G2.2 | Terrestrial Reference Frames: systematic errors in modern space geodetic observation techniques, determination and applications
EDI
Terrestrial Reference Frames: systematic errors in modern space geodetic observation techniques, determination and applications
Convener: Xavier Collilieux | Co-conveners: Mathis Bloßfeld, Claudio Abbondanza, Susanne Glaser, Radosław Zajdel, Jan Kodet, Eva Hackmann
Orals
| Mon, 15 Apr, 16:15–18:00 (CEST)
 
Room G2, Tue, 16 Apr, 08:30–10:15 (CEST)
 
Room -2.91
Posters on site
| Attendance Tue, 16 Apr, 16:15–18:00 (CEST) | Display Tue, 16 Apr, 14:00–18:00
 
Hall X2
Orals |
Mon, 16:15
Tue, 16:15
The Terrestrial Reference Frame (TRF) is critical for monitoring the Earth's rotation in space, as well as for many Earth science applications that need absolute positioning and precise orbit determination of near-Earth satellites. TRFs are determined by modeling space geodetic observations at a ground network of stations and require the estimation of a large number of parameters including station positions and Earth Orientation Parameters. Nowadays, the major limiting factors for a reliable interpretation of the observed quantities and consequently a resilient monitoring of global change phenomena are measurement biases, systematic errors caused by measurement electronics, as well as imperfect background models.
This session welcomes contributions that develop strategies to overcome systematics in space geodetic observing systems such as long-term mean range biases in SLR observations, gravitational deformation of VLBI antennas, GNSS phase center variations, local tie discrepancies in global TRF solutions, etc. This session also focuses on the handling of time in geodesy, delay-compensated time and frequency distribution, time transfer (via space-based atomic clocks and common ground clocks), systematic errors in measurement electronics, optical clocks and quantum sensing for geodetic applications. The integration of optical clocks in Geodesy provides novel opportunities and new challenges, while satellite missions like ACES provide the means to transfer time at unprecedented levels of accuracy.
The second objective of this session is to bring together contributions from individual technique services, space geodetic data analysts, ITRS combination centres and ITRF users, with a broad range of applications from geosciences to society, to discuss the results and scientific applications of ITRF2020 and other realizations. The determination of new local tie vectors at co-location sites, the assessment of observed non-linear station motions, including geocentre motion, through space geodetic techniques by comparison with physics-based deformation models is of particular interest. Furthermore, the handling of co-located instruments of individual techniques onboard satellite missions (space ties) for TRF realization is explored. In general, presentations evaluating the terrestrial reference frame, on new or improved combination strategies, or regarding any type of development that potentially improves future ITRF solutions are highly encouraged.

Orals: Mon, 15 Apr | Room G2

Chairpersons: Claudio Abbondanza, Eva Hackmann, Mathis Bloßfeld
16:15–16:20
16:20–16:30
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EGU24-10560
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On-site presentation
Zuheir Altamimi, Paul Rebischung, Xavier Collilieux, Laurent Métivier, and Kristel Chanard

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.

16:30–16:40
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EGU24-14591
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On-site presentation
Guilhem Moreaux, Frank Lemoine, Hugues Capdeville, Petr Štěpánek, Michiel Otten, Samuel Nahmani, Arnaud Pollet, and Patrick Schreiner

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.

16:40–16:50
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EGU24-19991
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On-site presentation
David Sarrocco, Cinzia Luceri, Antonio Basoni, Mathis Bloßfeld, Keith Evans, Magda Kuzmicz-Cieslak, Frank Lemoine, and Giuseppe Bianco

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.

16:50–17:00
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EGU24-5732
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On-site presentation
Manuela Seitz, Mathis Bloßfeld, Matthias Glomsda, Detlef Angermann, Sergei Rudenko, and Julian Zeitlhöfler

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.

17:00–17:10
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EGU24-16188
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On-site presentation
Miguel Angel Muñoz de la Torre, Carlos Fernández, Marc Fernández, Heike Peter, Pierre Féménias, and Carolina Nogueira Lodd

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.

17:10–17:20
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EGU24-13524
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Virtual presentation
Bruce Haines, Willy Bertiger, Shailen Desai, Matthias Elmer, Michael Heflin, Da Kuang, Gabor Lanyi, Mark Miller, Chuck Naudet, Athina Peidou, Paul Ries, Alex Tolstov, Xiaoping Wu, and Nikki Zivkov

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.

17:20–17:30
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EGU24-3821
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On-site presentation
Johannes Böhm, Helene Wolf, and Lisa Kern

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.

