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In the multi-technique integrated solution, different space geodetic techniques are combined via common parameters and external observations, such as global ties (Earth Orientation Parameters, EOPs) and local ties (LTs), respectively. Local ties, which directly link the coordinates of co-location stations, are currently used in determining terrestrial reference frames (TRFs). We investigate the impact of global and local ties in Global Navigation Satellite Systems (GNSSs) and Very Long Baseline Interferometry (VLBI) integrated solutions in five VLBI continuous (CONT) campaigns. Compared to a baseline solution where GNSS and VLBI have separate minimal constraints implemented (no-net-rotation and no-net-translation, NNR+NNT), applying additional GTs and LTs improves the VLBI TRF and EOP estimates. As LTs are supposed to transfer the datum information between GNSS and VLBI, we further investigate the scenario where the VLBI datum is linked to GNSS via LT, without NNR+NNT applied to the VLBI network. We demonstrate that in this scenario the VLBI solution is deteriorated, especially the east-west component of station coordinates and UT1-UTC estimates, mainly due to the insufficient accuracy of the currently used LTs. We thus emphasize the importance of improving the accuracy of LT measurements and applying additional ties, for example, space ties and atmospheric ties.

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) envisages stringent goals for the International Terrestrial Reference Frame (ITRF) realization in terms of accuracy (1 mm) and precision (0.1 mm/year). These requirements entail that the Earth Orientation Parameters (EOP) should be estimated with similar accuracy. The conventional International Terrestrial Reference Frame (ITRF) is based on the combination of solutions from four space geodetic techniques, including observations until the end of 2020, incorporating updated data and models. On the other hand, the Celestial Reference Frame (CRF) is a VLBI-only solution based on data until 2015, provided by one sole VLBI-analysis centre. Additionally, the current conventional EOP series, IERS 14 C04, is also produced in a separate process following a different analysis and combination strategy. It is based on a combination of monthly EOP estimates obtained by the combination centres of each space geodetic technique. These disparate approaches might cause a slow degradation of the consistency among EOP and the reference frames or a misalignment of the current conventional EOP series. The recent release of the ITRF2020 brings an exciting opportunity to investigate this topic. In this work, we empirically assess the consistency among the conventional terrestrial reference frame (TRF) and celestial reference frame (CRF), and EOP through the analysis of Very Long Baseline Interferometry (VLBI) historical data, taking different TRFs as alternative settings in the analysis: ITRF2020, VTRF2020, ITRF2014, and the terrestrial frame consistent with the newest Celestial Reference Frame (i.e., ICRF3). Additionally, Helmert transformations are computed to evaluate to which extent the behaviour that may be found in the previous point can be attributed to orientation differences of the TRFs themselves. Finally, different CRF realizations (ICRF2 and ICRF3) are tested to study their impact on the EOP, especially in the long term, paying attention to the appearance of biases and trends among the EOP series. This study allows evaluation if the selection of the TRFs and/or the CRFs has a significant impact on the consistency of the estimated EOP and assesses its agreement with the conventional EOP series.

Polar Motion is the movement of the Earth's rotational axis relative to its crust, reflecting the influence of the material exchange and mess redistribution of each layer of the Earth on the Earth's rotation axis.The real-time estimation of Polar Motion (PM) is needed for the navigation of Earth satellites and interplanetary spacecraft. However, it is impossible to have real-time information due to the complexity of the measurement model and data processing.Various prediction methods have been developed. However, the accuracy of PM prediction is still not satisfactory even for a few days in the future. Therefore, a new technique or a combination of the existing methods needs to be investigated for improving the accuracy of the prediction PM.In this study, we combine the 1D Convolutional Neural Network with the Singular Spectrum Analysis (SSA). The computational strategy follows multiple steps, first, we model the predominant trend of the PM time series using SSA. Then, the difference between the PM time series and its SSA estimation is modeled using the 1D Convolution Neural Network. However, we developed a Multivariate Multi step 1D-CNN Model with a Multi-output strategy to predict at the same time both components (Xp, Yp) of the PM. . We introduce to the Model: the Ocean Angular Momentum, Atmospheric Angular Momentum, and Hydrological Angular Momentum (OAM+AAM+HAM) to improve the results. Multiple sets of PM predictions which range between 1 and 10 days have been performed based on an IERS 14 C04 time series to assess the capability of our hybrid Model. Our results illustrate that the proposed method can efficiently predict both (Xp, Yp) of PM.

The Bureau of Products and Standards (BPS) is a key component of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG). It supports GGOS in its goal to provide consistent geodetic products needed to monitor, map, and understand changes in the Earth’s shape, rotation, and gravity field. In its current structure, the Committees “Contributions to Earth System Modeling” and “Definition of Essential Geodetic Variables” as well as the Working Group “Towards a consistent set of parameters for the definition of a new Geodetic Reference System (GRS)” are associated to the BPS. This poster contribution presents the role of the BPS and it highlights some of the recent activities, which are focused on the updating of the IERS Conventions, mainly related to Chapter 1 "General definitions and numerical standards", the updating of the BPS inventory of standards and conventions used for the generation of IAG products to incorporate the latest developments in the field, the compilation of user-friendly descriptions of geodetic products published at the GGOS website, and the contribution to GGOS films for specific products. The BPS activities also comprise the interaction with IAG and other entities in the field of standards and conventions, such as the IAG Services, the IERS Conventions Center, the Commission A3 “Fundamental Standards” of the International Astronomical Union (IAU), ISO/TC 211 and the Working Group ``Data Sharing and Development of Geodetic Standards” of the UN-GGIM Subcommittee on Geodesy (SCoG).

