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%.

Arctic sea ice leads, which are narrow linear openings between sea ice, are critical to polar climate and ocean–atmosphere interactions. However, detecting multi-width leads accurately under the case of cloud interference remains challenging for thermal infrared (TIR) data. In this article, we propose an improved U-Net-based model–CTMLU-Net–which is designed to extract multi-width leads, including internal bright leads, from FY-3D MERSI-II 250-m TIR imagery under varied cloudy conditions. The model incorporates a dynamic fast Fourier transform (FFT) module to distinguish leads from clouds in the frequency domain and introduces cloud categories into the training labels to improve the lead detection accuracy. A dual-loss module and multi-window brightness temperature anomaly (BTA) inputs further enhance the sensitivity to different lead widths. Under cloudy conditions, CTMLU-Net achieved a precision of 86.82% and a recall of 71.73%, outperforming the traditional BTA method by 39.26% and 23.95%, respectively. For multi-width leads, it achieved a precision of 92.10% and a recall of 87.73%, and for bright leads, the corresponding scores were 90.59% and 84.75%, respectively. Compared to the original U-Net model, the FFT module improved the precision and recall by 4.80% and 17.66% in cloudy conditions, respectively. Compared to single-loss setting, the dual-loss module further enhanced the multi-width lead detection precision and recall by 23.64% and 9.92%. CTMLU-Net was also applied to 7045 scenes of MERSI-II TIR images to generate monthly Arctic lead frequency maps from November 2019 to April 2020. The spatiotemporal patterns of the leads can capture dynamic events, such as the collapse of the Beaufort High in spring 2020. Overall, CTMLU-Net offers a robust and accurate solution for multi-width sea ice lead detection under cloudy conditions.

The third generation Microwave Temperature Sounder (MWTS-III) onboard Fengyun-3E (FY-3E) can obtain the atmospheric temperature profile in the early morning time, which is valuable for numerical weather prediction (NWP), climate analysis and other environment studies. However, calibrated brightness temperature (BT) of the surface-insensitive sounding channels (i.e. channels 7-17) have a sea-land contrast bias. It is 0.2 K-0.3 K for channels 9-17 and -1.33 K and -2.86 K for channels 7 and 8, respectively. The bias characteristic of channels 9-17 is similar to that of the corresponding channels of FY-3C, which is caused by the interference from a window channel and can be corrected by empirical method. Moreover, the spectral response functions of channels 7 and 8 were examined over a wider frequency range by measuring the instrument of similar design and construction to the on-orbit MWTS-III. The measured data shows that channels 7 and 8 have out-of-band response located between 50 GHz to 52 GHz, which close to the response range of channel 4. A linear function developed in this letter is effective for the BT bias correction. The sea-land contrast bias of these two channels has been decreased to -0.35K and -0.16K. And the daily mean bias of channels 7 and 8 are improved from -1.8K and -2.3K to -0.5K and -0.4K, respectively. This correction function has been integrated into the FY-3E MWTS-III operational calibration program in December 7, 2022. Meanwhile, the subsequent MWTS-III has been further improved to avoid the sea-land contrast bias.

The MEdium Resolution Spectral Imager (MERSI) onboard the Fengyun-3 (FY-3) series satellites can provide the long-term series data with favorable spectral and spatial resolution on the global scale since 2008. Such datasets are valuable for the studies of climate change. However, due to the lack of stable and reliable onboard calibration equipment and inconsistent in-orbit calibration methods, the MRESI historical data have poor long-term stability and unreliable accuracy, which affects the quantitative application of the data. This study reveals the overall status of the FY-3A/B/C MERSI-I historical data and proposes the recalibration methods for the reflective solar bands (RSBs) and thermal emission bands (TEBs). For the RSBs, by using FY-3A as the radiative transfer reference, an integrated transfer calibration method is developed for the calibration of FY-3B, which is then used to recalibrate FY-3C. The degradation tracking model of FY-3 MERSI-I is established first in the recalibration process by integrating multiple calibration methods. Then, based on the overlapping observations over the Libyan Desert, the linear consistency transfer model of the reference and target satellites is established, and the consistent correction coefficient between them is obtained. For the TEBs, a retrospective transfer recalibration scheme is proposed to achieve the reevaluation of the in-orbit radiometric calibration parameters based on intercalibration and to conduct the recalibration of historical data without permanent dependence on reference instruments. All the historical data of FY-A/B/C MERSI-I (from February 2008 to March 2017) are reprocessed with the same calibration method. The reprocessed datasets show remarkable improvements in calibration accuracy and stability compared with the operational datasets. The overall radiometric biases are found to be small and highly stable during the entire mission cycle of the instrument. The calibration biases of reprocessed data are less than 3% and 0.5 K for the RSBs and TEBs, respectively, much better than those of the operational datasets. There are also substantial improvements in the seasonal fluctuations and deviation discontinuities. This reprocessed long-term MERSI data with high intersensor consistency can provide valuable insights into global climate monitoring and model assessment.

