Based on the methods of low Earth orbit (LEO) satellite orbit determination and inter-satellite single-difference ambiguity resolution, this paper utilizes uncalibrated phase delay (UPD) and observable-specific signal bias (OSB) products to achieve precise orbit determination for eight LEO satellites, namely GRACE-C/D, SWARM-A/B/C, and SENTINEL-3A/3B/6A, and evaluates the impact of these two products on orbit determination accuracy. The results indicate that both products achieve a nearly 100% fixed rate for wide-lane ambiguities; except for the GRACE satellites, which have a narrow-lane fixed rate of approximately 85%, the remaining satellites achieve rates close to 95%. Ambiguity resolution significantly enhances orbit determination accuracy, with the fixed solutions obtained using UPD and OSB products showing comparable three-dimensional accuracy, reaching 1.5 cm, 1.7 cm, and 0.9 cm for the GRACE-FO, SWARM, and SENTINEL series satellites, respectively, representing improvements of up to 50% compared to the float solutions. Additionally, a multi-system analysis of the SENTINEL-6A satellite reveals that the fixed solution accuracy for the combined GPS/Galileo orbit determination is 0.9 cm, representing improvements of 10% and 18% compared to single-GPS and single-Galileo systems, respectively. This study confirms that ambiguity resolution can effectively improve the orbit determination accuracy of LEO satellites and provides a reference for future research on orbit determination for large-scale LEO constellations.

By employing fundamental methods from the theory of curves and surfaces, the concepts of different types of latitude and longitude (reduced latitude and longitude, geometric latitude and longitude, and geodetic latitude and longitude) at an arbitrary point on a triaxial ellipsoidal surface are proposed. Theoretical relationship models are established between Cartesian coordinates (spatial rectangular coordinates) and geodetic coordinates (geodetic latitude, longitude, and height), as well as between Cartesian coordinates and geometric coordinates (geometric latitude, longitude, and height), for any point in space both inside and outside the ellipsoid. Based on the theoretical relationships among the different latitude and longitude types, approximate geodetic coordinates of the study point are derived. Using these approximations as initial values, a novel method for the inverse transformation of coordinates is presented via Newton's iterative approach. Extensive numerical calculations demonstrate that, for any arbitrary point in space, the proposed method accomplishes the transformation from Cartesian to geodetic coordinates nearly instantaneously; even for points near the Earth's surface, only three iterations are required to achieve convergence.

The synchronization accuracy of the pseudo-range single-point positioning method is low, making it unable to meet the high-precision time synchronization requirements of low Earth orbit (LEO) satellites. Meanwhile, the inter-satellite link time synchronization method faces challenges in large-scale LEO constellation applications due to factors such as complex payloads, high equipment costs, and susceptibility to interference from the space environment.This study utilizes onboard GPS observation data from the GRACE-FO satellites to design and investigate an inter-satellite time synchronization method for low Earth orbit satellites based on precise point positioning (PPP). Experimental results show that the orbit determination accuracy (RMS) of the GRACE-FO satellites in all directions is approximately 7 cm, and the GNSS timing accuracy (STD) of the two satellites is 0.78 ns and 0.77 ns, respectively, with short-term stability (1 280 s) reaching 2.22×10^-13 and 2.13×10^-13, and long-term stability (10 240 s) reaching 2.69×10^-14 and 3.21×10^-14. Meanwhile, the inter-satellite time synchronization accuracy (STD) is 0.46 ns, with short-term stability (1 280 s) of 2.24×10^-13 and long-term stability (10 240 s) of 3.47×10^-14. These results validate the feasibility of the proposed algorithm and provide an effective approach for high-precision inter-satellite time synchronization in LEO satellite systems.

Aiming at the limitation of real/near-real-time GNSS PWV retrieval under missing meteorological parameters, three PWV estimation models without the need for measured meteorological parameters were established in the Guangxi region based on the XGBoost model. First, the XGBZ-PWV model was developed with inputs including station time (DOY and HOD), location (Longitude, Latitude, and Height), and GNSS ZTD, and the output feature being GNSS PWV. Then, based on the XGBZ-PWV model, two empirical PWV values were incorporated to establish the XGBZG-PWV and XGBZE-PWV models, respectively. For comparison, the GPT3 model was used to provide pressure and temperature for PWV retrieval based on GNSS ZTD (GPT3-PWV model). The accuracy of the established models was validated using GNSS PWV retrieved from GNSS ZTD, ERA5 surface pressure, and temperature in the Guangxi region in 2022 as the reference value. The results show that, compared to the GPT3-PWV model, the estimation accuracy of the XGBZ-PWV, XGBZG-PWV, and XGBZE-PWV models improved by 22.98%, 29.03%, and 31.45%, respectively, with the XGBZE-PWV model performing the best. During two extreme rainfall events in 2022, the spatiotemporal evolution characteristics of PWV and rainfall were analyzed. The results demonstrate that the XGBZE-PWV model maintains good applicability even under extreme weather conditions.