Funding Information: We would like to thank Arthur Niell very much for the comments during the reviews. The results reported in this paper used the data coordinated by the International VLBI Service (IVS) and its international self‐funded member organizations. We are grateful to the IVS stations at Hartebeesthoek (South Africa Radio Astronomical Observatory), Ishioka (Geospatial Information Authority of Japan), Kokee Park (U.S. Naval Observatory and NASA GSFC, USA), Ny‐Ålesund (Norwegian Mapping Authority, Norway), Onsala (Onsala Space Observatory, Chalmers University of Technology, Sweden), Shanghai (Shanghai Astronomical Observatory, China), Wettzell (Bundesamt für Kartographie und Geodäsie and Technische Universität München, Germany), and Yebes (Instituto Geográfico Nacional, Spain), to the staff at the Bonn Correlator, the Washington Correlator, the Onsala Observatory Correlator, and the MIT Haystack Observatory correlator for performing the correlations and the fringe fitting of the data, to the NASA GSFC VLBI group and the BKG VLBI group for doing the geodetic solutions, and to the IVS Data Centers at BKG (Leipzig, Germany), Observatoire de Paris (France), and NASA CDDIS (Greenbelt, MD, USA) for the central data holdings. This research has made use of the Generic Mapping Tools package, the pgplot library, and the SAO/NASA Astrophysics Data System. The work was supported by the Academy of Finland project No. 315721. Open Access funding enabled and organized by Projekt DEAL. Funding Information: We would like to thank Arthur Niell very much for the comments during the reviews. The results reported in this paper used the data coordinated by the International VLBI Service (IVS) and its international self-funded member organizations. We are grateful to the IVS stations at Hartebeesthoek (South Africa Radio Astronomical Observatory), Ishioka (Geospatial Information Authority of Japan), Kokee Park (U.S. Naval Observatory and NASA GSFC, USA), Ny-Ålesund (Norwegian Mapping Authority, Norway), Onsala (Onsala Space Observatory, Chalmers University of Technology, Sweden), Shanghai (Shanghai Astronomical Observatory, China), Wettzell (Bundesamt für Kartographie und Geodäsie and Technische Universität München, Germany), and Yebes (Instituto Geográfico Nacional, Spain), to the staff at the Bonn Correlator, the Washington Correlator, the Onsala Observatory Correlator, and the MIT Haystack Observatory correlator for performing the correlations and the fringe fitting of the data, to the NASA GSFC VLBI group and the BKG VLBI group for doing the geodetic solutions, and to the IVS Data Centers at BKG (Leipzig, Germany), Observatoire de Paris (France), and NASA CDDIS (Greenbelt, MD, USA) for the central data holdings. This research has made use of the Generic Mapping Tools package, the pgplot library, and the SAO/NASA Astrophysics Data System. The work was supported by the Academy of Finland project No. 315721. Open Access funding enabled and organized by Projekt DEAL. Publisher Copyright: © 2023. The Authors. We report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observations rather than dedicated short-baseline experiments. An analysis of phase delay observables at X band from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hr VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. Position ties are also obtained for other short baselines between legacy antennas and nearby, newly built antennas. They are critical for maintaining a consistent continuation of the realization of the terrestrial reference frame, especially when including the new VGOS network. The phase delays of the baseline WETTZ13N–WETTZELL enable an investigation of sources of error at the sub-millimeter level. We found that a systematic variation of larger than1 mm can be introduced to the Up estimates of this baseline vector when atmospheric delays were estimated. Although the sub-millimeter repeatability has been achieved for the baseline vector WETTZ13N–WETTZELL, we conclude that long term monitoring should be conducted for more short baselines to assess the instrumental effects, in particular the systematic differences between phase delays and group delays, and to find common solutions for reducing them. This will be an important step toward the goal of global geodesy at the 1 mm level.

Very Long Baseline Interferometry (VLBI) intensive (INT) sessions are critical for the rapid determination and densification of Universal Time 1-Coordinate Universal Time (UT1-UTC), which plays an important role in satellite geodesy and space exploration missions and is not predictable over longer time scales. Due to the limited observation geometry of INT sessions with two to three stations observing about 1 hr, tropospheric gradients cannot be estimated, which degrades the UT1-UTC precision. We investigate the impact of tropospheric ties at Global Navigation Satellite Systems (GNSSs) and VLBI co-located stations in INT sessions from 2001 to 2021. VLBI and GNSS observations are combined on the observation level. The results are evaluated by using both UT1-UTC and Length of Day (LOD) from consecutive sessions. We demonstrate a better agreement of 10%–30% when comparing the derived LOD to GNSS LOD for INT1, INT2, and VGOS-2 sessions; whereas, the agreement is not improved when directly comparing UT1-UTC to the IERS Earth Orientation Parameters (EOPs) product, potentially because INT sessions also contribute to IERS EOP products. The major impact comes from tropospheric gradient ties, whereas applying zenith delay ties does not improve or even deteriorate UT1-UTC agreement. Gradient ties also introduce systematic biases in UT1-UTC by around −3 to −5 μs, except for the Russian INT sessions. Regression analysis shows that the east gradient introduces systematic effects in UT1-UTC for sessions involving Germany and USA (Hawaii), whereas for Germany–Japan and Russian sessions, the north gradient also contributes systematically.