The challenges of utilizing the 250-m resolution thermal infrared (TIR) data obtained from the Medium Resolution Spectral Imager-II (MERSI-II) onboard the Chinese Fengyun-3D (FY-3D) satellite are bowtie effect and nonuniform brightness stripe noise. While previous solutions have addressed these issues separately, this article introduced a more integrated two-step image quality enhancement strategy for MERSI-II TIR images. It considered the interactions between the two issues and overcame the excessive or inadequate destriping in existing models due to the ideal stripe-type assumption. Specifically, for the bowtie effect, a rigorous geometric model suitable for MERSI-II was constructed by considering the Earth’s curvature and adjusting the preset image width. For the nonuniform brightness stripe noise, a novel adaptive multiscale frequential (AMSF) algorithm was developed. The multiscale spectral detection effectively captured the anomaly frequency, and the adaptive threshold dynamically adjusted the detection range, profiting in preserving details. The proposed strategy was validated on MERSI-II TIR images, outperforming existing methods in quantitative and qualitative assessments with higher efficiency on both bowtie effect and stripe noise removal. Further experiments conducted on Moderate Resolution Imaging Spectroradiometer (MODIS) data demonstrated the AMSF algorithm’s applicability to different data. In addition, the 250-m MERSI-II FY-3D data can help us understand the rapid variations of Arctic sea ice leads, which are key features within the sea ice. Using the quality-enhanced images to extract the sea ice leads in winter Arctic Baffin Bay improved the overall accuracy from 0.88 to 0.95, thereby providing more accurate and reliable sea ice lead data.

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 output of the FengYun-3 B (FY-3B) satellite's medium resolution spectral imager (MERSI) visible on-board calibrator (OBC) degrades with time, causing concerns regarding its reliability in radiometric calibration. Special efforts have been made to extract the variation in information of the interior lamps. The specifications include: 1) digital count retrieval from the OBC files and an exponential plus linear fit applied to determine the relative change in the MERSI response; 2) anchoring to cross calibration with MODIS to acquire the absolute radiometric calibration coefficients; 3) a time series calibration tracking method based on the pseudoinvariant target site applied to monitor the degradation of the FY-3B MERSI instrument; and 4) “net” interior lamp radiometric output (ILRO) obtained by subtracting long-term pseudoinvariant tracking results from the scaled visible on-board calibrator-based model gain coefficients. The results show that the decay rates of the ILRO are wavelength dependent and the shortwave outputs are found to experience a large degradation, particularly at 470, 550, 412, 443, 490, 520, and 565 nm. The largest annual degradation rate is greater than 15% in the first year. The implementation of the current procedure combined with a detailed analysis will provide a basis for further determining lamp changes, leading to improved absolute calibration accuracy of MERSI products.

In the above paper[1], the name of the institution of the authors Dandan Zhi, Tanqi Yu, and Yan Pan were incorrect, they are with the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Key Laboratory of Optical Calibration and Characterization, Hefei 230031, China, and also with the University of Science and Technology of China, Hefei 230026, China (e-mail: [email protected]).

The advanced MEdium Resolution Spectral Imager (MERSI II) is a major instrument onboard the Chinese FengYun 3D satellite, which was launched in November 2017. Extensive measurements were performed during MERSI II prelaunch testing to ensure effective characterization for on-orbit calibration. This paper gives a brief overview of the prelaunch performance testing conducted for MERSI II, as well as its improvements in terms of instrument design compared to MERSI I. The prelaunch calibration methodology and radiometric performance are detailed, including dynamic range, signal-to-noise ratio, noise equivalent differential temperature, linearity, and response uniformity. The assessment results indicate that most bands perform effectively with mirror specification noncompliances in a few reflective solar bands (RSBs). In addition, investigation of the stability and uniformity of the spherical integrating source indicates that they have a critical impact on the performance assessment and prelaunch calibration of RSBs. The temperature-dependence features of thermal emissive bands' performances are also discussed in terms of their sensitivity to the operating temperature of the focal plane assembly and instrument circumstance. This paper also shows that the self-stability of the calibration source and the representation of the assessment methods are important as they affect the results of instrument performance evaluation.

