Abstract. This study uses observations from COSMIC-2, FY-3, GRACE-C/D, KOMPSAT-5, MetOp-A/C, PAZ, PlanetiQ, Sentinel-6A, Spire, TerraSAR-X/TanDEM-X, TM-1, and YY-1 during 1–7 January 2026 to compare their spatial distributions, grid coverage, local-time sampling, and retrieval accuracy; it also assesses the benefits of multi-mission integration and the potential effects of COSMIC-2 retirement on the integrated observing capability. The results show that orbital configuration determines latitudinal and local-time sampling, whereas event abundance and constellation size primarily control grid coverage. Large constellations such as YY-1 and TM-1 provide the strongest global continuity, while missions with different orbital inclinations offer complementary sampling across latitude bands. Combining all missions substantially reduces spatial gaps and provides near-global coverage at 1.0° and 2.0° resolutions. Within 6 h windows, joint coverage reaches 70.98 % at 2.0° resolution. The combined observations also broaden local-time sampling, whereas COSMIC-2 remains particularly important for equatorial coverage. Therefore, the retirement of COSMIC-2 would reduce equatorial sampling capability and should be considered in the development of future multi-mission GNSS-RO observing systems. Comparisons with ERA5 indicate broadly consistent retrieval accuracy among most missions between 10 and 35 km, with larger differences in the lower troposphere. These findings demonstrate the value of integrating missions with diverse orbital configurations and highlight the need to sustain complementary sampling after the retirement of key missions.

Currently, GNSS radio occultation (RO) detection technology has been successfully implemented in satellite missions, including CHAMP, GRACE, COSMIC, Metop, the FengYun-3 series, et al., and has been operationally integrated into numerical weather prediction systems. Despite its technical maturity, the lack of standardized observation-independent exchange format remains a significant challenge for effective data exchange and unified processing of occultation data. To address this limitation, the National Space Science Center (NSSC) of the Chinese Academy of Sciences has proposed and developed the GNSS Radio Occultation Observation Independent Exchange Format (ROEX), aiming to improve data sharing and enhance the application of RO technology in atmospheric sounding and weather forecasting. Based on this, this paper develops a python-based open-source software tool for ROEX file preprocessing, named PyROEX. The software is a graphical user interface (GUI) tool which provides functions for file observation monitoring, observation scientific combination, data integrity checking, and file cutting. Users can freely select the observation they wish to view or combine, and analyze the file quality through plotted graphs. This paper introduces the ROEX format and the functional modules of PyROEX, as well as the analysis results of its operation. It is expected that this software will contribute to the widespread adoption of the ROEX format, thereby reducing the economic costs, time costs, and personnel costs associated with data exchange, communication, and software development caused by inconsistent data formats, and accelerate the operational application of GNSS occultation products.

The bottomside thickness parameter (B-0) is a critical component for accurately representing electron density profiles in the International Reference Ionosphere (IRI) model. This study presents a comprehensive validation and global analysis of B-0 derived from the Fengyun-3 (FY-3) GNSS radio occultation (RO) mission during the high solar activity period of 2022-2024. By integrating observations from the FY-3 constellation, this work effectively complements the data coverage of FORMOSAT-7/COSMIC-2 (F7/C2) by extending analysis to mid- and high-latitude regions. Validation against global digisonde measurements demonstrates that FY-3 derived B-0 achieves high reliability, yielding correlation coefficients exceeding 0.86 and an RMSE of approximately 20 km in low and midlatitudes. Comparisons with three IRI-2020 model options reveal that the ABT-2009 option offers the best overall agreement with observations, particularly in reproducing hemispheric asymmetries, whereas the Gul-1987 and Bil-2000 options exhibit notable deficiencies in capturing geomagnetic modulation and spatial variability. Global morphological analysis identifies a synchronized but inversely correlated relationship between B-0 and peak electron density (NmF2) in the equatorial ionization anomaly region. Furthermore, distinct longitudinal structures are observed, characterized by wavenumber-4 patterns during equinoxes, and wavenumber-2 and -3 patterns during summer and winter solstices, respectively. In high latitudes, observations suggest a B-0 enhancement near the South Magnetic Pole during the Southern Hemisphere winter, which likely reflects the influence of geomagnetic control. These findings confirm the utility of FY-3 RO data for characterizing the global ionosphere and offer valuable constraints for future refinements of the IRI model.

