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Very Long Baseline Interferometry (VLBI) is the only technique that connects the celestial and the terrestrial reference frames. Because of its essential role, it is important to investigate the quality of VLBI observations and their effect on the estimated parameters. The fundamental goal of the quality assessment using the sensitivity is to find out to what level VLBI can detect station displacements in the adjusted coordinates. However, the global VLBI station network is worse in terms of the number and global distribution of stations when compared to the GNSS station network. In this study, we aim to improve the capacity of VLBI to detect station displacements through an inter-technique combination with GNSS. The effect of the selection of co-location sites in the inter-technique combination is investigated with four scenarios and we investigate how the sensitivity is affected by the assumed standard deviations of the local ties. The lower the standard deviations of the local ties, the higher the obtained sensitivity level. When larger standard deviations are assumed, the sensitivity levels of the VLBI stations are obtained up to 3 mm. When a smaller deviation is assumed for a local tie having various standard deviations obtained from different local tie observations, the sensitivity levels change too. If local ties with standard deviations of 1 cm and more are excluded, the detection capacities of the VLBI stations based on the gross errors improve by around 1 mm. When smaller standard deviations are assumed among the local ties, which are used in the inter-technique combination, the sensitivity levels drop below 2 mm. Hence, this study quantifies how important the selection of the co-location sites and the standard deviations of the local ties are for the accuracy of geodetic products obtained by inter-technique combination.

The combined satellite clocks are one of the products provided by The International GNSS Service (IGS). These products are the result of a combination process that takes as input the individual solutions computed by the Analysis Centers (ACs). So far, only GPS and GLONASS combined products are distributed by the IGS. Since new constellations have been declared operational, such as Galileo in 2016 and BeiDou in 2018, a Multi-Constellation combination has been on focus. Therefore, we propose a method for a Multi-GNSS clock combination based on a least-squares framework, where the weights are determined by least-squares variance component estimation. The necessary alignments implemented in order to harmonize the solutions from the ACs will be demonstrated, such as radial correction between orbits and clocks, alignment of AC clocks to a reference AC through estimation of trends and offsets, and the inter-system bias correction between different systems. In addition, the outlier detection will be shown, which facilitates a reliable definition of a reference AC and consistent AC solutions. The results show that Galileo and GPS clocks have, on average, the best agreement between ACs and the combination, while GLONASS and QZSS have large variations. A validation of the combination is performed using the combined products as input in a Precise Point Positioning (PPP) processing, where the average residuals are below 10 mm, agreeing with the performance of the individual ACs., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

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., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Introducing precise atmosphere information into precise point positioning enables rapid ambiguity resolution and introduces a significant accuracy improvement. However, it can only be implemented in regions with dense networks and stable communication links. For larger areas, e.g., an intercontinental level, there is a conflict between the accuracy of corrections and the amount of atmosphere information to be disseminated. We develop a hierarchical augmentation mode to combine the advantages of the fitting model and region interpolation model to relieve the communication burden. Relying on the fitting model with fewer coefficients applied over large areas as the essential information, the unmodeled errors are calculated at each reference station, and further correction information is optional compensation depending on the magnitude of the unmodeled residuals. We perform the proposed models on 103 EUREF Permanent Network stations with 200-km station spacing and 84 stations as the external validation. The ionosphere and troposphere fitting models have an average accuracy of about 4.2 and 1.3 cm, respectively, under meteorologically calm conditions. The unmodeled error transmission determined by the magnitude of residuals can be reduced by 61% and 96% for the ionospheric and tropospheric delays, respectively, with respect to the legacy interpolation mode. Further compensation implemented, i.e., unmodeled residuals, can achieve instantaneous convergence for 83.6% of all solutions, and the overall initialization time is within 1.0 min. Thus, the proposed hierarchical positioning mode satisfies real-time positioning convergence requirements and significantly reduces massive corrections in communication.