17:30–17:50
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EGU24-9298
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solicited
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On-site presentation
Karl Ulrich Schreiber

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.

17:50–18:00
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EGU24-10691
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ECS
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On-site presentation
Julian Zeitlhöfler, Mathis Bloßfeld, Alexander Neidhardt, and Johann Eckl

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.

Orals: Tue, 16 Apr | Room -2.91

Chairpersons: Radosław Zajdel, Jan Kodet, Susanne Glaser
08:30–08:35
08:35–08:45
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EGU24-6472
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ECS
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On-site presentation
Jari Simon Widczisk, Benjamin Männel, and Jens Wickert

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.

08:45–08:55
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EGU24-11287
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On-site presentation
Peter Dunn, Van Husson, and Christopher Szwec

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.

08:55–09:05
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EGU24-8612
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ECS
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On-site presentation
Alexander Kehm, Manuela Seitz, and Susanne Glaser

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.

09:05–09:15
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EGU24-6189
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On-site presentation
Krzysztof Sośnica, Filip Gałdyn, Joanna Najder, Radosław Zajdel, and Dariusz Strugarek

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.

09:15–09:25
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EGU24-897
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ECS
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On-site presentation
Joanna Najder, Alexander Kehm, Mathis Bloßfeld, Krzysztof Sośnica, and Matthias Glomsda

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.

09:25–09:35
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EGU24-17665
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On-site presentation
Tobias Kersten, Andria Bilich, Igor Sutyagin, and Steffen Schön

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.

09:35–09:45
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EGU24-9527
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On-site presentation
Halfdan Pascal Kierulf

Arctic areas are heavily affected by climate change. The temperature is increasing, the permafrost is melting, the sea ice is disappearing, and the glaciers are retreating. The elastic response of the changes in the glacier affects the earth crust. Locally on Greenland or Svalbard the uplift can reach several centimetres per year. The ice melting in Greenland is so large that it affects the land uplift in large parts of the Northern hemisphere.

The geodetic observatory in Ny-Ålesund is a key station in the global geodetic network. It is the northern most fundamental station, containing all the main geodetic techniques and important for the realisation of the ITRF. However, its stability has been questioned. The observatory experience variations in the uplift on seasonal, inter-annual, decadal and longer timescales. The uplift for a moving window of 5-years periods has increased from below 6 mm/yr in the 1990 to more than 12 mm/yr today. This has challenged the realisation and stability of global and regional reference frames. 

We have modelled the elastic response of glacier changes based on various glaciological sources. These results will be presented. We will in particular compare the elastic uplift with geodetic time-series from Ny-Ålesund and other GNSS in Svalbard and discuss how this could affect reference frames. Could for instance the VLBI scale issue in ITRF2020 be related to glacial changes? 

We found that the variations in the uplift can be explained by the glacier changes and close to 50% of the VLBI scale drift can be explained by glacier related accelerating uplift. 

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.

09:45–09:55
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EGU24-19608
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ECS
|
On-site presentation
Karl Heidrich-Meisner, Eric Buchta, and Mirko Scheinert

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.

09:55–10:05
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EGU24-13398
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On-site presentation
Mohammad Ali Goudarzi and Michael Craymer

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.

10:05–10:15
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EGU24-14503
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On-site presentation
Yu Hu, Xing Fang, and Wenxian Zeng

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.

Posters on site: Tue, 16 Apr, 16:15–18:00 | Hall X2

Display time: Tue, 16 Apr, 14:00–Tue, 16 Apr, 18:00
Chairperson: Xavier Collilieux
X2.1
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EGU24-16905
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ECS
Lisa Kern, Hana Krasna, Johannes Böhm, and Axel Nothnagel

With the establishment of the International Terrestrial Reference Frame 2020 (ITRF2020), investigations revealed an unexpected positive VLBI scale drift after about 2014.0. Given the crucial role of Very Long Baseline Interferometry (VLBI) in determining the ITRF scale, this peculiar behavior raises concerns. Within the VLBI community, several studies have been conducted to decipher the cause behind this pattern. A recent study by the Onsala Space Observatory (OSO) explored the introduction of additional discontinuities in the station positions of NYALES20 and/or in the positions of MATERA, WETTZELL, and ONSALA60 due to repairs or replacements. They found that the introduction significantly mitigated the scale drift with respect to ITRF2020.

Utilizing our newest state-of-the-art combination software, VieCompy, developed at the Vienna Center for VLBI, we independently assess the impact of these additional breaks on session-wise estimated scale through a combination of VLBI sessions at the normal equation level.