To address the issues of reduced accuracy and misidentification of single water bodies caused by interpolation in large-scale flood inundation mapping using spaceborne GNSS-R technology, this paper proposes a dynamic monitoring method based on the reflectivity change rate using period-matched observation-based interpolation (POBI). Utilizing CYGNSS reflectivity data during the rainy seasons from 2019 to 2023 and China's 1 km resolution monthly precipitation products, we constructed a POBI spatial interpolation model. By combining this with the bistatic radar equation to retrieve surface reflectivity, we systematically analyzed the flood evolution process during the extreme rainfall event in Guilin in June 2024. The results show that, for reflectivity interpolation in unsampled areas, the root mean square error (RMSE) decreased by an average of 20.98% compared to the natural neighbor method. The inundation identification mechanism based on the change rate effectively avoids the subjectivity of traditional threshold methods. The monitoring results indicate that the inundated area in the central urban district expanded by 48.68% from June 19 to June 12, with spatial evolution highly consistent with changes in SMAP soil moisture. This study provides reliable methodological support for refined flood monitoring in highly dynamic, low-coverage areas using spaceborne GNSS-R.

Landslide disasters pose a continuous threat to the safety of life and property of mountain residents. Especially under heavy rainfall, landslide areas are often accompanied by severe surface deformation, resulting in serious incoherence of synthetic aperture radar interference data, which restricts the deformation monitoring capability of traditional timing InSAR technology in this type of area. To address this problem, this paper takes the large landslide that occurred in Nanyu township, Zhouqu county, Gansu province in July 2018 as the research object, and prensents the distributed scatterer InSAR (DS-InSAR) deformation monitoring method based on statistical homogeneity detection and time phase link optimization. This method introduces a statistical homogeneity determination model under the traditional DS-InSAR framework, and identifies clusters of pixels with stable scattering characteristics through non-parametric testing, thereby expanding the monitoring coverage of low-coherence areas. At the same time, the shortest path constraint strategy is used to optimize the interferogram network structure to improve connection density and temporal coherence. It is also combined with a time-Phase Linking joint estimation algorithm to optimize the interference phase as a whole, enhancing the temporal continuity and stability of deformation information. The research results show that this method significantly improves the distribution density and temporal coherence of monitoring points in the collapse area of Nanyu landslide. The number of effective coherent points in creased from 23 232 to 43 463, an increase of about 87%, and the average posterior coherence coefficient was improved by 0.3, which effectively revealing the dynamic evolution characteristics of the landslide main body. The optimized annual average deformation rate chart shows that the maximum sliding rate of the main deformation area of the landslide exceeds 70 mm/a, and the timing curve of the characteristic point is highly consistent with the rainfall process. This study verifies the applicability and effectiveness of the timing phase optimization method in high-voltage landslide areas, and provides strong technical support for InSAR monitoring of landslides in complex mountainous areas.

Based on over 20 years of MODIS satellite remote sensing infrared data, we extract brightness temperature low-frequency information using anomaly method and spatial anomaly superposition method, and investigate the temporal and spatial evolution and characteristics of infrared radiation anomalies before the Wenchuan M_S8.0 earthquake on May 12, 2008. The results show that there was a significant radiation enhancement anomaly before the earthquake. In terms of time, from 2006 to 2007, there was a trend of radiation enhancement in the core area located west of the epicenter. Conversely, from 2008 until the earthquake occurrence, this radiation enhancement exhibited a weakening trend. Spatially, the core area of radiation enhancement is located in the eastern section of Bayan Har and Qiangtang blocks, encompassing approximately 5.8×10⁵ km². The spatial distribution characteristics are consistent with the dynamic background of Wenchuan earthquake.