The location of the Earth's principal axes of inertia is a foundation for all the theories and solutions of its rotation, and thus has a broad effect on many fields, including astronomy, geodesy, and satellite-based positioning and navigation systems. That location is determined by the second-degree Stokes coefficients of the geopotential. Accurate solutions for those coefficients were limited to the stationary case for many years, but the situation improved with the accomplishment of Gravity Recovery and Climate Experiment (GRACE), and nowadays several solutions for the time-varying geopotential have been derived based on gravity and satellite laser ranging data, with time resolutions reaching one month or one week. Although those solutions are already accurate enough to compute the evolution of the Earth's axes of inertia along more than a decade, such an analysis has never been performed. In this paper, we present the first analysis of this problem, taking advantage of previous analytical derivations to simplify the computations and the estimation of the uncertainty of solutions. The results are rather striking, since the axes of inertia do not move around some mean position fixed to a given terrestrial reference frame in this period, but drift away from their initial location in a slow but clear and not negligible manner.

This article focuses on the new generation of geodetic very long baseline interferometry (VLBI), the VLBI global observing system (VGOS), and measurements carried out during the CONT17 campaign. It uses broadband technology that increases both the number and precision of observations. These characteristics make VGOS a suitable tool for studying the atmosphere. This study focuses on the effects of the ionosphere on VGOS signals using a model that incorporates and extends ideas originally published in Hobiger et al. (2006, https://doi.org/10.1029/2005RS003297). Our investigation revealed that the differential total electron content (dTEC) data product calculated with the VGOS post-processing software had a sign error that fortunately, does not change the final values of the phase and group delay. Therefore, this study was a way to identify this problem within the dTEC product. After diagnosing and solving this problem, the underlying model was modified such that instead of considering a single unknown for the latitude gradient of the ionosphere, a time series of latitude gradients were considered that enhanced the resulting vertical total electron content (VTEC) estimates. For evaluation purposes, time series of VTEC at each station during the CONT17 campaign were compared with VTEC obtained from the global navigation satellite system (GNSS). The final agreement between VGOS and GNSS was between 1.1 and 5.9 TEC units (TECU).

Global Geodetic Observing System (GGOS) requires an accuracy of 1 mm and a stability of 1 mm/decade for the Terrestrial Reference Frame (TRF), which is not achieved yet. Global Navigation Satellite Systems (GNSS) play a critical role in TRF determination, thanks to its continuous observations collected from hundreds of globally distributed stations. GNSS observations at the user side are affected by antenna, receiver, and the local environment. The impact of the antenna is more significant to carrier-phase observations and handled with the phase center offset and variation corrections, meanwhile that of the receiver is more significant to the pseudo-range observations. In this study, we investigate the long-term agreement of station coordinates at co-located GNSS stations, i.e., more than one GNSS stations within a few hundreds of meters. We demonstrate that (1) the coordinate difference between co-located GNSS stations could have seasonal variations and long-term trends, (2) discontinuities are often observed after instrument changes, especially the antenna change, and (3) different receiver types could cause station coordinate bias up to 1 mm. A preliminary investigation of the possible reasons is carried out and addressed as well.

Tropospheric delay modeling is challenging in high-precision Very Long Baseline Interferometry (VLBI) analysis due to the rapid water vapor variation and imperfect observation geometry, where observations from Global Navigation Satellite Systems (GNSS) co-locations can enhance the VLBI analysis. We investigate the impact of tropospheric ties in the VLBI and GNSS integrated processing during the CONT05–CONT17 campaigns, and present a method that automatically handles the systematic tropospheric tie biases. Applying tropospheric ties at VLBI–GNSS co-locations enhances the observation geometry and improves the solution reliability. The VLBI network is stabilized, with station coordinate repeatability improved by 12% horizontally and by 28% vertically, and the network scale improved by 32%. The Earth Orientation Parameters (EOP) improve by up to 20%. Both zenith delay and gradient ties contribute to the improvement of EOP, whereas the gradient ties contribute mainly to the improvement of length of day and celestial pole offsets.

Machine Learning (ML) algorithms are used to learn from data and make data-driven predictions. These algorithms consider pattern recognition and computational learning on the data. Earth Orientation Parameters (EOP) are the monitoring parameters for the Earth’s rotation. UT1-UTC is an EOP that monitors the time required by the Earth to complete a rotation versus atomic time. This parameter is indispensable for many applications like precise satellite orbit determination, interplanetary space navigation, etc. In this study, we will use novel ML algorithms to predict the UT1-UTC (IERS C04) time series and investigate its performance against each other and also, w.r.t. the conventional prediction fitting methods, like Least Squares (LS), Auto-Regressive (AR), Multivariate Autoregressive (MAR) methods, etc. In this work, a diversity of advanced ML algorithms will be tested: Random Forest (RF), Generalized Linear Model (GLM), Gradient Boosted Model (GBM), K-means and prophet algorithms. We would like to optimize the UT1-UTC prediction technique to work well with the short-term prediction up to 10 days. Finally, these ML predictions will be compared against those from the last Earth Orientation Parameters Prediction Comparison Campaign (EOP PCC) from October 1, 2005 to February 28, 2008. This detailed study would be useful to understand the performance of ML techniques on the UT1-UTC time series and would lead to further development of better prediction models using ML algorithms. Key words: Machine Learning, Earth Orientation Parameters (EOP), UT1-UTC, predictions

The quality of a geodetic network is classically determined with the precision criteria computed from the cofactor matrix of the unknown parameters. One of the precision criteria having more information compared to the Helmert position error and the mean error of the unknown parameters is the Helmert mean error ellipsoid. In two-dimensional networks – and the celestial reference frame realized by extragalactic radio sources can be considered as such - the Helmert mean error ellipse consists of three parameters which are the semi-major and semi-minor axis of the error ellipse and the direction of the semi-major axes. In a well-designed geodetic network, the error ellipses should have homogenous structures. In other words, the semi-axes of the error ellipses for all radio sources should be similar. In this study, daily IVS sessions of the CONT17 were evaluated with The Potsdam Open Source Radio Interferometry Tool (PORT) and the parameters of the Helmert mean error ellipses were computed for the radio sources in each session. Also, the results are compared with the number of observations and the angular position of the radio sources. As a result of this study, it can be seen how the precision criteria are affected depending on the angular position of the radio sources.

