ALBERT

Description: Earthquakes are generally known to alter the stress field near seismogenic faults. Observations using YRY-four-gauge borehole strainmeters within Yushu (YSH) borehole near the Ganzi-Yushu fault in eastern Tibetan Plateau shows that the azimuth variation of maximum horizontal stress (SH) first decreased and then increased substantially when the earthquakes occurred during the measurement period from January 1, 2009 to December 31, 2018. In this period, 38 earthquakes (M ≥ 3) were detected near the fault and the SH orientation showed a drastic change after the 2010 Ms 7.3 Yushu mainshock. We present a discrete element modelling using Particle Flow Code 2D (PFC2D) to simulate a dynamic fault rupturing process and to use the modelling results for interpretation of the stress reorientation. The modelling reveals that dilatation and compression quadrants are formed around a fault rupturing in strike-slip model, resulting in different spatiotemporal changes of the orientation of maximum horizontal stress (Δθ). The value of Δθ in the compression quadrants shows a sharp drop at the time of coseismic slip, then approaches slowly to an asymptotic value. In the dilatation quadrants, Δθ drops by coseismic slip, then increases sharply and finally reaches a stable value. The modelled Δθ by coseismic fault slip agrees with in-situ observations at YSH borehole during 2010 Ms 7.3 Yushu mainshock. It is also found that, the value of Δθ decreases with increasing distance from the rupturing source. We modelled the effect of fault geometry and host rock properties on the Δθ, and found that structural complexity and off-fault damage by coseismic fault slip have significant impact on the stress field alteration near the rupturing source.

Description: The determination of the ocean mass change associated with terrestrial water and land ice changes (referred to as LMIOM) is of great significance for understanding the driving factors of regional sea level change. GRACE/GRACE Follow-On (GFO) satellite gravity data can be used to assess the spatial and temporal distribution of land components and determine LMIOM change. However, the limitation of signal leakage error leads to the challenge of accurately estimating the LMIOM change. Among the various signal recovery methods, Forward Modeling (FM) have proven to be more suitable for global mass change recovery. But the two commonly used constraint models (uniform thin-layer, UTL; sea level fingerprint, SLF) in FM have some problems, respectively: (1) The UTL model does not consider the spatial distribution characteristics of LMIOM change in detail. (2) The SLF model constructs the LMIOM change through sea level equation but ignores the influence of the unmodeled ocean dynamics signal. The effects of the above problems on the recovered global mean mass change rates are about 0.13 mm/year and 0.14 mm/year, respectively, and they will further affect the determination of LMIOM change. Therefore, an FM method with constraints of combining the output from multiple ocean models is proposed, which can largely avoid the problems existing in UTL and SLF constrained models. The results of the comparative analysis show that the joint constrained FM method outperforms the results of the UTL and SLF constrained models in the mesoscale open oceans such as the South Atlantic and the South Indian Ocean.

Description: The orbit maneuver detection is crucial in Global Navigation Satellite System (GNSS) precise orbit determination, which is necessary for adjusting data processing strategies. The frequency of orbit maneuvers for the BeiDou Navigation System is significantly higher than that of other navigation systems, especially for geosynchronous orbit (GEO) and inclined geosynchronous orbit (IGSO) satellites. We propose a novel real-time and postprocessing method for detecting orbit maneuvers for BeiDou satellites based on the orbit differences between the epoch-updated orbit estimated using square root information (SRIF) and the predicted orbit according to the precise orbit estimated during non-maneuver period, as well as the orbital state difference during maneuver and non-maneuver periods. This method has significant advantages over using observation residuals and it is not affected by observation outliers, thus improving the accuracy and timeliness of orbit maneuver detection. We demonstrated that 32 orbit maneuver events of BeiDou satellites were successfully detected in 2022, of which 1 was for medium Earth orbit (MEO), 7 were for IGSO with an average detected maneuvering time of 7–8 min, and 24 were for GEO satellites with an average detected time of 4–5 min. Moreover, our method can be easily integrated into current real-time filter-based precise orbit determination (POD) processing without any extra task line, which simplifies the overall data processing. The data used in this method can be accessed easily, including GNSS observation data, broadcast ephemeris, and other open-source information files.

Description: A set of Global Navigation Satellite Systems (GNSS) satellite orbit and clock offset are an essential prerequisite for precise application. However, abrupt changes in accuracy at the boundaries are prevalent in products provided by international GNSS services, resulting in decreased orbit interpolation precision near the daily boundary. In addition, the effect of this phenomenon is reflected in the deterioration of accuracy and the fluctuations in subsequent applications. In this study, time-weighted and equal-weighted calibrated methods were utilized for adjacent Global Positioning System (GPS) satellite orbits and the orbit variations were then corrected for the clock offset to ensure their consistency. The calibration method is evaluated based on the accuracy and smoothness of post-processing kinematic precise point positioning (PPP) and low earth orbit (LEO) precise orbit determination (POD) near the day boundary. In a variety of scientific applications, the results indicate that the proposed calibration method can effectively reduce the excessive differences near the day boundary between adjacent days. Near the boundary, maximum improvements for post-processing kinematic PPP, dynamic LEO precision orbit, kinematic LEO precision orbit are 41.5%, 9.4%, and 20.5%, respectively.

