The ITRF2020 marked considerable innovations compared with previous versions of the ITRF, by modeling nonlinear station motions (seasonal signals and Post-Seismic Deformation –PSD– for sites subject to major earthquakes). It also confirmed the stability of the CM-based frame origin, as sensed by SLR, at the level of or better than 5 mm and 0.5 mm/yr. For the first time in the ITRF history, the scale agreement between SLR and VLBI solutions submitted to ITRF2020 is at the level of 0.15 ppb (1 mm at the equator) at epoch 2015.0, with no drift. Motivated by these results, and for a number of reasons that will be exposed in this paper, the ITRS Center decided to regularly (yearly) update the ITRF2020, with a first update expected to be released around mid-2024. Depending on the input data availability that will be submitted by the four technique services, we expect some preliminary results that will be discussed in this presentation. Emphasis will be given to answer some critical questions, such as: how does the VLBI scale drift evolve after 2021.0? Do the SLR and VLBI scales remain in agreement? Does the SLR origin drift or not from the ITRF2020 origin after 2021.0?

How to cite: Altamimi, Z., Rebischung, P., Collilieux, X., Métivier, L., and Chanard, K.: ITRF2020 Updates : motivation and expectation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10560, https://doi.org/10.5194/egusphere-egu24-10560, 2024.

In anticipation of the first update of the 2020 realization of the International Terrestrial Reference Frame (ITRF2020), the International DORIS Service (IDS) Combination Center is participating in the estimation of DORIS stations positions/velocities as well as Earth Orientation Parameters (EOPs), using DORIS data. These computations are based on the latest weekly multi-satellite series from all four IDS Analysis Centers and two IDS Associated Analysis Centers, from January 2021 to December 2023.

The primary objectives of this study are to analyze the DORIS contribution to this first update of the ITRF2020 in terms of: (1) geocenter and scale solutions, (2) station positions and week-to-week repeatability, and (3) Earth Orientation Parameters (EOPs).

Comparisons with the IDS 19 series time extension (contributing to ITRF2020) will highlight the benefits of the new models, including the latest DORIS missions (e.g. HY-2C, HY-2D, Sentinel-6A MF), and the addition of two new IDS contributors. Additionally, this study will assess the impact of new strategies designed to mitigate perturbations caused by the South Atlantic Anomaly (SAA) on certain DORIS missions.

How to cite: Moreaux, G., Lemoine, F., Capdeville, H., Štěpánek, P., Otten, M., Nahmani, S., Pollet, A., and Schreiner, P.: IDS contribution to the first update of the ITRF2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14591, https://doi.org/10.5194/egusphere-egu24-14591, 2024.

The contribution of the International Laser Ranging Service (ILRS) to the most recent realization of the International Terrestrial Reference System (ITRS) was the result of an analysis strategy with two major modifications compared to the operational products: a modeling for stations long-term systematic errors (biases reported in the ILRS Data Handling File) and an updated model of the target signature error (satellite Centre of Mass model). Both refined models were used as input information for the ILRS contribution to the ITRF2020 (International Terrestrial Reference Frame 2020).

Thereafter, the ILRS Analysis Standing Committee (ASC) focused its effort on implementing the new reference frame in its operational products, define a strategy to improve the ongoing monitoring of the systematic errors, compute the ILRS contribution to the planned ITRF2020 update, and to include LARES-2 among the considered satellites for the operational products.

The ILRS ASC implemented the ITRF2020/SLRF2020 into all its official operational products (TRF, Earth Orientation Parameters, predicted and combined satellite orbits) and its impact was evaluated. The operational products benefit from the continuous monitoring of the station systematic errors and the frequent updates of the Data Handling File whenever a significant change in the station systematic error is observed. In the future, a change-point detection algorithm, jointly estimating the times and the number of discontinuities, will be implemented to detect potential new discontinuities in the range bias series.

The inclusion of LARES-2 among the satellites whose data are operationally analyzed will furtherly increase the robustness of the estimated parameters. Finally, the ILRS ASC activities include the benchmarking of a new analysis center (CNES) which will formally begin its own contribution in 2024.

