This paper introduces the Chinese Tianmu-1 GNSS-R constellation of 22 small satellites launched in 2023–2024 and comprehensively evaluates the latest version of the in-orbit data. First, the mission design and instrument technology are described, which largely builds on the Fengyun-3/GNOS-II missions. Notable innovations include full-GNSS compatibility and dual-polarization antenna. Then, the spatiotemporal characteristics of the constellation are analyzed—specifically, coverage percentage and mean revisit time at different latitudes. Next, the accuracy of its science products including ocean surface winds and land soil moisture has been assessed, with two application cases demonstrating the mission’s utility for monitoring tropical cyclones and flooding. Finally, this paper for the first time evaluates Tianmu’s polarized observations including Horizontal (H), Vertical (V), Left-Hand Circularly-Polarized (LHCP) and Right-Hand Circularly-Polarized (RHCP). Analysis of the signal-to-noise ratio and reflectivity shows that the dual-polarimetric observations follow the trend of theoretical models and hold promise for advancing land remote sensing.

Fengyun-3 (FY-3) series are operational satellite missions that can provide global GNSS-R observations using multiple GNSS systems. This abstract highlights the advancements in GNSS-R product development from FY-3E, FY-3F, and FY-3G. Currently available to the public are operational Level 1 and Level 2 wind products with a 25-km resolution. Additionally, a raw intermediate frequency product is accessible for scientific research. Upcoming product developments include the release of Level 2 12.5-km wind, Level 2 land soil moisture, Level 2 sea ice thickness, and Level 3 wind. Their algorithms and scientific impact will be discussed.

The degradation issues significantly restrict the automotive application of proton exchange membrane fuel cell (PEMFC), while the dynamic characteristics of PEMFC stack with multiple cells play a critical role in fuel cell's lifetime and reliability. But it is challenging to efficiently and accurately predict the multi-cell voltages and performance uniformity under dynamic fuel cell operations due to the complex structure and multi-physical processes inside the stack. In this paper, we develop a hybrid quasi-2D equivalent circuit model to study the nonuniform dynamic characteristics of PEMFC stack. The 140-cell PEMFC stack with active area of 406 cm2are experimentally studied under various load changing processes to validate the proposed model. The proposed multi-cell equivalent circuit network could reproduce the dynamic cell voltage distributions with the error less than 15 mV (~2.1%) under varied operating conditions. The efficient prediction of dynamic cell voltage distributions of the stack is beneficial for control strategy optimization of fuel cell loading operations and improved stack lifetime.

Reflected global navigation satellite system (GNSS) signals from Earth surface can be received by receivers at low Earth orbit for the remote sensing of geophysical parameters. While the technique has been studied for around 30 years, most early spaceborne GNSS reflectometry missions only adapted to receive GPS signals and the studies of reflected Galileo (GAL) signals in space are limited. The Navigation Satellite System Occultation Sounder II (GNOS-II) payload onboard the FY-3E satellite is the first mission that can operationally receive reflected GPS, BeiDou (BDS), and GAL signals at the same time. This letter presents the GAL reflectometry measurements from GNOS-II together with their calibration and wind speed (WS) retrieval methods. Results show that while GAL has a different signal modulation, the observables can be used to retrieve WSs using the same geophysical model functions (GMFs) of GPS after a dedicated calibration. The retrieved WSs from GAL also have a comparable accuracy as those from GPS and BDS.

With low signal power, Global Navigation Satellite System (GNSS) reflections in space are vulnerable to radio frequency interference (RFI) across the globe which can contaminate observations of GNSS reflectometry (GNSS-R) and introduce bias in the retrieved geophysical parameters. This paper presents a comprehensive analysis of the RFI in the Fengyun-3E Navigation Satellite System Occultation Sounder II (FY-3E/GNOS-II) GNSS-R data over the ocean. It was found that major sources of interference are the reflections of regional GNSS and Satellite-Based Augmentation Systems (SBAS), and external L-band interference signals from the ground. Furthermore, interference can have different impacts on the observations through increasing the noise power, decreasing the signal-to-noise ratio (SNR) or biasing observables depending on the type of interference and retrieval method. Some geography-dependent biases in the retrieved wind speeds were found to be related to RFI. An RFI mitigation method is then presented to calibrate observables and reduce geographical wind speed bias for the operational retrieval system. The bias and root-mean-square error of retrieved wind speeds from a region that is affected by RFI were reduced from 1.26, 2.78 to 0.04, 1.59 m/s, respectively.

Global Navigation Satellite System Reflectometry (GNSS-R) is a burgeoning remote sensing observation technology that can retrieve global sea surface wind speeds using satellite signals reflected from the sea surface. The improvement of data quality and the accumulation of data volume of this technology provides data support for constructing interdisciplinary-based retrieval models. This article constructs a hybrid deep neural network model based on deep learning for wind speed retrieval, which can receive and perform feature mining on the entire delay waveform while simultaneously supporting multiple auxiliary features input and achieving joint fitting. Then a bias correction method based on cumulative distribution function (CDF) matching is introduced to mitigate bias, especially at high wind speeds. We verify the contribution of different attribute features in wind speed retrieval by designing a feature ablation analysis. The fluctuation variation of the retrieval accuracy in the time dimension and the retrieval results distribution in space are compared and analyzed. The root mean square error (RMSE) of retrieval results is 1.486m/s and can reach 1.399m/s under the 94.25% wind speed condition. After bias correction based on CDF matching, the retrieval accuracy at high wind speed is improved by 7.19%. Besides, this model also has good temporal stability and can reproduce large-scale wind fields while effectively mitigating retrieval bias on a global scale, showing great potential for operational applications.