The Global Navigation Satellite System Reflectometry (GNSS-R) technique provides global ocean surface wind observations unaffected by rainfall with high spatiotemporal resolution. The Fengyun-3E (FY-3E) mission, as the first operational GNSS-R satellite in China, offers low-latency data suitable for numerical weather prediction (NWP). However, the dense along-track sampling of GNSS-R winds poses challenges for observation error specification in data assimilation. In this study, FY-3E GNSS-R winds are assimilated into the Weather Research and Forecasting (WRF) model to investigate the impacts of different observation error configurations. Both static and dynamic error specifications, with and without data thinning, are evaluated through a sensitivity experiment and subsequent Observing System Experiments (OSEs). The results indicate that using a static observation error of 6 m/s without data thinning achieves the best performance. Under this configuration, GNSS-R winds influence atmospheric analyses from the surface up to approximately 700 hPa in a single assimilation case, while cycling experiments further extend the impact vertically and spatially. These findings highlight the importance of appropriate observation error specification for dense GNSS-R data and provide a practical reference for their assimilation in WRF, with potential applicability to other NWP systems.

This paper proposes a passive in-orbit calibration method for phased array antennas using GNSS carrier-phase measurements. By performing synchronous observation and exploiting the short-baseline property between the positioning antenna and array elements, the first differencing operation eliminates space propagation errors and clock biases. By further utilizing receiver channel consistency, the second differencing operation cancels out the receiver channel errors, thereby extracting the relative receive-chain phase error of the element under test. Under typical operating conditions, the calibration accuracy can reach an RMS error of approximately 3.02mm, corresponding to a phase accuracy of 5.72 degrees in the GPS L1 band. This accuracy is close to the 5.625 degrees minimum phase step of a 6-bit digital phase shifter, and can be further improved under higher C/N0 and well-controlled residual error conditions. Without requiring a dedicated GNSS band excitation signal, this method avoids co-frequency self-interference with the positioning antenna, which provides an auxiliary approach for in-orbit calibration of phased array receive chains.

This article introduces the Chinese Tianmu-1 Global Navigation Satellite System Reflectometry (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 (CP) 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 (TCs) and flooding. Finally, this article, 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 (SNR) and reflectivity shows that the dual-polarimetric observations follow the trend of theoretical models and hold promise for advancing land remote sensing.

The Fengyun-3 G (FY-3 G) satellite equipped with a Global Navigation Occultation Sounder II (GNOS-II) is capable of tracking BeiDou Navigation Satellite System (BDS) and Global Positioning System (GPS) dual-frequency signals by its zenith precise orbit determination antenna. Currently, most global navigation satellite system (GNSS)-related studies derived the topside total electron content (TEC) only by GPS signals, while there are relatively few studies using BDS signals. In addition, as FY-3 G satellite operates at a lower orbit with an altitude of 410 km, it is closer to the pivotal regions of the ionosphere, particularly the F2 layer. This grants it the capability of capturing dynamic changes and intricate details of the ionosphere with enhanced resolution and sensitivity. In this article, we derive the topside TEC from the six months of FY-3 G GNOS-II data and analyze the performance of differential code bias (DCB). The final retrieved topside BDS and GPS TEC has also been evaluated. The internal assessment of the FY-3 G receiver DCB is conducted by evaluating the long-term stability, which shows that the standard deviation (STD) of the FY-3 G receiver DCB reaches 0.86 TECU for the GPS C1C-C2W observation type and 0.83 TECU for BDS B2I-B6I type. The FY-3 G topside slant TEC (STEC) shows a high agreement with the space weather and atmospheric response mission TEC product, with a mean bias of 0.836 TECU and STD of 2.044 TECU. A comparative analysis in FY-3 G STEC between BDS and GPS satellites reveals no significant systematic bias, and the TEC retrieval performance of BDS-3 is superior to that of BDS-2. Moreover, the ionospheric characteristics of the monthly average FY-3 G topside vertical TEC map is consistent with the International GNSS service Global Ionospheric Map product in latitude, season, and local time. Above all, this research shows that FY-3 G GNOS-II can provide high-quality topside TEC observations, which can serve as a valuable dataset for research in ionospheric and plasma sciences.

