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.

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.

Calibration uncertainty is quantified for Microwave Humidity Sounder (MWHS-II) onboard Fengyun Meteorological Satellites (FY-3C and D) in this study. The on-orbit calibration equation is used to construct the uncertainty model and the observation period is during January to December 2020. The results show that, among the 15 channels of the microwave sound, Channel 2 has the largest observation uncertainty due to its narrow bandwidth. The maximum observing uncertainty values over the ocean and land onboard FY-3C are 0.98K and 0.97K, respectively, while the corresponding values onboard FY-3D satellite are 0.94K and 0.82K. The calibration uncertainties of the 183GHz channels onboard FY-3D are greater than that of FY-3C due to the harmonic interference.

Blackbox transfer attacks for image classifiers have been extensively studied in recent years. In contrast, little progress has been made on transfer attacks for object detectors. Object detectors take a holistic view of the image and the detection of one object (or lack thereof) often depends on other objects in the scene. This makes such detectors inherently context-aware and adversarial attacks in this space are more challenging than those targeting image classifiers. In this paper, we present a new approach to generate context-aware attacks for object detectors. We show that by using co-occurrence of objects and their relative locations and sizes as context information, we can successfully generate targeted mis-categorization attacks that achieve higher transfer success rates on blackbox object detectors than the state-of-the-art. We test our approach on a variety of object detectors with images from PASCAL VOC and MS COCO datasets and demonstrate up to 20 percentage points improvement in performance compared to the other state-of-the-art methods.

Calibration accuracy can affect the interpretation of microwave radiometer observations and it is the premise of quantitative microwave remote sensing. To calibrate the microwave radiometers is to establish a quantitative relationship between output (voltage or voltage digital) and input noise temperature. On-orbit calibration uses the calibration target of the sensor and the cold space as reference sources. It has been found that the performance of the same microwave instrument will be degraded with the aging components. Observations from microwave instruments of different satellite generations may be inconsistent, and with different systems the receiving radiance for the same atmospheric scene could be also different. To calibrate the microwave payloads onboard FengYun satellite, the calibration/validation is performed based on the global navigation satellite system (GNSS) retrievals, focusing on the sounding mechanisms of microwave and the GNSS occultation payloads on FengYun polar-orbiting meteorological satellites. For GNSS occultation data, the detection accuracy above 3 km is better than that of the microwave radiometer. Within the time window of 30 min and the spatial resolution of 0.5 degree both in latitude and longitude, the brightness temperature deviation is better than 3 K for clear sky. The results show that the FengYun meteorological satellite can not only provide global all-weather temperature and humidity profiles but also realize self-cross-calibration for microwave sounders with complementary instruments onboard.

This study presents precipitation and related microphysical processes observed by a self-developed ground-based five-band dual-polarization microwave radiometer in conjunction with disdrometer and vertically pointing micro rain radars (MRRs) during convective and orographically induced precipitation. The ground-based microwave radiometer has ten channels measuring in horizontal and vertical polarization at 10.65, 18.7, 23.8, 36.5 and 89GHz, where dual 89GHz channels can provide additional information on cloud-ice and melting cloud, which are rarely used in ground-based instruments so far. Measured brightness temperatures and polarization differences (vertical-horizontal) are validated by 3-D radiative transfer simulations (VDISORT) considering the role played by cloud-ice and melting particles, other than traditional cloud-rain particles. Observing measurements confirm the importance of the 89GHz polarization difference for separating the cloud-ice from rain and cloud-water component and quantitatively interpreting cloud-ice-rain concentration and distribution profiles by deep learning combing additional measurements from MRR and disdrometer.

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.

To analyze the characteristic distribution of the MWRI rainfall algorithm, six record observations of typhoon Mufia are selected as the case study for this study. Six track records of MWRI observations and AMSR-E GPROF2010 rainfall products are matched and selected to make the analysis and comparisons. Averaged radial rainfall distribution from MWRI and AMSR-E rainfall products are consistent in six records. The results indicate that the MWRI retrievals are coincident with AMSR-E rain rates in temporal and spatial evaluation. MWRI radiometer can observe the entire process from formation, maturity to extinction for typhoons.

In this study, along-scan performance of FengYun-3C microwave radiation imager (MWRI) is examined utilizing one-year data over ocean. Generally-good uniformity along azimuth scan position is found with a TB bias less than 1 K at the edge-of-scan. However, discernible errors can be still found and the behaviors of the scan-position-dependent bias are different for ascending and descending phases of the orbits. It is thus suggested that descending and ascending passes should be taken into account, separately, when correcting antenna pattern in calibration processes.

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.

This paper analyzes the effects of cloud supercooled liquid water (SCLW) on the polarization difference (PD) and brightness temperature (TB) generated by horizontally oriented snow particles at microwave band. Radiative transfer (RT) calculations from six realistic snowfall profiles selected from European Centre for Medium-Range Weather Forecasts (ECMWF) datasets indicate, that the existence of SCLW has noticeable impact on PD and TB at three window frequencies (150 GHz, 243 GHz and 664 GHz) for, respectively, light, median and heavy snowfall in dry and wet air conditions. It is implied that accurate information on liquid water is required to interpret polarimetric observations of ice clouds and snowfall for future satellite missions.

This paper presents the effects of uncertainties in ice permittivity on emerging brightness temperatures at three different zenith angles at the frequencies of Advanced Microwave Sounding Unit-B (AMSU-B), 89 GHz, 150 GHz and 183 GHz. The absolute error of brightness temperature, due to an uncertainty of ±20% in the imaginary part of ice permittivity and of ±5% in the real part, is explored with vector discrete ordinate radiative transfer (VDISORT) method at the zenith angle of 0°, 30° and 53° in this study. Particles in clouds are taken to be spheres, of which the diameters are uniformly distributed in the range of 40 ¿m to 4000 ¿m in Gamma-size distribution. We found that ±5% uncertainty in the real part of ice permittivity results in more than 1 K variation in upwelling brightness temperature, higher than ±20% uncertainty in the imaginary part when the effective diameter in Gamma-size distribution is 150 ¿m. And the absolute error of brightness temperature caused by uncertainties in ice permittivity is up to more than 2 K at the zenith angle of 53°, greater than the corresponding value at the other two zenith angles.