How to cite: Sarrocco, D., Luceri, C., Basoni, A., Bloßfeld, M., Evans, K., Kuzmicz-Cieslak, M., Lemoine, F., and Bianco, G.: ILRS analysis activities after the adoption of ITRF2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19991, https://doi.org/10.5194/egusphere-egu24-19991, 2024.

The latest realizations of the ITRS, specifically the ITRF2020, the JTRF2020 and the DTRF2020, have been computed using input data series provided by the IAG technique services IVS, ILRS, IGS and IDS. They cover the entire observation period of the individual techniques until the end of 2020. Since 1996, recalculations of the ITRF have been performed approximately every 3 to 6 years. The main reason for recalculation is to ensure a high accuracy of the ITRF for current applications. In particular, seismic events that occure after an ITRF release as well as the general increase of the ITRF extrapolation error with time are key factors that cause the increase of the ITRF uncertainty.

To enhance the frequency of ITRS realizations and consequently improve the accuracy of the ITRF, the ITRS Product Center plans to calculate annual updates of the ITRF2020 starting in 2024. The IAG technique services will provide three additional years of analyzed observations (2021-2023) collected after the end of the ITRF2020 observation period in February 2024. As an ITRS Combination Center, at DGFI-TUM, we will analyze the data series w.r.t. discontinuities, post-seismic deformations and their consistency with the input data series provided for the ITRS 2020 realizations. Model changes performed in between by the individual technique services, e.g. new PCO (phase center offsets) for GNSS satellites, updated mean long-term range biases for SLR satellites or gravitational deformation models for some more VLBI antennas, are expected to have an impact on the relevant ITRF parameters (station coordinates, EOP and datum parameters). Its order of magnitude and the effect of possible inconsistencies on the DTRF solution need to be investigated. We will present the first results of our analyses and draw preliminary conclusions regarding the accuracy of a possible DTRF2020 extension.

How to cite: Seitz, M., Bloßfeld, M., Glomsda, M., Angermann, D., Rudenko, S., and Zeitlhöfler, J.: First results of the analysis of the input data series provided by the IAG technique services for the extension of xTRF2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5732, https://doi.org/10.5194/egusphere-egu24-5732, 2024.

The new ITRF20 includes a geocenter motion (GCM) model and seasonal station corrections, both with annual and semi-annual periods. The GCM model is recommended to be applied when processing space-geodetic observation data and the seasonal station corrections should be applied if no non-tidal modelling is applied for the station coordinates. In the case of the Copernicus Precise Orbit Determination (CPOD) Service this applies to the processing of GNSS data of the Sentinel satellites for precise orbit determination (POD) and to the processing of Satellite Laser Ranging (SLR) tracking data for validation of the estimated Sentinel orbits.

The CPOD Service delivers, as part of the Ground Segment of the Copernicus Sentinel-1, -2, -3, and -6 missions, orbital products and auxiliary data files for their use in the corresponding Payload Data Ground Segment (PDGS) processing chains at ESA and EUMETSAT, and to external users through the newly available Copernicus Data Space Ecosystem (https://dataspace.copernicus.eu/). It generates routinely several types of orbital products for Sentinel-1, -2, -3 and -6: predictions, near-real time (< 10 min), short-time critical (< 1.5 days) and non-time critical (< 25 days).

The POD quality control within the service is based on comparing the CPOD orbit products to orbit solutions provided by members of the accompanying Copernicus POD Quality Working Group (QWG). A combined orbit generated by CPOD from all available orbit solutions serves as reference for the comparisons. The new ITRF20 modelling opens the door to different Centre-of-Network (CoN) or Centre-of-Mass (CoM) frame realisations, so in order to avoid inconsistencies between the solutions when doing the combination and comparisons, special care must be taken.

This study aims to analyse the impact of the new ITRF20 GCM model and the seasonal station corrections on the CPOD Service products. Sentinel orbit comparisons and the corresponding processing metrics are analysed when applying the GCM model or not, with a focus on the geocenter motion modelling used by the different CPOD QWG centres.