This study proposed a novel monitoring scheme that integrates GNSS interferometric reflectometry (GNSS-IR) with GNSS positioning for monitoring coastal absolute sea level changes. Using over 10 years of observational data from seven coastal stations in Hong Kong as an example, research on absolute sea level change monitoring in nearshore areas was conducted. The results indicated that after excluding the stations HKSL and KYC1, which had lower data quality, the GNSS-IR-derived relative sea level changes from the remaining stations showed good agreement with tide gauge data in their monthly averages. For most stations, the RMSE was less than 6 cm, the correlation coefficient was greater than 0.86, and the difference in estimated sea level rise trends was less than 1 mm/a compared to tide gauges (with a regional average difference of only 0.036 mm/a). In absolute sea level monitoring, after applying the dynamic atmospheric correction (DAC), the difference in regional average rates between coastal GNSS and satellite altimetry at corresponding points was reduced from -3.40 mm/a to -0.76 mm/a, indicating highly consistent trends. Compared with 50-year long-term absolute sea level data from Hong Kong tide gauges, the deviations were mostly less than 1 mm/a, and all stations fell within reasonable error margins for regional absolute sea level monitoring. The research demonstrates that the fusion of GNSS-IR and GNSS positioning can effectively address traditional monitoring gaps, providing a scalable new technical approach for monitoring coastal sea level changes and conducting risk assessments.

Based on the observation data of VP-type broadband vertical pendulum inclinometers from 5 stations in Inner Mongolia, this study adopted a method combining the wave propagation theoretical model and the deep learning prediction framework to preliminarily reveal the multi-frequency quantitative influence mechanism of the overburden thickness of observation caves on the observation signals. The results show that the overburden thickness is significantly negatively correlated with background noise (especially in the north-south direction), but has a weak impact on low-frequency signals. It exerts a directional regulatory effect on the amplitude of M₂ tidal waves: the amplitude of the north-south component increases linearly with the increase of thickness, while the east-west component shows a unique resonance amplification effect at the thickness of 20 m (the peak value is about 3.6 times higher than that at 10 m). When the thickness is greater than or equal to 20 m, it can effectively suppress the seasonal fluctuation of data, significantly reduce the variation amplitude, delay the peak value by 1-2 seasons, and simultaneously decrease the barometric admittance coefficient remarkably. Verified by the random forest model, it is found that the optimal thickness range of the cave overburden is 18-22 m (with 20 m as the optimal value). Lithology is a secondary influencing factor, while the overburden thickness and its interaction with the density of mountain lithologic media constitute the dominant control parameters. In this study, 20 m of overburden thickness was selected as the engineering threshold, which provides an important design basis for effectively suppressing background noise, ensuring the fidelity of tidal signals and resisting seasonal interference, and gradually promotes the construction of cave-based observation at stations from an empirical mode to a quantitative and engineering-oriented direction.

Under an elastic Earth model, clear analytical solutions exist for surface gravity changes induced by a single strike-slip fault. However, the quantitative relationship between multiple fault zones and their geometric parameters with the resulting surface gravity change patterns remains unclear. Based on dislocation theory in an elastic half-space, this paper systematically investigates the quantitative connection between the geometric parameters of parallel strike-slip dislocation sources and the characteristics of the surface gravity change patterns through numerical simulations. Using the control variable method, we separately analyze the influence of fault length (L), width (W), slip amount (K), number of faults (N), dip angle (δ), and top depth (dep) on the four-quadrant distribution pattern and amplitude of the gravity anomaly. The results show that: 1) Fault length L is linearly positively correlated with the distance between gravity extremal points along the fault strike (X-direction). 2) Increasing fault width W elevates the gravity anomaly amplitude, but the growth trend diminishes, without altering the four-quadrant structure. 3) The strike-slip width K is linearly positively correlated with the distance between gravity extremal points along the Y-direction, but negatively correlated with the gravity anomaly amplitude. 4) An increase in the number of strike-slip faults N leads to a linear increase in the gravity change amplitude. 5) Variations in the dip angle δ affect the symmetry of the gravity change pattern. 6) An increase in the top depth dep causes a significant increase in the distance between extremal points in the Y-direction. 7) The fault center position (s_x, s_y) coincides with the four-quadrant symmetric center of the gravity change pattern, and the line connecting a pair of extremal points in the X-direction aligns with the fault strike angle α. In summary, the main geometric parameters of multiple dislocation sources can be determined based on observed gravity change patterns.