For the second Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-2) Global Positioning System radio occultation (GPS RO) neutral atmosphere retrieval, ionosphere error correction on bending angle is performed by combining either L2C or L2P signals with L1 at the University Corporation for Atmospheric Research (UCAR), Boulder, CO, USA. While the L2C is a civil signal, the encrypted L2P signal from high-noise occultations will lead to low-accuracy bending angle profiles. This study distinguishes high-noise COSMIC-2 L2P occultations using the L2P signal-to-noise ratio (SNR) and proposes an alternative ionosphere correction method by fitting and extrapolating the difference between the L1 bending angle and the bending angle derived from the MSIS 00 model above 35 km ( ${\alpha }_{\text {L1+M}}$ ). Statistical validations are performed based on over 110159 L2P occultations from January 2023 to June 2023. The comparison of $ {\alpha }_{\text {L1+M}}$ and L1 and L2P bending angle, derived from L2P occultations with SNR below 200 v/v ( ${\alpha }_{\text {{L}1}+{\text {ML}}}$ and ${\alpha }_{\text {{L}1+L}{2}{\text {PL}}}$ ), shows that the percentage passed quality control increased from 40.8% (39.5%) to 73.6% (69.0%) for rising (setting) occultations. The mean standard deviation (SD) of ${\alpha }_{\text {L1+M}}$ is 2.0% (1.9%) on impact heights between 19 and 30 km for rising (setting) occultations, which is obviously smaller than 2.6% (2.5%) for ${\alpha }_{\text {L1+L2PL}}$ profiles. Hence, it can be concluded that the proposed ionosphere correction method can be utilized for retrieving COSMIC-2 L2P occultations, and combined with L2P high SNR occultations, the COSMIC-2 L2P occultations can further enrich the amounts of effective profiles on numerical weather prediction (NWP) and climate research.

Global Navigation Satellite System (GNSS) Radio Occultation (RO) and GNSS Reflectometry (GNSS-R) are the two major spaceborne GNSS remote sensing (GNSS-RS) techniques, providing observations of atmospheric profiles and the Earth’s surface. With the rapid development of GNSS-RS techniques and spaceborne missions, many experiments and studies were conducted to assimilate those observational data into numerical weather-prediction models for tropical cyclone (TC) forecasts. GNSS RO data, known for its high precision and all-weather observation capability, is particularly effective in forecasting mid-to-upper atmospheric levels. GNSS-R, on the other hand, plays a significant role in improving TC track and intensity predictions by observing ocean surface winds under high precipitation in the inner core of TCs. Different methods were developed to assimilate these remote sensing data. This review summarizes the results of assimilation studies using GNSS-RS data for TC forecasting. It concludes that assimilating GNSS RO data mainly enhances the prediction of precipitation and humidity, while assimilating GNSS-R data improves forecasts of the TC track and intensity. In the future, it is promising to combine GNSS RO and GNSS-R data for joint retrieval and assimilation, exploring better effects for TC forecasting.

Thermospheric mass densities are investigated to explore their responses to solar irradiance and geomagnetic activity during the period from 31 October to 7 November 2021. Utilizing data from the Global Navigation Satellite System (GNSS) payload and an ionization gauge mounted on the Orbital Neutral Atmospheric Detector (OAD) payload onboard the QQ-Satellite, thermospheric mass densities are derived through two independent means: precise orbit determination (POD) and pressure measurements. For the first time, observations of these two techniques are compared and analyzed in this study to demonstrate similarities and differences. Both techniques exhibit similar spatial–temporal variations, with clear dependences on local solar time (LT). However, the hemispheric asymmetry is almost absent in simulations from the NRLMSISE-00 and DTM94 models compared with observations. At high latitudes, density enhancements of observations and simulations are shown, characterized by periodic bulge structures. In contrast, only the OAD-derived densities exhibit wave-like disturbances that propagate from two poles to lower latitudes during geomagnetic storm periods, suggesting a connection to traveling atmospheric disturbances (TADs). Over the long term, thermospheric mass densities derived from the two means of POD and the OAD show good agreements, yet prominent discrepancies emerge during specific periods and under different space-weather conditions. We propose possible interpretations as well as suggestions for utilizing these two means. Significantly, neutral winds should be considered in both methods, particularly at high latitudes and under storm conditions.

The spatial–temporal sampling errors arising from the differences in geographical locations and measurement times between co-located Global Navigation Satellite System (GNSS) radio occultation (RO) and radiosonde (RS) data represent systematic errors in the three-cornered hat (3CH) method. In this study, we propose a novel spatial–temporal sampling correction method to mitigate the sampling errors associated with both RO–RS and RS–model pairs. We analyze the 3CH processing chain with this new correction method in comparison to traditional approaches, utilizing Fengyun-3E (FY-3E) GNSS Occultation Sounder II (GNOS II) RO data, atmospheric models, and RS datasets from the Hailar and Xisha stations. Overall, the results demonstrate that the improved 3CH method performs better in terms of spatial–temporal sampling errors and the variances of atmospheric parameters, including refractivity, temperature, and specific humidity. Subsequently, we assess the error variances of the FY-3E GNOS II RO, RS and model atmospheric parameters in China, in particular the northern China and southern China regions, based on large ensemble datasets using the improved 3CH data processing chain. The results indicate that the FY-3E GNOS II BeiDou navigation satellite system (BDS) RO and Global Positioning System (GPS) RO show good consistency, with the average error variances of refractivity, temperature, and specific humidity being less than 1.12%2, 0.13%2, and 700%2, respectively. A comparison of the datasets from northern and southern China reveals that the error variances for refractivity are smaller in northern China, while temperature and specific humidity exhibit smaller error variances in southern China, which is attributable to the differing climatic conditions.