Description: Precise orbit products are fundamental for real-time Global Navigation Satellite Systems (GNSS) applications. Currently, the ultra-rapid orbits are provided via batch processing and predicted over a certain period at most of the IGS Analysis Centers. As the orbit accuracy degrades along with the prediction time, especially for satellites in eclipse, reducing orbit update latency becomes increasingly important but is very challenging due to the multi-GNSS constellation with about 130 satellites. In this study, a new ultra-rapid obit processing strategy is proposed to realize orbit update every hour or even half-hour for integrated processing of all GNSS and QZSS satellites. By utilizing information of previously processed sessions, only the most recent (sub-)hourly observations are processed with undifferenced ambiguity resolution. Through a rigorous combination approach of the historic and the recent (sub-)hourly information equivalent solutions to that of the 24/48-hour session are achieved. With the proposed strategy, a latency of 30-min is available and ensures that the accuracy of the real-time available orbits is improved to 3.5 cm, 10.2 cm, 3.9 cm and 7.2 cm for GPS, GLONASS, Galileo and BDS, respectively, which is around 6.5%-22% better than the 2-hour updated orbits.

Description: Real-time Global Navigation Satellite Systems (GNSS) are critical for precise and reliable positioning, navigation, and timing (PNT) services, including both civil applications with an accuracy of meter-level and geohazard early warning systems with an accuracy of cm- or even mm-level. For high-precision real-time GNSS applications, the precise orbits, clocks, hardware delays, and atmospheric information are estimated and disseminated in real time by the service provider. In this study, we introduce the current status and recent developments of the GFZ GNSS real-time analysis center. We introduce the data processing strategy for generating satellite orbits and clocks, hardware delays, and atmospheric information, and present the accuracy of the online operational products. We focus on the recent developments in improving the accuracy and reducing the latency of real-time products, such as shortening the ultra-rapid orbit processing time and improving the predicting accuracy, estimating orbits with real-time streaming data, and adopting the un-differenced ambiguity resolution in the estimation of real-time clocks and orbits. Lastly, we evaluate the real-time positioning performance using the GFZ real-time products, and demonstrate with the atmospheric augmentation information, the cm-level accuracy can be achieved within a few minutes.

Description: The incorporation of multi-frequency signals into global navigation satellite systems (GNSS) has presented new possibilities for precise positioning and rapid ambiguity resolution. Inter-frequency clock bias (IFCB) pertains to the time-varying biases among distinct frequencies within carrier phase observations in GNSS signals. The appropriate handling of IFCB is critical in enhancing the accuracy and convergence time of precise point positioning (PPP) solutions. The focus of this study is on the proper modeling of phase IFCB in multi-GNSS multi-frequency PPP. In this paper, the optimal IFCB power spectral density value of 0.6 m/sqrt(s) is first determined. To obtain the optimal stochastic model for IFCB, a thorough comparison and analysis of the product correction and parameter estimation methods is conducted. Additionally, experiments are conducted on the effect of IFCB modeling on the performance of undifferenced and uncombined PPP using data from 130 multi-GNSS experiment stations across the globe over a period of one week in January 2022. The study reveals that the optimal power spectral density for IFCB is within [60, 0.006] m/sqrt(s), modeling IFCB as a random walk is feasible, and the PPP is comparable for the three IFCB processing schemes of product correction, random walk, and white noise. Meanwhile, it is not reasonable to treat IFCB as a random constant or neglect it in the multi-GNSS multi-frequency PPP. In the absence of product correction or for users who require immediate and continuous positioning solutions, modeling IFCBs as random walks can lead to more reliable positioning results and improved convergence performance.

Description: High-precision Global Navigation Satellite Systems (GNSS) orbits are critical for real-time clock estimation and precise positioning service; however, the prediction error grows gradually with the increasing prediction session. In this study, we present a new efficient precise orbit determination (POD) strategy referred to as the epoch-parallel processing to reduce the orbit update latency, in which a 24-h processing job is split into several sub-sessions that are processed in parallel and then stacked to solve and recover parameters subsequently. With a delicate handling of parameters crossing different sub-sessions, such as ambiguities, the method is rigorously equivalent to the one-session batch solution, but is much more efficient, halving the time-consuming roughly. Together with paralleling other procedures such as orbit integration and using open multi-processing (openMP), the multi-GNSS POD of 120 satellites using 90 stations can be fulfilled within 30 min. The lower update latency enables users to access orbits closer to the estimation part, that is, 30–60-min prediction with a 30-min update latency, which significantly improves the orbit quality. Compared to the hourly updated orbit, the averaged 1D RMS values of predicted orbit in terms of overlap for GPS, GLONASS, Galileo, and BDS MEO are improved by 39%, 35%, 41%, and 37%, respectively, and that of BDS GEO and IGSO satellites is improved by 47%. We also demonstrate that the boundary discontinuities of half-hourly orbit are within 2 cm for the GPS, GLONASS, and Galileo satellites, and for BDS the values are 2.6, 15.5, and 9.8 cm for MEO, GEO, and IGSO satellites, respectively. This method can also be implemented for any batch-based GNSS processing to improve the efficiency.