How to cite: Kern, L., Krasna, H., Böhm, J., and Nothnagel, A.: Verifying the impact of additional breaks in station coordinates on VLBI scale drift, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16905, https://doi.org/10.5194/egusphere-egu24-16905, 2024.

X2.2
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EGU24-16881
Francesco Matonti, Adam Miller, and Joanna Wnuk

Absolute positioning is a requirement for public and private GNSS networks with a global coverage due to integrations of positioning solutions into mid-tier and mass-market solutions, such as autonomous driving systems, together with an ever-increasing demand for global positioning applications. 
Global correction service providers need to support and deliver the service in a global reference frame whilst still maintaining the local (regional) reference frames. 

A quality check of the RINEX data from the HxGN SmartNet GNSS Network is performed before a computation of a daily solution, which is calculated using precise orbits and following the guidelines of the EPN Analysis Centres and Bernese GNSS Software (Dach, 2015). 

The non-linear station movements are checked to assess the stability of each station of the network. In case of a position change, a coordinate update is evaluated and applied. A periodic computation in ITRF2020 is also performed to update the coordinates in the GNSS network. A regular update and maintenance of the coordinates of the stations in the network is crucial to maintain the high precision of the correction service. The HxGN SmartNet GNSS network consists of more than 5300 GNSS reference stations and is used to provide RTK corrections at global and regional scale. Additionally, it provides support for PPP and SSR based solutions. The ITRF2020 coordinates of some sites are presented and discussed to evaluate examples of non-linear motion and what solution has been adopted to keep a homogeneous set of coordinates, whilst also considering the increasing impact of ionospheric activity. 

KEYWORDS: GNSS reference station network, Bernese GNSS 5.2, Leica CrossCheck, Leica GNSS Spider, HxGN SmartNet 

References: 
Dach, R., S. Lutz, P. Walser, P. Fridez (Eds); 2015: Bernese GNSS Software Version 5.4. User manual, 
Astronomical Institute, University of Bern, Bern Open Publishing. DOI: 10.7892/boris.72297; ISBN: 978-3-906813-05-9. 

 

How to cite: Matonti, F., Miller, A., and Wnuk, J.: ITRF2020 usage and maintenance in a global dense network for commercial applications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16881, https://doi.org/10.5194/egusphere-egu24-16881, 2024.

X2.3
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EGU24-6698
Claudio Abbondanza, Toshio M Chin, Richard S Gross, and Michael B Heflin

In recent years, new determinations of the ITRF based on full-blown reanalyses of frame inputs from the four space-geodetic techniques have been produced at intervals of 3-6 years. Between frame determinations, ITRF users must rely on predictions of station positions of the reference stations included in the frame whose accuracy rapidly degrades over time, thus causing errors in the products derived from such predictions.    
JTRF2020 is the most recent TRF solution computed at JPL by assimilating the frame input data submitted by  IGS, IVS, ILRS, and IDS for ITRF2020. Determined with a square-root information filter and Dyer-McReynolds smoother algorithm, JPL frame products lend themselves to being updated rather easily as long as frame inputs from the four technique centers consistent with the frame-defining data set are readily available. 
In this presentation, we will discuss and test SREF (Square-root Reference frame Estimation Filter) updating capabilities in relation to JTRF2020. We will upload state estimate and its covariance computed at the last step of JTRF2020, and update them by assimilating at daily intervals the extended frame inputs made available by IGS (Repro3 extension), IVS (BKG operational combined series with loading effects restored using loading information from the NASA GSFC solution), ILRS (v170 and v171), and IDS (wd20) from 2021 through the end of 2022.   
Discussions will focus on the peculiarities of the extended frame inputs in relation to the data submitted for the ITRF2020 computation, and in particular on the data pre-processing and transformations we’ve applied to the extended frame inputs in order to ensure consistency with JTRF2020. We’ll also assess the quality of the JTRF2020 updates in terms of frame-defining parameters.     

How to cite: Abbondanza, C., Chin, T. M., Gross, R. S., and Heflin, M. B.: Updating JTRF2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6698, https://doi.org/10.5194/egusphere-egu24-6698, 2024.