Since 2020 a few research groups have reported new estimations of nutation amplitudes or precession parameters, usually presented as corrections to the nutation and precession theories IAU2000 and IAU2006, as well as new empirical models for the free core nutation (FCN). The main effect of those proposed corrections is reducing the unexplained variance of the celestial pole offsets (CPO) time series. The improvement of the accuracy of CPO models was encouraged by Resolution 5 of the 2019 General Assembly of the International Association of Geodesy (IAG), included among the recommendations of the GGOS/IERS Unified Analysis Workshop held that year, and also supported by Resolution B2 of the International Astronomical Union (IAU) in 2021.The variance reduction attainable by refining the precession offsets and rates and the amplitudes of the forced nutations is noticeably larger than that resulting from the corrections needed for the consistency of the precession and nutation theories (Escapa et al 2018). However, the amount of the improvement depends strongly on the various fitting strategies, the input CPO series and the time periods chosen for benchmarking.That fact is shown through the accuracy assessment of a selection of correction models proposed by the authors and other teams, either supplemented with FCN models or not, and for a selection of input CPO data and time spans. The work of JMF, AE, and TB was partially supported by Spanish Projects PID2020-119383GB-I00 funded by MCIN/AEI/10.13039/501100011033 and PROMETEO/2021/030 (Generalitat Valenciana); and SB was supported partially by Generalitat Valenciana (SEJIGENT/2021/001) and the European Union˜NextGenerationEU (ZAMBRANO 21-04) .

Knowledge of the Earth orientation parameters (EOP) is essential for numerous practical and scientific applications including, positioning and navigating in space and on Earth.The LOD (length of day), which represents the variation in the Earth's rotation rate, is the most difficult to forecast since it is primarily affected by the torques associated with changes in atmospheric circulation. Therefore, accurate LOD predictions are an ongoing challenge and are the focus of this work.Consequently, there is a compelling need to identify next-generation time series prediction algorithms to be integrated into an operational processing chain. Of specific interest is the emergence of deep learning methods. These methods tend to behave as highly adaptive and versatile fitting algorithms and can thus replace conventional fitting functions for enabling more accurate predictions.In this study, the 1D-Convolutional Neural Networks (1D-CNN) is introduced to model and to predict the LOD using the IERS EOP 14 C04 and the axial Z component of the atmospheric angular momentum (AAM) taken from the German Research Centre for Geosciences (GFZ), since it is strongly correlated with the LOD changes. The prediction procedure operates as follows: First, we detrend the LOD and Z-component series by using the LS method, then, we obtain the residuals series of each one to be used in the 1D-CNN prediction algorithm. Finally, we analyzed the results before and after introducing the AAM function. These results prove the potential of the proposed method as an optimal algorithm to successfully reconstruct and predict LOD for up to 7 days.Keywords1D-Convolutional Neural Networks (1D-CNN); Length of the day; atmospheric angular momentum(AAM) function; prediction

The international celestial and terrestrial reference frames (ICRF and ITRF) are two important realizations of the global geodetic reference frame (GGRF). As the basis for high-accuracy astrometry and space exploration, ICRF is currently determined by the Very Long Baseline Interferometry (VLBI) technique solely and independently from the ITRF, whereas a consistent determination of TRF and CRF by a combination of different techniques is highly desirable. We conduct the Global Navigation Satellite Systems (GNSS) and VLBI integrated processing on the observation level and investigate the impact of applying global ties (that is, Earth Orientation Parameters, EOP), local ties, and tropospheric ties on the precision of both TRF and CRF. The GNSS and VLBI observations in VLBI continuous campaigns from CONT05 to CONT17 are processed simultaneously in the common least-squares estimator using the Positioning And Navigation Data Analyst (PANDA) software. We present that the precision of the VLBI station coordinates is significantly improved in the integrated solution, such as the horizontal components by global ties and the vertical components by local and tropospheric ties. Focusing on the precision of active galactic nuclei (AGN) coordinates, we demonstrate that the global ties can slightly reduce the AGN coordinate formal errors by up to 4%, and the local ties mainly improve the declination precision by about 10%. As for the tropospheric ties, the formal error of AGN coordinate can be reduced by 10% on average, and the repeatability can also be improved, especially the declination (10%). Moreover, the southern AGN are more improved than the northern ones, due to the observation geometry of the VLBI ground station distribution.

The Continuous Very Long Baseline Interferometry (VLBI) campaign 2017 (CONT17) differs from previous CONT campaigns as there are three independent networks observing in parallel; two legacy VLBI networks observing in S/X band (Legacy-1 and Legacy-2) and one VGOS (the next generation VLBI system: VLBI2010) network observing in broadbrand. Co-located VLBI radio telescopes across the networks could be combined to strengthen the geodetic solutions from CONT17 by using local ties. Moreover, it is widely known that the co-located VLBI radio telescopes observe common effects such as the tropospheric delays in a similar way. Therefore, we can not only combine the station coordinates, but we can also combine the tropospheric parameters. In this work, we focused on the impact of combining the tropospheric parameters obtained at co-located VLBI telescopes on the estimated geodetic parameters during CONT17. We considered three case studies where we combined: (i) the Legacy-1 and the Legacy-2 networks, (ii) the Legacy-1 and the VGOS networks, and (iii) the Legacy-1, the Legacy-2, and the VGOS networks. The results show an improvement in station position repeatability when combining the tropospheric parameters w.r.t. applying only local ties by 28%, 15%, and 26% respectively. Station coordinates at the VLBI radio telescopes of the Legacy-1 network were not affected by this approach as they were used for the datum definition. In addition, the baseline length repeatabilities show no improvement when combining the tropospheric parameters. Moreover, the agreement between the tropospheric parameters from VLBI and tropospheric parameters from independent GNSS data analysis and ray-tracing through ERA5 improves due to the network combination.

