful rain information due to more sensitive to precipitation than sounders. A remapping methodology is described in this paper to make full use of data from sounders and imager simultaneously. Observations and products from MWRI can thus be remapped to field of view of sounders. Comprehensive information obtained in this way can be used to not only assimilation in numerical weather prediction models but also satellite observations quality monitoring.

InfraRed Atmospheric Sounder (IRAS) instruments were successfully launched onboard the FengYun-3A (FY3A) and FengYun-3B (FY3B) satellites on May 27, 2008, and November 5, 2010, respectively. They aim at providing multichannel radiances within the spectral range of visible to infrared (IR) wavelengths for many environmental applications, including data assimilation and retrievals of global atmospheric temperature and humidity profiles. However, the velocity of the filter wheel of the first IRAS onboard FY3A is unstable and, therefore, induced a discontinuity in the measurement. The IRAS onboard FY3B works well in normal and stable operational mode since its launch without any anomaly. A variety of postlaunch calibration/validation tasks are conducted using on-orbit data during a period of three months. This paper presents on-orbit verification of IRAS instrument performance, including long-term trends of the space and warm calibration counts and noise equivalent delta radiance. The Earth scenes observed simultaneously by IRAS and Meteorological Operational Satellite Programme (METOP)/Infrared Atmospheric Sounding Interferometer were obtained and compared to demonstrate a close similarity between the two measurements. Furthermore, the IR channel observations from FY3B/IRAS are compared with those from National Oceanic and Atmospheric Administration-19/High Resolution Infrared Radiation Sounder (HIRS) equivalent channels and simulations from a radiative transfer model. The results show that some of IRAS IR channels perform very well, particularly for channels 1-10, 15, 19, and 20, compared to those of HIRS. Several channels, such as 13, 16, and 18, however, display some large biases. The causes of these increased biases are still under investigation.