The GGOS Bureau of Products and Standards (BPS) supports the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (GGOS) in its goal to provide highly-accurate, consistent and long-term stable geodetic products needed to monitor, map, and understand changes in the Earth’s shape, rotation and gravity field. A key objective of the BPS is to keep track and to foster homogenization of adopted geodetic standards and conventions across all IAG components for the generation of geodetic products. This includes the interaction with IAG and other entities that deal with standards and conventions, such as the IERS Conventions Center, the IAU Commission A3 ”Fundamental Standards”, ISO/TC 211 and the Working Group ”Data Sharing and Development of Geodetic Standards” of the UN-GGIM Subcommittee on Geodesy. This contribution presents the role of the BPS, and it highlights some of the recent activities, which are focused on updating of the BPS inventory of standards and conventions used for the generation of IAG products, the revision of the IERS Conventions, mainly related to Chapter 1 ”General definitions and numerical standards” and the compilation of user-friendly product descriptions published at the GGOS website (www.ggos.org). The BPS also contributes to the generation of short GGOS videos to make other disciplines and society aware of geodesy and its beneficial products., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Throughout the earthquake cycle at fault zones, Earth's crust undergoes deformations. GNSS coordinate time series reveals tectonic motion, seismic displacements, and periodic signatures. Additional motion patterns are usually summarized as transient tectonic signals, i.e., unexpected accelerations with respect to standard trajectory models. As the number of permanent stations increases and as time series grow, we are increasingly able to recognize transient tectonic signals. Since some of these suspected tectonic transients have subtle magnitudes or sometimes unusual spatiotemporal features, we need to develop methods for determining which transients are artifacts and which are of a tectonic origin.Here, we investigate the impact of certain GNSS processing choices and how they affect the appearance of transients in the GNSS displacement time series solutions. In our study, we choose data from Cascadia, a region for which the occurrence of transients in the GNSS time series is well known. To control our database, we processed data based on network solutions from 150 carefully selected GNSS stations for the time between 2015 and 2020. Then, we have an opportunity to discriminate between tectonic transients and processing artifacts in the GNSS displacement time series. We will discuss the detected transients against selected GNSS-related indicators (data quality, observation residuals, troposphere delays, etc.) to identify potential GNSS artifacts aliasing into the transient signals., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Project “SaptaRishi” was initiated by the National Centre for Geodesy (NCG), IIT Kanpur to set up the modern space geodetic infrastructure in the country. This project primarily focuses on the establishment of the Indian Geodetic VLBI network, and is currently with the national funding agency and has not yet been approved. Thus, the purpose of this work is to demonstrate the benefits of the project plan to the national and international scientific communities. The project proposes to establish three VLBI stations along with a correlator at one facility. In this work, we looked at how adding proposed VLBI antennas to India affected the global terrestrial reference frame by showing Helmert transformation parameters, u-v coverage of radio sources, and the ability to track NAVIC (Navigation with Indian Constellation) satellites. Three 24-hour sessions, IVS-R1, IVS-VGOS, and IVS-AOV, were studied to examine the change in Helmert parameters between the terrestrial reference frames caused by the addition of the proposed VLBI antennas. Furthermore, the u-v coverage of some radio sources from the southern hemisphere in observing networks with and without the proposed antennas was compared. The proposed VLBI stations were also scheduled and simulated to observe the NAVIC satellites along with quasars to assess their potential contribution to the Indian Navigation system. As a result, this work helps to establish that the development of the geodetic VLBI infrastructure in India will improve the necessary information for Earth observation missions, location-based applications, and the realization of terrestrial and celestial reference frames with international collaboration., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