In the SLR validation, station range biases are routinely estimated as part of the residuals analysis. Preliminary results reveal that the estimates of these range biases show smaller seasonal variations when applying the seasonal station corrections. Detailed analyses will be shown and discussed.

How to cite: Muñoz de la Torre, M. A., Fernández, C., Fernández, M., Peter, H., Féménias, P., and Nogueira Lodd, C.: Copernicus POD Service – Impact of ITRF2020 Modelling Changes on Orbit and Validation Results, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16188, https://doi.org/10.5194/egusphere-egu24-16188, 2024.

We describe the development and assessment of a new terrestrial reference frame (TRF) based on combining GPS, SLR and VLBI at the observation level over the period 2010–2022. Included in the solution, in addition to station coordinates and precise orbit solutions for all participating satellites, are Earth orientation parameters (EOP) and low-degree zonal coefficients (J2 and J3) of the geopotential. The overall solution concept grew from earlier efforts to realize a TRF using GPS data alone, capitalizing on GPS receivers on the ground and in low-Earth orbit (LEO). Here we add observations from both the SLR and VLBI techniques, which provide the foundation for traditional realizations of the TRF.

In linking the GPS and SLR techniques, our approach dispenses with traditional ground survey ties, relying exclusively on space ties from the GRACE and Jason LEO missions. In addition to SLR from these satellites, we include observations from the dedicated LAGEOS satellites, which prove particularly important for recovering low-degree gravity. A major evolution of our approach is the addition of VLBI at the observation level. Lacking a robust tie in Earth orbit for VLBI observations, we apply as constraints the published ground survey ties to nearby GPS stations, enforcing inclusion of the corresponding tracking data in the solutions. The VLBI effort is in the exploratory phase, and further tuning of the strategy is needed to better exploit collocations with both GPS and SLR. About 40% of the participating solution arcs (spanning 2010–2022) now include VLBI and support accurate recovery of UT1 as part of the EOP solution.

Though the resulting TRF solution is based on only 12.6 years of data, it is competitive with ITRF2020 in terms of fundamental frame parameters (origin and scale) and their temporal evolution, both linear and seasonal. The relative rates of origin (3D) and scale (at Earth's surface) are 0.2 mm yr^-1 and 0.1 mm yr^-1 respectively. Absolute scale (at epoch 2015.0) and 3D origin both differ by 2 mm. One advantage of our technique is that precise orbit solutions for both GRACE and Jason missions, defined in the realized TRF, are byproducts of the overall solution. We use the Jason orbit solutions to characterize the impact of contemporary TRF errors on sea level variations (both global and regional) and discuss the implications of these results.

How to cite: Haines, B., Bertiger, W., Desai, S., Elmer, M., Heflin, M., Kuang, D., Lanyi, G., Miller, M., Naudet, C., Peidou, A., Ries, P., Tolstov, A., Wu, X., and Zivkov, N.: A New Realization of the Terrestrial Reference Frame: Combining GPS, SLR and VLBI at the Observation Level from 2010–2022, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13524, https://doi.org/10.5194/egusphere-egu24-13524, 2024.

Mission Genesis of the European Space Agency (ESA) has been approved for launch in 2027. Genesis will be the first satellite in orbit to have a dedicated Very Long Baseline Interferometry (VLBI) transmitter on board, next to Global Navigation Satellite System (GNSS) and Doppler Orbitography and Radiopositioning Integrated on Satellite (DORIS) receivers as well as a Satellite Laser Ranging (SLR) reflector; consequently, Genesis will realize a space tie combining all geometric space geodetic techniques. If perfectly calibrated, the space tie will enhance and improve local ties measured on the ground. The following scenario is possible: If the orbit of Genesis is determined from the satellite techniques alone, the station coordinates of the VLBI radio telescopes in the "satellite frame" can be derived by VLBI observations to Genesis, thereby assessing the tie with the "VLBI frame", realized with decades of VLBI observations to quasars.