High-resolution thermospheric mass density low Earth orbit (LEO)-based measurements are valuable for accurately estimating shortterm atmospheric abrupt disturbances triggered by solar flux forcing. To investigate the enhancing status of solar cycle 25 between August 29 and December 31, 2020, we processed Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) 10-s accelerometer-derived thermospheric mass density (TMD) measurements normalized at 500 km altitude by the NRLMSIS-2.0 empirical model. These 4-month enhancing disturbance observations suggest a shift from relative quiescence to a much more active solar phase, revealing unexpected dependencies on temporal and spatial characteristics. The results indicated that the dominant driver is solar extreme ultraviolet radiation (EUV) during this ascending phase. Density enhancement was symmetric in both hemispheres around the autumn equinox. After the equinox, the neutral density enhancement intensity in the Southern Hemisphere surpasses that in the Northern Hemisphere. Density maxima occurred from high to low latitudes, accompanied by a 2-3 h time lag. The Wygant function was applied to model the response to solar wind geomagnetic field changes and quantify the impact of geomagnetic activities on upper atmosphere density, verifying the time lag of density disturbances. All these findings could potentially improve our understanding of the solar cycle and LEO orbital drag. (c) 2024 COSPAR. Published by Elsevier B.V. All rights reserved.

Objectives: Using global navigation satellite system（GNSS） reflectometry（GNSS-R） signal to do remote sensing research has become a hotspot in recent years. We has simulated Fengyun 3 E the Ⅱgeneration of GNSS occultation sounder（GNOSⅡ） GNSS-R parameters, which includes the average number of specular points,the maximum number of specular points,the average distance of specular point（SP）-GNSS,the average distance of SP-low Earth orbit（LEO）,the average path lost,the average incident angle on LEO and the average reflected angle on SP.Methods:The relationships between these simulated parameters and antenna parameters（antenna angle, beam width, and inclination direction） are analyzed, and the corresponding results are demonstrated using snow flake method.Results: Through the statistical analysis results, It can be easily learned the conclusion that the beam width has the maximum influence on all of these antenna observations, the antenna angle is the second, and the inclination direction has the minimum influence.Conclusions:The snow flake method mentioned is able to provide a new method for data analysis of GNSS-R especially space-borned ones, and may help to clear some obstacles which obstruct the successful development of corresponding payloads.

The Global Navigation Satellite System Occultation Sounder II (GNOS-II) payload onboard the Chinese Fengyun-3E (FY-3E) satellite is the world’s first operational spaceborne mission that can utilize reflected signals from multiple navigation systems for Earth remote sensing. The satellite was launched into an 836-km early-morning polar orbit on 5 July 2021. Different GNSS signals show different characteristics in the observations and thus require different calibration methods. With an average data latency of less than 3 h, many near real-time applications are possible. This article first introduces the FY-3E/GNOS-II mission and instrument design, then describes the extensive calibration methods for the multi-GNSS measurements, and finally presents application results in the remote sensing of ocean surface winds, land soil moisture and sea ice extent. Especially, the ocean surface wind product has been used in operational applications such as assimilation in the numerical weather prediction model and monitoring of tropical cyclones. Currently, GNOS-II has been carried by FY-3E, FY-3F (launched in August 2023) and FY-3G (launched in April 2023). It will be also carried by future follow-on FY series and a more complete multi-GNSS reflectometry constellation will be established.

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 1.486 m/s and can reach 1.399 m/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.