The relationship between the length of day (LOD) and El-Niño Southern Oscillation (ENSO) has been well studied since the 1980s. LOD is the negative time-derivative of UT1-UTC, which is directly proportional to Earth Rotation Angle (ERA), one of the Earth Orientation Parameters (EOP). The EOP can be determined using Very Long Baseline Interferometry (VLBI), which is a space geodetic technique. In addition, satellite techniques such as the Global Navigation Satellite System (GNSS), Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) can provide Earth Rotation Parameters, i.e., polar motion and LOD. ENSO is a climate phenomenon occurring over the tropical eastern Pacific Ocean that mainly affects the tropics and the subtropics. Extreme ENSO events can cause extreme weather like flooding and droughts in many parts of the world. In this work, we investigated the effect of ENSO on the LOD from January 1979 to April 2022 using the wavelet coherence method. This method computes the coherence between the two non-stationary time-series in the time-frequency domain using the real-valued Morlet wavelet. We used the Multivariate ENSO index version 2 (MEI v.2) which is the most robust series as the climate index for the ENSO, and LOD time-series from IERS (EOP 14 C04 (IAU2000A)). We also used Oceanic Niño and Southern Oscillation index in this study for comparison. The results show strong coherence of 0.7 to 0.9 at major ENSO events for the periods 2–4 years between LOD and MEI.v2.

The real-time Earth orientation parameters (EOP) estimation is needed for many applications, including precise tracking and navigation of interplanetary spacecraft, climate forecasting, and disaster prevention. However, the complexity and time-consuming data processing always lead to time delays. Accordingly, several methods were developed and applied for the EOP prediction. However, the accuracy of EOP prediction is still not satisfactory even for prediction of just a few days in the future. Therefore, new methods or a combination of the existing approaches can be investigated to improve the predicted EOP. To assess the various EOP prediction capabilities, the international Earth rotation and reference systems service (IERS) established the working group on the 2nd Earth Orientation Parameters Prediction Comparison Campaign (2nd EOP PCC).Our EOP prediction team provides the full set of EOP predictions weekly for one year ahead. The SSA+Copula method and the empirical free core nutation (FCN) model (named B16) are used for Earth rotation parameters and celestial pole offsets (CPO) prediction, respectively. Our preliminary results illustrate an improvement in EOP prediction compared to the current EOP prediction methods, especially on CPO. Additionally, the comparison with other method results indicates that the proposed techniques can efficiently and precisely predict the EOP at different terms (short, mid, and long term).

We report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observation...

This article focuses on the new generation of Very Long Baseline Interferometry (VLBI), the VLBI Global Observing System (VGOS), and measurements carried out during CONT17 campaign. It uses broadba...

Atmospheric ties are theoretically affected by the height differences between antennas at the same site and the meteorological conditions. However, there is often a discrepancy between the expected zenith delay differences and those estimated from geodetic analysis, potentially degrading a combined solution employing atmospheric ties. In order to investigate the possible effects on GNSS atmospheric delay, this study set up an experiment of four co-located GNSS stations of the same type, both antenna and receiver. Specific height differences for each antenna w.r.t the reference antenna are given. One antenna was equipped with a radome at the same height and type as a antenna close to the ground. In addition, a meteorological sensor was used for meteorological data recording. The results show that tropospheric ties from the analytical equation based on meteorological data from GPT3, Numerical Weather Model, and in-situ measurements, and ray-traced tropospheric ties, reduced the bias of zenith delay roughly by 72 %. However, the in-situ tropospheric ties yield the best precision in this study. These results demonstrate, that the instrument effects on GNSS zenith delays were mitigated by using the same instrument. In contrast, the radome causes unexpected bias of GNSS zenith delays in this study. Additionally, multipath effects at low-elevation observations degraded the tropospheric east gradients.

Atmospheric ties are the differences of atmospheric parameters between antennas or stations at the same site and meteorological conditions. However, there is often a discrepancy between the expected zenith delay differences and those estimated from geodetic analysis, potentially degrading a combined solution employing atmospheric ties to constrain atmospheric delay differences. To investigate the possible effects on GNSS atmospheric delay, this study set up an experiment with four co-located GNSS stations of the same type, both antenna and receiver. Specific height differences for each antenna w.r.t. one reference antenna have been measured. One antenna was equipped with a radome of the same height and type as an antenna close to the ground. Additionally, a meteorological sensor was used for meteorological data recording. The results show that tropospheric ties from the analytical equation based on meteorological data from Global Pressure and Temperature 3 (GPT3) model, Numerical Weather Models, in-situ measurements, and ray-traced tropospheric ties, reduced the bias of zenith delay roughly by 72 %. However, the in-situ tropospheric ties yielded the best precision in this study. These results demonstrate that the instrument effects on GNSS zenith delays were mitigated using the same instrument. In contrast, although the effects of the radome on atmospheric delays are well known, the magnitude of the effects determined in this study is unexpectedly large. Additionally, multipath effects at low-elevation observations degraded the tropospheric gradients. To extract the instrument effect, we set up another experiment with three GNSS stations and four different antennas. The height differences between the three stations were on one centimeter level. One of the three stations could be adjusted in height to control the height displacement after changing antenna. We succeeded in keeping the shift in the GNSS zenith delays within 2 mm level. The bias on GNSS zenith delays and tropospheric gradients agrees with the result of the previous experiment in this study. Moreover, we successfully detected the antenna-dependent effect on both the GNSS zenith delays and gradients from this experiment.