We present our plans to devise observing strategies for VLBI to reach accuracies as defined in the Genesis white paper. We start with our findings for VLBI transmitters on Galileo satellites, before we show the simulation results for the VLBI transmitter on Genesis. We illustrate the advantages of the Genesis satellite at 6000 km altitude compared to Galileo satellites in terms of sky coverage and accuracy of station coordinates, but also in terms of orbit estimation. Furthermore, we provide an outlook on geodetic parameters, which could not be determined with VLBI so far but will be possible with Genesis.

How to cite: Böhm, J., Wolf, H., and Kern, L.: Benefits for the terrestrial reference frame with VLBI observations to Genesis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3821, https://doi.org/10.5194/egusphere-egu24-3821, 2024.

The techniques of space geodesy, comprising the four techniques, Global Navigation Satellite Systems (GNSS), Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Doppler Orbitography and Ranging Integrated by Satellite (DORIS) are currently reaching a measurement resolution in the range of 1 part per billion for the terrestrial reference frame. However, a small set of discrepancies remain evident within each of the techniques as well as in the combination of different techniques. Systematic measurement errors are causing this and problems in the local ties between the reference points of the various measurement systems and biases in the atmospheric refraction correction have long been suspected as the main contributors. 

However, it turns out that errors in the internal delay compensation of the measurement systems are also a significant contributor. They are extremely hard to detect, since they are small and come with different characteristics. It is understood that the experienced delay variation is related to a complex pattern of ambient temperature variation inside of the electronic devices. These changes relate to the micro-climate of the electronic signal path and can both be slow and highly variable. With the advent of high bandwidth mode-locked lasers and active delay compensation in the optical domain, it is now possible to utilize coherent time as an independent probe for instrumental signal delays. 

The research unit FOR5456 of the German National Science Foundation (DFG) has been formed in 2022 in order to apply and investigate active delay compensation to the techniques of space geodesy. This talk introduces the application of coherent time in space geodesy and its potential to act as a novel tie in fundamental stations.

How to cite: Schreiber, K. U.: Clock Ties: A novel approach for the reduction of systematic errors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9298, https://doi.org/10.5194/egusphere-egu24-9298, 2024.

The observations of the four space-geodetic techniques (GNSS, SLR, VLBI, DORIS) are used for the computation of the International Terrestrial Reference Frame (ITRF) and for the realisation of its geodetic datum as well as (except VLBI) for the precise orbit determination of Earth-observing satellites. The ITRF itself is fundamental for a broad variety of scientific and societal applications such as Earth monitoring and positioning, relevant for satellite companies, logistics, and finally new techniques like autonomous driving. However, all four techniques still suffer from unknown systematic measurement or modelling errors which makes the estimation of bias parameters inevitable.

The Geodetic Observatory Wettzell (GOW), Germany, with its unique and ideal test environment comprising multiple GNSS antennas, three VLBI antennas, two SLR telescopes, a DORIS beacon, and numerous other sensor systems provides the opportunity to systematically identify, quantify, understand, and compensate system-specific measurement errors. The installation of a so-called common target (CT) in 2017 and the realisation of a common clock (CC) enables a profound analysis of space-geodetic measurements. The CT is connected to the clocks of the space-geodetic techniques via delay-compensated fibre links which allows so-called ‘closure in time’ experiments.

Within the recently established DFG research unit ‘Clock Metrology: Time as a New Variable in Geodesy’, one project at DGFI-TUM focuses on the investigation of time-related and technique-specific errors using ‘closure in time’ experiments at the GOW. Therefore, completed experiments involving the CT are analysed and enhanced experiments will be planned and conducted. With the aid of the CC, the source of present measurement discrepancies will be investigated and resolved. We present the current state of our work with the main focus on the analysis of VLBI and SLR observations of the two SLR and three VLBI systems at GOW.

How to cite: Zeitlhöfler, J., Bloßfeld, M., Neidhardt, A., and Eckl, J.: Identification of system-specific observation errors for SLR and VLBI telescopes at GOW, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10691, https://doi.org/10.5194/egusphere-egu24-10691, 2024.