As wide-area atmosphere correction models often struggle to attain high-precision and uniformity, especially in regions experiencing rapid atmospheric changes, they are usually as constraints to the user-end atmospheric parameters. The proper constraint is crucial for rapid or instantaneous convergence in Precise Point Positioning with Ambiguity Resolution (PPP-AR). However, the empirical uncertainty information commonly adopted cannot accurately represent the actual quality of a real-time established model. To address this problem, we propose an approach to determine atmospheric uncertainty information using the residuals of the atmosphere delays fitting to a polynomial model. Two sets of 2°×2° grids with a 15-minute update rate are generated based on the fitting residuals of tropospheric and satellite-wise slant ionospheric delays derived using PPP-AR at all reference stations. The proposed approach is verified using 166 EUREF Permanent Network stations with a 150km station-spacing and validated using 113 additional stations. The utilization of wide-area grid uncertainty information leads to an average convergence time that reduces to 1.5min, i.e., improved by up to 36% compared to the legacy empirical constraints. Additionally, an improvement of 9% in the fixing rate compared to the empirical constraint solutions is observed. In conclusion, the provision of atmosphere uncertainty information accurately characterizes the performance of the fitting model in all coverage areas, resulting in rapid positioning performance with lower computational resources., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Integrated processing of high-rate GNSS and accelerometer data can overcome the disadvantage of each individual sensors and to increase the quality of derived co-seismic displacement. However, the contribution of accelerometer is usually underestimated by estimating baseline shifts epoch-wise which in fact happened very rarely. To take full advantage of both sensors, we propose a sliding window based Kalman filter to detect baseline shifts according to the disagreement of GNSS and accelerometer data and to estimate only the detected baseline shifts. The relationship of the window width, minimal detectable baseline shift and the displacement accuracy is investigated. The performance of the proposed approach is demonstrated by datasets collected during Samos Island earthquake (Mw 7.0, 30th October 2020). The results show that the baseline shifts in accelerometers can be precisely detected and estimated according to the very good agreement of the displacement integrated from accelerometer data after applying baseline shift corrections and that estimated from high-rate GNSS. Furthermore, the baseline corrected accelerometer data provides tight and reliable constraints on position and velocity to facilitatecorrect PPP ambiguity resolution. Thanks to the proposed approach, the complementary of GNSS and accelerometers is fully employed, consequently the co-seismic displacements of the tightly integrated processing are significantly improved., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The GFZ VLBI working group develops, maintains and supports the VLBI data analysis software "PORT", the Potsdam Open Source Radio Interferometry Tool. PORT development commenced in August 2018 with the initial import of the existing code base into a "Git" version-controlled software repository. Following its antecedent software packages "VieVs" and "VieVS@GFZ", PORT is focused on the timely processing of VLBI sessions, as well as post-processing activities supporting the generation of celestial and terrestrial reference frames; notably, GFZ contributed to the most recent ITRF realization, the ITRF2020. Furthermore, PORT provides a framework for research and development projects and is part of the educational tool set used in training and promotion of young researchers. Within its activities as an IVS analysis centre, GFZ employs PORT to estimate Earth orientation parameters, radio source, VLBI station positions, and tropospheric parameters using standard least squares adjustment and, in future, Kalman filter techniques. A refined graphical interface allows the user to manually insert clock breaks and to flag outlier data points. Weighting of group/phase delays, amplitudes and position constraints can be applied to stations, radio sources and baselines on an individual basis. Results are displayed on screen, written to disk for later usage (e.g. as input to global solutions) and the corresponding normal equations are stored in PORT-specific and SINEX data formats. Current PORT development activities focus on both, consolidation of the legacy code base and revision of its design, code structure and implementation in order to improve robustness, modularity, readability, maintainability and facilitate innovation., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Space geodetic techniques, particularly Global Navigation Satellite Systems (GNSS), have contributed significantly to neotectonic research on continental plates over the last decades, allowing us to observe crustal motions in all three spatial directions. Meanwhile, the previous studies of German neotectonics have mainly focused on several areas associated with the European Cenozoic rift system. Thus, this study investigates the entire German territory to identify the neotectonic motion characteristics based on GNSS-integrated data and Machine Learning (ML). Parallelly, high-precise GNSS time series processing techniques are applied to reflect actual motions. Our results show that the German Earth’s crust tends to uplift, especially in the active graben regions moving with a speed of almost +1.0 mm/yr. In contrast, the local subsidence of around -0.8 mm/yr is concentrated in the large groundwater extraction regions and the river basins such as the Rhine, Ems, Elbe, Northern Oder, and Danube. In the horizontal movements, the German Earth’s crust moves with an average intra-plate velocity of ~0.8 mm/yr, while some places reach up to ~2.5 mm/yr. Furthermore, ML detects the noticeable surface deformation regions, namely Lower Rhine Bay, Upper Rhine Graben, Eifel volcano, and Thuringian-Vogtland slate mountains. Our findings have filled the overall picture of German neotectonics and provide a valuable material source for infrastructure management toward sustainable development in Germany.

Global Navigation Satellite Systems (GNSS) Real-time Precise Point Positioning (RT-PPP) strongly relies on the precise satellite orbits and clocks, especially the latter requires high update rate, e.g., five seconds, due to its limited prediction accuracy. Such a high-rate update frequency is a burden to both the data analysis and communicating, and interruption in communicating is almost unavoidable. For the new constellations such as Galileo and BDS-3 with high-stability hydrogen clocks onboard, it is possible to predict satellite clocks to a longer arc. Moreover, recent developments in multi-GNSS ultra-rapid precise orbit determination enables the half-hourly update, ensuring the availability with a prediction arc of 30–60 min. We investigate multi-GNSS RT-PPP using half-hourly predicted products and demonstrate that a 3-D accuracy of 2.9 and 11.3 cm can be achieved for static and simulated kinematic solutions, respectively. We present the different clock prediction accuracies of different types of satellites and propose a satellite-specific weighting strategy in PPP, which exploits the benefits of the satellites of good performance. The method is based on the prediction accuracy of both, satellites and clocks, and shows an improvement of 15 to 60% compared to those without satellite-specific weighting or with simplified weighting strategies. We also demonstrate that Galileo satellites contribute the most in the quad-constellation solution, thanks to the highly stable satellite clocks.

We show the extent of the variance of CPO time series solutions from VLBI that can be reduced by adding suitable corrections to the precession model, and provide information concerning nutations.