In 2020 new estimations of nutation amplitudes or precession parameters have been published or presented at main meetings. The derivation of corrections to improve the current precession-nutation models was encouraged by Resolution 5 of the 2019 General Assembly of the International Association of Geodesy (IAG). Besides, the GGOS/IERS Unified Analysis Workshop held in October 2019 recommended that effort to be prioritized among the tasks of the current IAU/IAG Joint Working Group on Improving Theories and Models of the Earth’s rotation (JWG ITMER).This presentation is intended to present comparisons of some of those new semi-empirical and semi-theorical precession-nutation models developed by different authors from either VLBI solutions of individual analysis centers or combinations of them. The models recently introduced by the authors that were reported at the AGU 2020 Fall Meeting are included in this assessment.

Besides the natural extra-galactic radio sources, observing an artificial Earth-orbiting radio source with the Very Long Baseline Interferometry (VLBI) permits to extend the geodetic and geodynamic applications of this highly accurate interferometric technique. Furthermore, combining aforementioned observations provides a promising method to determine the satellite orbit and delivers the new type of observations such as group delay and delay rate which might be employed to validate the orbit independent of other space geodetic techniques.In this research, the potential of the interferometric satellite tracking for the Precise Orbit Determination (POD) has been explored based on simulated observations for different scenarios with various VLBI networks, satellite orbits (eccentric low Earth orbits or circular medium Earth orbits) and error sources. POD of the Earth-orbiting satellites is studied on the basis of daily VLBI sessions where satellite observations are scheduled together with the quasar observation for regionally or globally distributed legacy as well as next generation VLBI station networks. In order to simulate VLBI to satellite observations, the influence of the most prominent random error sources in VLBI as well as mismodelling of different force models acting on the satellite are utilized. This study indicates that POD is feasible with VLBI observations and the accuracy mainly depends on the observation geometry.

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) provides the geodetic infrastructure needed to monitor the Earth system.. The understanding of forced temporal variations of celestial pole motion (CPM) could bring us significantly closer to meeting the GGOS goals (i.e. 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the ITRF defining parameters). Besides astronomical forcing, CPM excitation depends on the processes in the fluid core and the core-mantle boundary. The same processes are responsible for the variations of the geomagnetic field (GMF). This study investigates the interconnection between the celestial pole offset (CPO) and effective geophysical processes that contribute to the Earth's rotational variation. We use the CPO time series obtained from very long baseline interferometry (VLBI) observations together with the latest GMF data such as geomagnetic jerk and magnetic dipole moment, and a state-of-the-art geomagnetic field model to explore the correlation between CPM and GMF. Our results confirm the findings of previous studies, revealing that substantial free core nutation (FCN) disturbance occurred at the epochs close to the GMJ events. The results also reveal some common features in the FCN and GMF variation, which show the potential to improve knowledge regarding the GMF's contribution to the Earth's rotation.

This presentation gives a summary of the role and the activities of the Bureau of Products and Standards (BPS) to support IAG’s Global Geodetic Observing System (GGOS) in its goal to provide observations and consistent geodetic products needed to monitor, map and understand changes in the Earth’s shape, rotation and mass distribution. In its present structure, the two Committees “Earth System Modeling” and “Essential Geodetic Variables” as well as the Working Group “Towards a consistent set of parameters for the definition of a new GRS” are associated with the BPS. A key objective of the BPS is to keep track and to foster homogenization of adopted geodetic standards and conventions across all IAG components as a fundamental basis for the generation of consistent geometric and gravimetric products. Towards this aim, an updated 2nd version of the BPS inventory of standards and conventions used for the generation of IAG products has been published in the Geodesist’s Handbook 2020. In the framework of the renewing of the GGOS website, the BPS supports the GGOS Coordinating Office in particular regarding the representation of geodetic products. Furthermore, the BPS contributes to the rewriting of the IERS Conventions as Chapter Expert for Chapter 1 “General definitions and numerical standards” and interacts with external stakeholders regarding standards and conventions, such as ISO, IAU, BIPM, CODATA and the UN GGIM Subcommittee on Geodesy, including its Working Group “Data Sharing and Development of Geodetic Standards”.

The terrestrial and celestial reference frames are linked by the Earth Orientation Parameters (EOP), which describe the irregularities of the Earth's rotation and are determined by the space geodetic techniques, namely, Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). The satellite geodetic techniques (SLR, GNSS, and DORIS) cannot determine the UT1-UTC or celestial pole offsets (CPO), rendering VLBI the only technique capable of determining full EOP set. On the other hand, the GNSS technique provides precise polar motion estimates due to the continuous observations from a globally distributed network. Integrating VLBI and GNSS provides the full set of EOP and guarantees a superior accuracy than any single-technique solution.In this study we focus on the integrated estimation of the full EOP set from GNSS and VLBI. Using five VLBI continuous observing campaigns (CONT05–CONT17), the GNSS and VLBI observations are processed concurrently in a common least-squares estimator. The impact of applying global ties (EOP), local ties, and tropospheric ties, and combinations thereof is investigated. The polar motion estimates in integrated solution are dominated by the huge GNSS observations, and the accuracy in terms of weighted root mean squares (WRMS) is ~40 μas compared to the IERS 14 C04 product, which is much better than that of the VLBI-only solution. The UT1-UTC and CPO in the integrated solution also show slight improvement compared to the VLBI-only solution. Moreover, the CPO agreement between the two networks in CONT17, i.e., the VLBA and IVS networks, shows an improvement of 20% to 40% in the integrated solution with different types of ties applied.