Global Navigation Satellite Systems (GNSS) are based on measuring the time that elapses between the signal’s transmission at the satellite and its reception on the ground. Therefore, clock information is required on both sides. While the GNSS satellites are equipped with atomic clocks, ground stations usually use the time information from the internal oscillator of their GNSS receiver, which has a much lower time-keeping performance compared to the satellite clocks. Nevertheless, some continuously operated tracking stations obtain their time information from an external atomic clock, as it is the case with many stations of the International GNSS Service (IGS).

To compensate for synchronization errors, current GNSS analysis models generally introduce clock biases for satellites and receivers into the observation equations. The often-made assumption of a pure white noise behavior for the estimated clocks may lead to high correlations with other geodetic parameters, such as the radial orbit error for the satellite clock, or the station height coordinate and tropospheric delay parameters for the station clock. A general solution to this problem is to reduce the amount of unknown clock parameters by modeling them in the adjustment process. In order to be modeled adequately, the corresponding clock must have a high degree of stability, which is particularly crucial for the ground stations.

In this contribution, we investigate the clock stability of globally distributed IGS tracking stations. Those IGS stations, that are steered by an external Hydrogen-Maser (H-Maser) clock, are considered in a global network analysis over a period of several weeks. The generated clock products are used to compare the frequency stabilities within the station network, as well as with the mean behavior of GPS and Galileo satellite blocks. After some further research on stations with significantly higher deviations, the final result of this contribution will be a set of reliable ground stations, that will serve as a basis for future clock modeling approaches at GFZ.

How to cite: Widczisk, J. S., Männel, B., and Wickert, J.: Investigations into GNSS clock biases in a global network of IGS H-Maser stations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6472, https://doi.org/10.5194/egusphere-egu24-6472, 2024.

Recent advances in SLR data analysis are described in ‘The ILRS contribution to ITRF2020’, Pavlis and Luceri (2022). Improved vertical resolution can now complement the more easily resolved horizontal motion.
We examine the emerging results from ITRF2020 and prioritize the most accurate geodetic products.
Height variations at SLR stations in tectonically stable regions of Australia and North America exhibit steady and consistent height rates. 
Robust long-term vertical motion models also enable precise monitoring of behavior at higher frequencies: annual, tidal, and diurnal. Tracking operation procedures have been implemented to reduce and monitor ranging measurement accuracy. Comparisons of the vertical signals in SLR, VLBI, DORIS and GNSS systems allow robust accuracy monitoring at co-located stations.
Data handling techniques are outlined to enhance the isolation of the geodetic signals and enable their application to Earth and Ocean Model development 

How to cite: Dunn, P., Husson, V., and Szwec, C.: Accurate Long Term Height Determination at SLR Stations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11287, https://doi.org/10.5194/egusphere-egu24-11287, 2024.

Highly accurate Terrestrial Reference Frames (TRF) – based on the combination of the four space-geodetic techniques Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Global Navigation Satellite Systems (GNSS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) – are the fundamental backbone for a broad range of applications like land surveying, the geodetic monitoring of geophysical processes within the Earth system or navigation on and near the Earth’s surface. Recent efforts at the Geodetic Observatory Wettzell (GOW), Germany, aim at a transition from the purely geometric link between space-geodetic techniques via local ties as the current standard to an innovative quasi-error-free combination based on a common clock (CC) and a common target (CT).

Once the CC/CT-based infrastructure at GOW is fully developed and operational, it will be possible to uncover systematics between the space-geodetic techniques as well as individual instruments. However, to guarantee the long-term accuracy and stability of the TRF, it is indispensable to know and, if possible, to eliminate the systematics over the entire observation period of the techniques. A prerequisite for this is to compile an inventory of the existing discrepancies between the techniques and their possible causes.

The DFG research unit ‘Clock Metrology: Time as a New Variable in Geodesy’ features a joint project by DGFI-TUM and Uni Bonn with focus on developing a new CC-/CT-based approach to combine the space-geodetic techniques. As a basis, we develop an approach to analyse and cross-compare station position time series from different instruments/techniques observed over several decades. Based on the example of GOW co-locating all four space-geodetic techniques, we investigate absolute station position time series consistently aligned to the datum of the DTRF2020, DGFI-TUM’s most-recent realisation of the International Terrestrial Reference System (ITRS), as well as differential time series eliminating datum-realisation-related variations in the time series of one technique. Finally, we prepare a pool of metadata (log files, data time series from meteorological sensors and weather models, estimated clock and tropospheric parameters, etc.) and include these data in the analysis to identify causes of systematics. 