High-resolution thermospheric mass density (TMD) measurements from Low Earth Orbit (LEO) Satellites are valuable to accurately estimate the short-term atmosphere abrupt disturbances, triggered by magnetospheric forcing. A good characterization of TMD variation ahead of the arrival geomagnetic storms can benefit LEO operations and crucial for both orbit propagation and collision avoidance. In this contribution, we will reveal the most probable feature of TMD variation during the initial stage of solar cycle 25, at the same time, we proved Wygant function as a better geomagnetic events indicator.In this study, GRACE-FO 10s accelerometer-derived TMD measurements were employed and normalized at altitude of 505km by the NRLMSISE-00 (Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere 2000) empirical atmosphere model to investigate the status of solar cycle 25 between September 1 and December 31, 2020. With the high-inclination orbit global coverage, three magnetic latitude regions were separated and divided into day and nighttime using magnetic local times (MLT). 4-month enhancing disturbances observations suggest solar activities will shift from its relatively quiet condition to a much more active behavior, which reveal unexpected dependencies on the temporal and spatial characteries. Our detailed analysis shows that (1) TMD spreads from high latitudes to low latitudes and as same as time lag, (2) TMD enhancement in the Southern hemisphere is more intense than in the Northern one, reaching peak value around 15:00 MLT; geomagnetic activities cause TMD to increase up to 0.86×10-13 kg/m3 at night side, 3.4×10-13 kg/m3 at day side, and (3) the TMD enhancement was symmetric in both N- and S- hemispheres before the equinox. In general, thermospheric mass density analysis reveals the significant impact of solar and geomagnetic activities, providing the most relevant and probable characteristic of the TMD disturbances driven by solar wind.Additionally, we try to use different geomagnetic indices for a complete description of geomagnetic storms and their phases. The S10.7 index is used as a proxy for solar irradiation. These indicators show high correlation with the TMD variation during recurrent geomagnetic activities. What’s more, the cross-correlation analysis reflects a high correlation of to the Wygant function EWAV found both at three latitude bins.Even thought our study is considered a minor to moderate geomagnetic storm of the upcoming solar cycle 25 maximum, the high-speed stream injection into the thermosphere still caused thermosphere expansion that significantly enhanced the neutral density in the LEO environment. Therefore, all these findings provide a possibility to improve our understanding of LEO orbital drag.

The FengYun-3E Global Navigation Satellite System (GNSS) occultation sounder II (FY-3E GNOS II) was launched on 5 July 2021. For the first time, based on the new GNOS II sensor, this mission realizes radio occultation (RO) and reflectometry observations using the navigation signals from the third-generation BeiDou System (BDS-3), and it is hence important to assess and analyze the BDS-3 remote sensing performances relative to other systems. Here, we assessed FY-3E GNOS II RO atmospheric retrievals by inter-comparing with corresponding data from the NCEP FNL global atmospheric analysis and FY-3D GNOS mission. The GNOS RO data quality and consistency of the different FY-3 meteorological satellites, i.e., FY-3D and FY-3E, as well as different GNSS systems (GPS, BDS-2, BDS-3) were analyzed. We find that the FY-3E GNOS II RO data exhibit better quality than FY-3D GNOS, particularly in the number, penetration height toward surface, and global coverage by BDS RO profiles, due to the integration of BDS-2 and BDS-3. Additionally, comparing with co-located NCEP FNL analysis profiles, the mean difference (and standard deviation) of the FY-3E GNOS II RO atmospheric refractivity profile retrievals is found to be smaller than 0.2% (and 1%), in the upper troposphere and lower stratosphere, from 5 to 30 km, and remains consistent at this accuracy and precision level with the FY-3D GNOS RO data. These features provide clear evidence for a high utility of the new GNOS II RO data for weather and climate research and applications.

全球定位系统(global positioning system,GPS)卫星的ⅡR和ⅡF卫星能够在各个信号分量之间重新分配其发送信号的功率,一个或多个GPS信号可以在指定区域根据需要进行功率调整或者关闭.分析GPS信号的变化特征对于地面和空间应用有重要的意义.风云三号D(FengYun-3D,FY-3D)卫星是中国极轨气象卫星之一,利用FY-3D卫星实际测量数据可以帮助GPS用户全面了解GPS功率调整的特点.首先,利用FY-3D运行轨道全球覆盖的特点分析GPS信号的强度,特别是GPS信号功率调整时间段信号变化的特点;然后,使用在轨数据研究了全球范围L波段信号干扰的特征,得到了干扰对全球导航卫星系统掩星探测仪掩星天线的自动增益控制和基底噪声的影响.结果表明:从2020-02-14开始的GPS功率调整以[35°N,37°E]和[35°N,69°E]为中心,覆盖半径约为7 500 km,在该区域内GPS P(Y)码功率增加约10 dB;GPS L1和L2频段在中东地区持续受干扰的影响,该区域的基底噪声比其他区域增加约3～10倍;干扰区域中心点和GPS功率调整区域中心点大致在同一位置.GPS卫星的功率调整和信号干扰对GPS用户,特别是低轨卫星的定位有明显的影响,在GNSS接收机设计时应引起重视.