How far apart can two space geodetic sites be located to consider the integrated water vapor (hereinafter IWV) trends as equal, from a statistical viewpoint? How to do efficient feature selection with a given IWV time series? To address these questions, we utilize spatio-temporal variations of long-term IWV trends that were estimated employing very long baseline interferometry (VLBI), Global Navigation Satellite Systems (GNSS), and numerical weather prediction data (ERA5 reanalysis). We estimate coefficients for several spatial covariance functions; Hirvonen's model proves to be the most precise for our area of interest, Greater Europe. We find that the effective spatial resolution is around 56 km (for error level (p) < 0.05). Our investigations indicate that among else, altitude and proximity to the ocean are key factors affecting the IWV trend decorrelation lengths. We find good agreement between the spatially varying decorrelation lengths and established climate classification systems such as the latest Köppen-Geiger model. Moreover, the IWV trend variation as a function of data span and temporal resolution has been investigated. We find that varying the temporal resolution from one hour up to 30 days does not yield a statistically significant difference (p < 0.05) in the IWV trend and its uncertainty, provided the inherent autocorrelation is factored in and the data span remains. We also find that given the IWV time series length, the spread calculated from the estimated trends varying the start point of the time series, follows an exponential decrease σ(Δt) = 22Δt -1.7 + 0.008.

The individual space geodetic techniques have different advantages and disadvantages. For instance, the global observing network of Very Long Baseline Interferometry (VLBI) consists of much fewer stations with a poorer distribution than the one of Global Navigation Satellite Systems (GNSS). In a combination thereof, this fact can be compensated, mainly to the benefit of the former.The sensitivity level provides information on the detection capacity of observing stations based on undetectable gross errors in a geodetic network solution. Furthermore, sensitivity can be understood as the minimum value of the undetectable gross errors by hypothesis testing. The location of the station in the network and the total weight of its observations contribute to the sensitivity levels thereof. Also, the total observation number of a radio source and the quality of the observations are critical for the sensitivity levels of the radio sources. Besides these criteria, a radio source having a larger structure index has a larger sensitivity level. In this study, it is investigated whether the sensitivity levels of VLBI stations in the CONT14 campaign improve by combination with GNSS. The combination was done at the normal equation level using 153 GNSS stations in total, 17 VLBI radio telescopes, and local ties at 5 co-located stations which are ONSA-ONSALA60, NYA1-NYALES20, ZECK-ZELENCHK, MATE-MATERA, and HOB2-HOBART26 during the CONT14 campaign spanning 15 days. To evaluate the observations of GNSS and VLBI, the software of EPOS8 and VieVS@GFZ (G2018.7, GFZ, Potsdam, Germany) were used respectively. In the VLBI-only solution, FORTLEZA shows the poorest sensitivity level compared to the other VLBI radio telescopes. As a result of the combination with GNSS, it can be seen that the sensitivity levels of FORTLEZA improved by about 60% in all sessions of CONT14. It can be concluded that VLBI stations, which are poorly controlled by the other radio telescopes in the network, can be supported by the other space geodetic techniques to improve the overall quality of the solution.

The new data release of the Gaia satellite operated by the European Space Agency recently published its 3rd data release (Early Data Release 3, EDR3). The dataset contains astrometric data of about 1.8 billion objects detected at optical frequencies and therefore it outperforms any catalog of astrometric information up to date. The reference frame defined by Gaia EDR3 is aligned to the International Celestial Reference System by referring to counterparts in its realization, the third International Celestial Reference Frame (ICRF3), which is calculated from very long baseline interferometry (VLBI) observations of extragalactic objects at radio frequencies. The Gaia dataset is known to be magnitude-dependent in terms of astrometric calibration. As the objects in ICRF3, although bright at radio frequencies, are mostly faint at optical frequencies, the optically bright Gaia frame has to be linked to ICRF3 by additional counterparts besides objects in ICRF3. The non-rotation of the optically bright Gaia frame is especially important as optically bright objects can, besides astrophysical studies, be used for navigation in space, where other geodetic systems like global navigation satellite systems are out of reach. Suitable additional counterparts are radio stars which are observed by VLBI relative to extragalactic objects in ICRF3. We discuss the orientation and spin differences between the optically bright Gaia EDR3 and VLBI data of radio stars and their impact on the Gaia data usage.

The Potsdam Open Source Radio Interferometry Tool (PORT) is the very long baseline interferometry (VLBI) analysis software developed and maintained at the GFZ German Research Centre for Geosciences. Chiefly, PORT is tasked with the timely processing of VLBI sessions and post-processing activities supporting the generation of celestial and terrestrial reference frames. In addition, it serves as a framework for research and development within the GFZ's VLBI working group and is part of the tool set employed in educating young researchers. Starting out from VLBI group delays, PORT estimates station and radio sources positions, as well as Earth orientation parameters, tropospheric parameters, and station clock offsets and drifts. The estimation procedures take into account all the necessary data analysis models that were agreed on for contributing to the ITRF2020 processing activities. The PORT code base is implemented in the MATLAB (R) and Python programming languages. It is licensed under the terms of the GNU General Public License and available for download at GFZ's Git server ., Publications of the Astronomical Society of the Pacific, 133 (1028)