From the analyses, discontinuities, time-variable drifts and the spectra of intra- and inter-technique position difference time series between individual instruments at GOW can be identified and interpreted. The result of the work is an inventory which lists both, known and previously unmodelled systematics, and, as far as possible, their causes, thus providing the basis for the consistent combination of techniques in a common space-time.

How to cite: Kehm, A., Seitz, M., and Glaser, S.: Analysis of long time series of space-geodetic techniques at co-location sites to identify technique- and instrument-specific systematics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8612, https://doi.org/10.5194/egusphere-egu24-8612, 2024.

LAser RElativity Satellite 2 (LARES-2) successfully joined the constellation of geodetic satellites tracked by Satellite Laser Ranging (SLR) stations on July 13, 2022. LARES-2 has a spherical shape and a very favorable area-to-mass ratio that minimizes the non-gravitational orbit perturbations. Due to very small retroreflectors, the spread of center-of-mass corrections for different detectors installed at SLR sites is much smaller than for LAGEOS satellites. LARES-2 orbits at a similar height as LAGEOS-1, however, with a complementary inclination angle of 70° forming a butterfly configuration together with LAGEOS-1.

Although the primary objective of LARES-2 is verification of the Lense-Thirring effect emerging from general relativity, the satellite also has a substantial impact on the geodetic parameters derived from SLR observations. We process 18 months of LARES-2 data and compare the LAGEOS-1/2 solutions with the combined LAGEOS-1/2+LARES-2 solutions. We show the impact of LARES-2 on the (1) SLR station coordinates, (2) pole coordinates, (3) length-of-day excess, (4) low-degree gravity field parameters focusing on C₂₀ and C₃₀ coefficients, (5) scale of the reference frame, (6) geocenter motion. We show that LARES-2 can especially improve the Z component of the geocenter coordinates and de-correlate C₂₀ from the length-of-day parameter. The secular drifts of the ascending nodes for LARES-1 and LAGEOS-1 caused by C₂₀ are the same in terms of absolute values but with opposite signs. This allows us to successfully separate the measurements of length-of-day excess (or the UT rate) from the C₂₀-induced changes. We also analyze the empirical accelerations acting on LARES-2 which result from unmodeled non-gravitational orbit perturbations, such as thermal effects, and compare them to those observed for LAGEOS satellites. The observation geometry of LARES-2 is especially beneficial for stations located at high and medium latitudes, which allows it to improve the estimation of station coordinates provided by LAGEOS-1/2. Therefore, LARES-2 substantially contributes not only to general relativity and fundamental physics but also to space geodesy improving the future realizations of the international terrestrial reference frames.

How to cite: Sośnica, K., Gałdyn, F., Najder, J., Zajdel, R., and Strugarek, D.: Contribution of LARES-2 to the realization of reference frames, deriving Earth rotation and gravity field parameters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6189, https://doi.org/10.5194/egusphere-egu24-6189, 2024.

The International Terrestrial Reference System is realised in the form of multi-year reference frames such as the International Terrestrial Reference Frame (ITRF) or in the form of epoch reference frames relying on short observation time spans up to several weeks. The realisation is based on the combination of space-geodetic techniques, namely the global navigation satellite systems (GNSS), satellite laser ranging (SLR), very long baseline interferometry (VLBI) and Doppler orbitography and radiopositioning integrated by satellite (DORIS). In some ITRF and epoch reference frame solutions, SLR and VLBI are responsible for realising the datum parameters origin (only by SLR) and the scale, while the orientation of the network with respect to the Earth’s body is maintained by a mathematical constraint. The integration of the techniques is achieved by introduction of local ties (LTs) at co-located sites, i.e., by ground-based measurements of difference vectors between the technique-specific reference points. High accuracy of current LTs between techniques and the establishment of new co-location sites are necessary to provide (and further improve) a reliable realisation of the geodetic datum. Co-location sites with the SLR technique are of particular significance as this is the only technique that enables the realisation of a terrestrial reference frame origin with a high level of accuracy. As previous studies demonstrate, the performance of the observational networks has a significant impact on the accuracy and stability of the corresponding datum realisation, especially for epoch reference frames.