The shortwave infrared (SWIR) bands (1.64 and 2.13μm) of FY-3A MERSI have multiple electronic gains. Unfortunately, the SWIR bands are influenced by anomaly electronic gain jumps possibly induced by space environment, which restricts the data application. In this paper, the long-term data series of space view (SV) observation and sensor temperature measurement of FY-3A MERSI are used to analyze the on-orbit variation of SWIR band gains, and a gain level auto identification algorithm is presented. Based on it, the SV data are normalized to the same gain level with good results. Besides, the long-term drifting of SV is revealed, as well as the temperature dependent feature of the radiometric response of 2.13 μm band, and a preliminary temperature correction model is obtained. Based on this research, continuous on-orbit radiometric calibration can be implemented and the application ability of the SWIR data could be recovered. The revealed features of SV long-term drifting and temperature dependency provide valuable information for the Chinese sensor design, manufacture and testing, as well as elaborated on-orbit radiometric calibration processing.

The MEdium Resolution Spectral Imager (MERSI) is the keystone instrument onboard FengYun-3 (FY-3). After FY-3A, FY-3B MERSI is the second launched in November 2010. Nineteen of the 20 MERSI spectral bands are the reflective solar bands, which cannot be absolutely calibrated onboard. The annual vicarious calibration (VC) based on synchronous in situ measurements at the Dunhuang site is the baseline calibration method for MERSI. To assure frequent and stable calibration updates, a multisite calibration tracking method is developed. This paper presents the FY-3B MERSI postlaunch daily calibration updating method based on multisite calibration tracking with the Dunhuang VC correction, the long-term sensor response on-orbit change, and the calibration performance evaluation. A reflectance-based method is used for the Dunhuang VC, and the reflectance calibration uncertainties are within 3% for most MERSI bands. The multisite calibration tracking method relies on simulated radiation over several stable sites without synchronous in situ measurements. A postlaunch daily calibration updating model is established using a linear function of days since launch to describe the long-term trend. The calibration updating model is validated by the Dunhuang VC, showing the relative bias within 3.5% for most bands. It is found that the shortwave channels of MERSI experience large degradation, particularly the 412-nm band with an annual degradation rate of approximately 18%, whereas most red and near-infrared bands are relatively stable. Using the calibration updating model with the Dunhuang VC correction, the recalibrated MERSI data are validated against Moderate Resolution Imaging Spectroradiometer by near synchronous-nadir-observation analysis, and good agreement is achieved.

The Medium Resolution Spectral Imager (MERSI) is a key instrument onboard Fengyun-3 (FY-3), the second generation of polar-orbiting meteorological satellites in China. This paper summarizes the knowledge of MERSI instrument in terms of sensor design, calibration algorithm, prelaunch and on-orbit characterization, and performance verification. The calibration monitoring of its reflective solar bands (RSBs) is primarily conducted using a visible onboard calibrator and found that it has a significant degradation on the order of 10% in its shorter RSB bands (<; 500 nm), with the largest in band 8 of about 20% during the past two years. However, the performance at longer wavelength bands is relatively stable with a change of less than 5%. It is shown that the postlaunch calibration of the two short-wavelength infrared bands has frequent fluctuations because of random jumps in their electronic gains. These results are consistently verified by two kinds of vicarious calibration (VC) methods: China Radiometric Calibration Sites VC and intercalibration using Terra/Moderate Resolution Imaging Spectroradiometer over Dunhuang desert. The overall uncertainty in the MERSI top-of-atmosphere radiance or reflectance is less than 5%. These results provide the important reference and evaluation for the update of the FY-3A/MERSI calibration coefficients.

The MEdium-Resolution Spectral Imager (MERSI), onboard the second-generation Chinese polar-orbit meteorological satellite FY-3A, is a MODIS-like sensor with 19 solar bands and one thermal infrared band. Although there is a visible onboard calibration device, it can only be used for tracking temporal instrument degradation. The vicarious calibration (VC) campaign at the Dunhuang site, conducted once a year, has been the main postlaunch absolute radiometric calibration method for MERSI in the solar bands. To increase the in-flight calibration frequency, a multisite radiometric calibration tracking method is presented. This method relies on simulated radiation over several stable sites, and a daily calibration updating model is built from long-term trending of calibration coefficient series. The MERSI calibration reference is evaluated against the observations of Aqua MODIS, showing mean relative biases within 5% from 0.4 to 2.1 μm . The short-wave channels of MERSI are found to experience large degradation, particularly the 412-nm band with an annual degradation rate of 9.7%, whereas the red and near-infrared bands are relatively stable with annual degradation rates within ±1%. Several approaches have been used to analyze the reliability of MERSI calibration results. A comparison of the calibration slopes shows that the relative biases between the multisite method and the annual Dunhuang VC campaign are below 3.8%. Aqua MODIS is used as a reference to monitor the data quality of the recalibrated MERSI. A double-difference analysis shows that the mean relative biases are almost within 5% over stable deserts, and the synchronous nadir observation analysis also reveals good agreement.