The next-generation, broadband geodetic very long baseline interferometry system, named VGOS, is developing its global network, and VGOS networks with a small size of 3--7 stations have already made broadband observations from 2017 to 2019. We made quality assessments for two kinds of observables in the 21 VGOS sessions currently available: group delay and differential total electron content ($\delta$TEC). Our study reveals that the random measurement noise of VGOS group delays is at the level of less than 2 ps (1 ps = 10$^{-12}$ s), while the contributions from systematic error sources, mainly source structure related, are at the level of 20 ps. Due to the significant improvement in measurement noise, source structure effects with relatively small magnitudes that are not overwhelming in the S/X VLBI system, for instance 10 ps, are clearly visible in VGOS observations. Another critical error source in VGOS observations is discrete delay jumps, for instance, a systematic offset of about 310 ps or integer multiples of that. The predominant causative factor is found to be related to source structure. The measurement noise level of $\delta$TEC observables is about 0.07 TECU, but the systematic effects are five times larger than that. A strong correlation between group delay and $\delta$TEC observables is discovered with a trend of 40 ps/TECU for observations with large structure effects; there is a second trend in the range 60 ps/TECU to 70 ps/TECU when the measurement noise is dominant., Comment: 25 pages, 11 figures, and accepted for publication in Journal of Geodesy

A new realization of the International Celestial Reference Frame (ICRF) is presented based on the work achieved by a working group of the International Astronomical Union (IAU) mandated for this purpose. This new realization, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry. The ICRF3 includes positions at 8.4 GHz for 4536 sources, supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources, for a total of 4588 sources. A subset of 303 sources among these, uniformly distributed on the sky, are identified as "defining sources" and as such serve to define the axes of the frame. Source positions are reported for epoch 2015.0 and must be propagated for observations at other epochs for the most accurate needs, accounting for the acceleration toward the Galactic center, which results in a dipolar proper motion field of amplitude 0.0058 milliarcsecond/yr (mas/yr). The frame shows a median positional uncertainty of about 0.1 mas in right ascension and 0.2 mas in declination, with a noise floor of 0.03 mas in the individual source coordinates. A subset of 500 sources is found to have extremely accurate positions at 8.4 GHz, in the range of 0.03 to 0.06 mas. Comparing ICRF3 with the Gaia Celestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames. Significant positional offsets between the three ICRF3 frequencies are detected for about 5% of the sources. Moreover, a notable fraction (22%) of the sources shows optical and radio positions that are significantly offset. There are indications that these positional offsets may be the manifestation of extended source structures. This third realization of the ICRF was adopted by the IAU at its 30th General Assembly in August 2018 and replaced the previous realization, ICRF2, on January 1, 2019., Comment: 28 pages, 22 figures

International audience; There has been growing interest in recent years in the use of homogeneously reprocessed ground-based GNSS, VLBI, and DORIS measurements for climate applications. Existing datasets are reviewed and the sensitivity of tropospheric estimates to the processing details is discussed. The uncertainty in the derived IWV estimates and linear trends is around 1 kg m−2 RMS and ± 0.3 kg m−2 per decade, respectively. Standardized methods for ZTD outlier detection and IWV conversion are proposed. The homogeneity of final time series is limited however by changes in the stations equipment and environment. Various homogenization algorithms have been evaluated based on a synthetic benchmark dataset. The uncertainty of trends estimated from the homogenized times series is estimated to ±0.5 kg m−2 per decade. Reprocessed GNSS IWV data are analysed along with satellites data, reanalyses and global and regional climate model simulations. A selection of global and regional reprocessed GNSS datasets and ERA-interim reanalysis are made available through the GOP-TropDB tropospheric database and online service. A new tropo SINEX format, providing new features and simplifications, was developed and it is going to be adopted by all the IAG services.

Atmospheric ties are affected by the differences of atmospheric parameters of space geodetic techniques at co-location sites. Similar to local ties, they could be applied along with local ties for a combination of space geodetic techniques to improve the realization of terrestrial reference frames (TRF). Theoretically, atmospheric ties are affected by the height differences between antennas at the same site and meteorological conditions. Therefore, atmospheric ties could be determined by analytical equation based on meteorological information from in situ measurements or weather model. However, there is often a discrepancy between the expected zenith delay differences and those estimated from geodetic analysis, thus potentially degrading a combined atmospheric ties solution. In this study, we analyse the time series of zenith delays from co-located GNSS antennas at Wettzell (height differences below 3 meters), for 11 years (2008–2018). GNSS observations were analyzed with Bernese GNSS software version 5.2 with double-differencing technique and relative tropospheric delay and gradients were estimated with L1, L2, and the ionosphere-free (L3) linear combination thereof. Atmospheric ties were derived analytically employing meteorological data from Global Pressure and Temperature model 3 (GPT3) and ERA5 reanalysis, as well as corrections derived from ray tracing (Potsdam Mapping Functions, PMF). The comparison shows that zenith delay differences are dominated by equipment changes. The discrepancies between atmospheric ties and estimated zenith delay differences are frequency dependent, with the L1 solutions being the least biased. For these small vertical differences, seasonal signals are not significant for all frequencies.

The quality of geodetic networks can be determined with the sensitivity and the internal reliability magnitudes when the mathematical model established for the network adjustment is questioned. Internal reliability is used for controlling an observation with the help of the other observations in the network and describes the magnitude of the undetectable gross errors by using hypothesis testing. The sensitivity level is explained as the minimum value of the undetectable gross error in the adjusted coordinate differences. In VLBI the observed sources are of major importance for the quality of the observations. In this study, it is investigated how the sensitivity levels of the sources impact the internal reliability of the observations during the continuous VLBI campaign CONT14. It is aimed to detect the poor sources and their effects statistically in the VLBI data analysis. If the sources having worst sensitivity value such as 0506-612, 3C454.3, NRAO150, and 3C345 have been excluded, the internal reliability values of the observations get better. For the rest of the sources the sensitivity distributions have been obtained as better. It can be concluded that the source structure might be significant for the quality of the observations.