This study aims to examine how improving the performance of the existing network of co-located SLR stations could affect the quality of determined datum parameters. The considered simulation scenarios study the performance of SLR stations co-located with the VLBI technique and improve the performance of those that do not meet the standards set by the International Laser Ranging Service (ILRS). Moreover, it is examined how significant the improvement of the datum parameters is in the case of extending the SLR network with stations located nearby existing VLBI telescopes (due to a ‘better’ datum transfer via a higher number of local ties).

How to cite: Najder, J., Kehm, A., Bloßfeld, M., Sośnica, K., and Glomsda, M.: Improved geodetic datum realization based on simulation studies for co-located SLR-VLBI stations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-897, https://doi.org/10.5194/egusphere-egu24-897, 2024.

Achieving a high-precision geodetic spatial reference depends on a thorough understanding of the equipment-specific sources of error of phase centre corrections (PCCs) of Global Navigation Satellite System (GNSS) receiver antennas. GNSS station operators and network analysers are constantly challenged regarding consistent PCCs, such as in the latest IGS Repo3 project. The challenges are, on the one hand, that not for all antennas in the network multi-GNSS calibrations are available. On the other hand, not all antennas are individually calibrated, so that type mean combinations with individual PCCs have to be used.

Even small differences between PCCs can significantly affect position accuracy, troposphere modelling, and GNSS time and frequency transfer. Such deviations manifest differently depending on used hardware, software, and data processing approach. A generalised and easily accessible benchmark for assessing the quality of PCCs remains difficult to find. There is a lack of easy-to-apply and common quality assessments of PCCs when comparing individual calibrations versus a type mean and results from the various calibration facilities and calibration methods among each other.

In response to this challenge, a global initiative involving nine calibration organisations has launched a comprehensive ring calibration campaign. By sharing six constructionally different antenna samples for calibration and presenting the subsequent results, this collaborative effort aims to enhance (1) the consistency of calibration methods and facilities, (2) develop a validation strategy, and (3) provide insights into the stability of receiver antenna calibrations.

This contribution provides an overview of the current status of this campaign, initiated one and a half years ago, outlines the calibration and evaluation concept for carrier phase patterns. First initial results from consulting contributors are presented and the roadmap towards a standardised, robust quality assessment framework for PCCs will be covered.

How to cite: Kersten, T., Bilich, A., Sutyagin, I., and Schön, S.: A Global Collaboration to Enhance GNSS Receiver Antenna Calibration: The IGS Antenna Ring Campaign, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17665, https://doi.org/10.5194/egusphere-egu24-17665, 2024.

How to cite: Kierulf, H. P.: Glacial induced variations in the uplift – a challenge for the reference frame , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9527, https://doi.org/10.5194/egusphere-egu24-9527, 2024.

Determining time series of bedrock motion from global navigation satellite systems (GNSS) in Antarctica is one method to investigate geodynamical processes (e. g. glacial isostatic adjustment (GIA)). To achieve coordinate time series with the highest possible precision, a stable realization of the International Terrestrial Reference Frame (ITRF) is a crucial precondition. Regional networks of GNSS stations have the advantage of exhibiting small common mode errors and allowing to infer accurate velocity estimates. However, to achieve high consistency, it is necessary to base the analysis on a global network of International GNSS Service (IGS) stations, constraining them to their ITRF positions. Various approaches can be found in the literature on how the global solution is constrained to the ITRF and how the regional solution is transformed to the global solution. These approaches have different effects on the resulting time series and inferred parameters, e. g. absolute coordinates, linear trends, and noise properties. In this study, approximately 30 Antarctic GNSS stations are processed together with a global GNSS network of overall 200 IGS stations. We investigate different approaches to realize the ITRF and discuss the inferred results. To be consistent w. r. t. the general processing, we use a consistent set of GNSS observation data, GNSS products (e. g. orbit corrections), and apply the Bernese GNSS Software v5.4. In this way, the residuals between the different coordinate time series can be assumed to be due to the differences in reference frame realization. In our time series analysis, we put particular emphasis on linear trends because these are most important for GIA studies.

