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.

To systematically evaluate the accuracy performance of the next-generation VLBI global observing system (VGOS) in determining UT1-UTC, we process synchronous observation data from the 2024 VGOS-INT-A and traditional S/X-band IVS-INT-1 sessions using identical strategies, with the international IERS C04 series serving as the reference for accuracy assessment. The results indicate that, under identical station counts and geometric configurations, the number of observations per session for VGOS-INT-A (approximately 65) is significantly greater than that for IVS-INT-1 (approximately 30). The weighted root mean square (WRMS) of the post-fit delay residuals for both systems is comparable. Regarding UT1-UTC solution accuracy, VGOS-INT-A demonstrates a clear advantage over IVS-INT-1, the RMS of its deviation with the C04 series is ±0.046 ms, superior to the ±0.060 ms of the latter. Its mean formal error is 14.26 μs, while that of IVS-INT-1 is 30.38 μs. Furthermore, preliminary analysis of data from China's VGOS stations shows good formal error (22.11 μs) and high stability in the solutions, although external agreement accuracy still lags behind international levels. This research provides quantitative evidence for the precision efficacy of VGOS technology in practical applications and highlights its potential for high-frequency Earth orientation parameter monitoring.

A high-precision seismic dataset was constructed using deep learning methods, and a regional three-dimensional velocity model was obtained through double-difference tomography. The study reveals a correlation between surface velocity structures and geological features: mountainous areas generally exhibit high-velocity P-wave anomalies, whereas depression zones are characterized by low-velocity P-wave anomalies. This pattern may be attributed to the widespread presence of bedrock with relatively high wave velocity in folded regions, in contrast to the Quaternary sedimentary layers with lower velocity in depression areas. Seismic epicenters are densely distributed along the transition zones between high- and low-velocity anomalies. These transitional regions experience significant stress gradients, which facilitate rock fracturing and lead to heightened seismic activity.The Luxi uplift is characterized by several near-vertical high- and low-velocity blocks, with some earthquake epicenters also displaying steeply dipping distributions, suggesting the presence of high-angle faults. This structural configuration may result from multiple phases of compression and extension under the influence of Pacific Plate subduction and the tectonic activity of the Tan-Lu fault zone. These processes have led to cyclic uplift and subsidence in the region, during which progressive compression and extension of rock folds gradually increased their dip angles, forming near-vertical velocity blocks and high-angle faults.Integrated analysis of P- and S-wave velocities suggests the presence of mantle-derived basaltic magmatic intrusions beneath both the Nishan area and the northeastern region of the Lushan. In the Nishan area, the intrusions are predominantly plutonic, whereas in northeastern Lushan, mantle-derived basaltic magma likely reached the surface, forming ancient volcanoes and extrusive rocks.

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.

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.