Proton exchange membrane fuel cells (PEMFCs) have been extensively studied and commercialized in many fields, such as electric vehicle and residential applications, but some operational and technical challenges limit their more widespread use. One of the challenges is that PEMFCs are prone to drying or flooding due to inappropriate operating conditions. However, there is a lack of simple and effective method to detect the faults in advance. To address the issue, this paper presents an early diagnosis approach by measuring the phase of single-frequency impedance. Impedance responses under different conditions of water content in the stack were studied in this paper. Experiments show that the impedance phase angles in the mid-frequency range (tens of Hz) are positively correlated with the water content in the stack. By analyzing the experimental results, a fuel cell fault diagnosis method based on single-frequency impedance phase angle is proposed. This method is simple, effective, and easy to implement. It can be used for early faults diagnosis and closed-loop water management to improve the performance, durability, and reliability of the fuel cell system.

The Global Navigation Satellite System Reflectometry (GNSS-R) technique can measure ocean surface winds and other geophysical parameters from forward scattered GNSS (GPS, BeiDou, Galileo, etc.) signals. However, most early spaceborne missions only captured GPS signals while the study of spaceborne reflectometry using BeiDou signals (BDS-R) is limited. The GNOS-II payload onboard China’s FY-3E satellite has been operationally collecting a large number of BDS-R data since July 10, 2021. BDS is different from GPS in the orbit, signal frequency, chipping rate and Effective Isotropic Radiated Power (EIRP). Furthermore, BDS satellites have different generations and orbits. This paper, for the first time, comprehensively characterizes the spaceborne BDS-R observations over the ocean using the FY-3E GNOS-II data from different BDS satellite types including BDS-2 IGSO, BDS-2 MEO, BDS-3 IGSO and BDS-3 MEO. Their spatial coverage, spatial resolution, effective scattering area, incidence angle, range corrected gain, EIRP and signal-to-noise ratio have been analyzed and compared to those of GPS-R. Spaceborne BDS-R shows a lot of uniquenesses compared to GPS-R. The BDS-R observables are then calibrated separately for each type. An intercalibration is also applied to correct extra calibration errors. After the calibration, BDS-R observables and retrieved winds are consistent between each type and compared to those of GPS-R. Calibrated observables from FY-3E GNOS-II are also evaluated by comparing them to those measured by the Cyclone Global Navigation Satellite System (CYGNSS) mission and a theoretical model. The results of this paper can provide a reference for future BDS-R studies and spaceborne GNSS-R mission design.

A data-driven strategy for diagnosing the water management failure in a Proton Exchange Membrane Fuel Cell (PEMFC) is proposed in this paper. In the proposed diagnosis approach, individual cell voltages are used as the variables for diagnosis. A dimension reduction tool, named principal component analysis (PCA), is used to extract important feature information from diagnostic variables collected at different time points. The pattern recognition tool, named support vector data description (SVDD), is then used to construct hyperspheres, each of which tightly contains a certain kind of data in the feature space. A multi-classification decision strategy, which considers the size of the hypersphere and the distance from the sample to the hypersphere center, is finally proposed to realize fault detection. The experimental results show that the PEMFC stack water management fault can be successfully diagnosed and distinguished based on the PCA and SVDD multi-classification fault diagnosis strategy.

The interferometric Global Navigation Satellite System Reflectometry (iGNSS-R) exploits the full spectrum of GNSS signal to improve the ranging performance for sea surface height applications. However, simultaneous reception of multiple satellite signals introduces the intersatellite interference degrading the altimetry accuracy, which occurs in both ground-based and spaceborne scenarios. By simulating situations of ground-based stations in Beijing and Hainan, the feasibility of ground-based iGNSS-R altimetry measurement without the intersatellite interference is verified, for more than half of the total observation areas are available and up to 34.73% and 74.64% of a day is suitable in Beijing and Hainan. Some antenna directions are recommended in the northern (southern) hemisphere: for long-time measurement, the lower (upper) parabola formed by GLONASS and the area of specular reflection points of GEO satellites are preferred; For continuous measurement, GEO satellites are recommended; For all-day measurement, the range of elevation and azimuth is 30°-60° and ±300(150°-210°).

For preventing the effects of sea ice bring the serious negative impact on the maritime transport, a full-scale detection of sea ice should be performed. The BeiDou GEO satellites could provide stable geometry and better coverage in mid- and low-latitude region where most of the sea ice occur. Based on this consideration, this paper evaluates the usage of BeiDou GEO Satellites reflected signals for accurate real-time Earth observation to study the changes in the sea surface state through remote sensing. BeiDou GEO signals received after reflection from Bohai Bay were analyzed for their sea ice content. The results are in good agreement with the cluster center of the sea ice reflected correlation power and the sea water reflected correlation power. The average of the cluster center on the sea ice surface is much smaller than that on the sea water surface.

In this paper, the SOMOSTA (Soil Moisture Monitoring Station) experiment on soil moisture monitoring by Global Navigation Satellite System Reflected signals(GNSS-R) at the Valencia Anchor Station is introduced.