How to cite: Heidrich-Meisner, K., Buchta, E., and Scheinert, M.: GNSS for geodynamics in Antarctica: Sensitivity to reference frame realizations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19608, https://doi.org/10.5194/egusphere-egu24-19608, 2024.

Canada and the US are collaboratively implementing a new dynamic, geometric reference frame for North America (NA) known as the North American Terrestrial Reference Frame of 2022 (NATRF2022). It will be a plate-fixed reference frame based on ITRF2020/IGS20 that is kept aligned to the NA tectonic plate using an Euler pole rotation. We have estimated Euler pole parameters (EPP) for NA based on the spherical model of Earth using different sets of stations and compared our results to other sources, including those recently obtained by Kreemer (2023) under contract to the U.S. National Geodetic Survey. The velocity field used for our analyses are those from Kreemer (2023) that were obtained for 4274 stations using GipsyX precise point positioning in the IGb14 reference frame and corrected for non-tidal and atmospheric loading as well as hydrological loading obtained from GRACE. A challenge for our analyses is the impact of ongoing glacial isostatic adjustment (GIA) on the horizontal velocities which can bias the EPP estimation. To mitigate such biases, we have used the horizontal component of the ICE-6G model to remove the GIA effect from the velocity field. Following Kreemer (2023),  we have determined a small set of homogeneously distributed stations that closely reproduce Kreemer’s EPP estimates. We also considered that users of the new reference frame would prefer that the intra-plate motions be minimized across the entire continent for conventional use and have therefore computed the best fitting EPP that minimizes the overall intra-plate motions across the entire continent.  In addition, we chose only good stations that met certain statistical criteria for the residuals and had stable monumentation, preferably anchored to bedrock. Finally, we compare the EPP estimates for all these sets of stations with and without GIA removed using statistical tests and descriptive statistics. The differences in resulting intra-plate velocities across the continent are also discussed for each test.

How to cite: Goudarzi, M. A. and Craymer, M.: Estimating Euler pole parameters for NATRF2022, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13398, https://doi.org/10.5194/egusphere-egu24-13398, 2024.

Multi-station (or time series) stacking is a fundamental task in the realization of a terrestrial reference frame (TRF), which is, however, rank-deficient in nature due to the arbitrarily selected target frame. In practice, such a model is approximated as a linear form and the classical free network theory is applied. It is known that the one-step adjustment only works in cases where the nonlinearity (measured by curvature) is moderate, and the initial point is very good; for TRF, it requires the deformable networks with a small enough time span for the network shape to be nearly unaltered. However, these assumptions can be nullified for the cases such as large time span and the integration of some local survey results. To address these limitations, we propose to solve the geodetically meaningful and numerically exact least-squares (LS) solution for the multi-station stacking model. The contributions are summarized as follows:

• The original nonlinear LS objective for the multi-station stacking model is investigated and its characteristics are analyzed;
• The nonlinear Baarda’s S-transformation is formulated for such a problem, which transforms different LS solutions that share the same network configuration;
• Two ways to obtain the geodetically meaningful solution are proposed, i.e., the minimally-constrained solution and the nearest-solution, where the latter originates from the inner-constraint solution in the linear case.
• The iterative schemes to obtain the two types of solutions are derived, which are shown to be essentially the truncated Gauss-Newton method. In addition, some techniques such as finite differences are employed to enhance numerical stability.

The developed theory is verified by the real examples.

How to cite: Hu, Y., Fang, X., and Zeng, W.: Nonlinear least-squares solution for the multi-station stacking problem in realizing a terrestrial reference frame, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14503, https://doi.org/10.5194/egusphere-egu24-14503, 2024.