X2.4
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EGU24-8737
Maria Karbon, Santiago Belda, Esther Acuze, Mariana Moreira, Alberto Escapa, and Jose Manuel Ferrándiz

Geodesy provides the highest precision and accuracy International Terrestrial Reference Frame, International Celestial Reference Frame and Earth Orientation Parameters. However, in our processing chain, we take mathematical shortcuts, drop higher order polynomials, assume linearity where it is no longer valid, omit correlations and colored noise, and use outdated models. If intra- or inter-technique combinations are done, they happen at different stages, and different methods are employed. The datums applied to the reference frames are inherited over decades, accumulating all uncertainties of their predecessors. Dependencies between the reference frames and the EOP are largely ignored. Finally we inflate our errors by a predefined factor, to somehow account for all of that.
This is just a short list of the inconsistencies within our main products. Even for a specialist it will be almost impossible to list them all, for a mere end-user its an insurmountable task. In this work we will investigate these central products of geodesy, focusing mainly on the errors, their derivation, and significance from a user perspective. We look exemplarily at various official IVS and IAG products, and their reported errors. We investigate how transparent their nature and derivation is for the final user, if the parameters in question follow our physical understanding of the matter, and what insight we might gain from them.

How to cite: Karbon, M., Belda, S., Acuze, E., Moreira, M., Escapa, A., and Ferrándiz, J. M.: A critical look at the reported errors of geodetic products, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8737, https://doi.org/10.5194/egusphere-egu24-8737, 2024.

X2.5
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EGU24-20115
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ECS
Roland Hohensinn and Yehuda Bock

Through a parametric fit to daily vertical displacement time series from European Permanent GNSS stations, we conducted a statistical sensitivity analysis focusing on Vertical Land Motion (VLM) – specifically, station velocity (linear trend). We compared two independent corrections to raw observed displacements: non-tidal atmospheric, oceanic, and hydrological loading displacements, as well as a correction for common mode errors (CME). Our methodology involved selecting the most realistic stochastic models based on information criteria, analyzing GNSS-observed displacements and identifying discrepancies with loading model predictions. We also employed restricted maximum likelihood estimation (RMLE) to mitigate low-frequency noise biases, enhancing the reliability of velocity uncertainty estimates.

Our results demonstrate that 1) an autoregressive, power-law, and white noise model combination is preferred for uncorrected GNSS VLM data, 2) when compared to the corrected cases, this model choice yields lower improvement rates in trend sensitivity than previously reported, and 3) RMLE reveals that for many stations, noise is optimally modeled by a combination of random-walk, flicker-noise, and white noise. We report median trend sensitivity and detection rates of about 0.5 mm/year (with best results for the CME-corrected case), approaching the GGOS goal of a 0.1 mm/year precision, crucial for sea level studies and other applications.

How to cite: Hohensinn, R. and Bock, Y.: Gauging the Sensitivity of GNSS for Resolving Vertical Land Motion Over Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20115, https://doi.org/10.5194/egusphere-egu24-20115, 2024.

X2.6
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EGU24-4333
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ECS
Dariusz Strugarek and Radosław Zajdel

The Satellite Laser Ranging (SLR) technique is used to independently validate the microwave-based satellite orbit products. In the so-called SLR validation, the orbit quality is assessed based on the analysis of the SLR residuals, which are the discrepancies between the direct SLR range measurements and the station-satellite vector calculated based on the SLR station positions and the evaluated orbits in Earth-fixed reference frame. Therefore the results of SLR validation are strongly related to the SLR station coordinates. In 2022, the new realization of the International Terrestrial Reference Frame – ITRF2020 – has been released, which considers a few innovations, mainly, an extended model of post-seismic deformations, and the seasonal station coordinate variations in form of annual and semi-annual terms.  In this study, we investigate the impact of recent advancements in ITRF into the SLR-based orbit validation of LEO and GNSS satellites.      

We perform the SLR validation of LEO orbit (Swarm-ABC, Sentinel-3A/B, Jason-3) products provided by European Space Agency (ESA) Copernicus Service and Technical University of Graz for one year. Also, we validate Galileo and BeiDou-3 orbit products delivered by ESA and Center for Orbit Determination in Europe in 2023. 

We incorporate the latest ITRF2020 realization into the SLR validation processing, contrasting the outcomes with solutions that involve the previous ITRF2014 release to illustrate the impact of TRF aging on validation results. Additionally, we examine the influence of including seasonal station motions on SLR validation outcomes. Furthermore, a comparison is made between SLR validation results when utilizing the most recent alternative TRF realizations, namely DTRF2020 and JTRF2020. We discuss the dependency of residuals on different measurement conditions, such as elevation angle and azimuth angle, and their time variability.

How to cite: Strugarek, D. and Zajdel, R.: Impact of terrestrial reference frame on the SLR validation results of GNSS and LEO orbits, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4333, https://doi.org/10.5194/egusphere-egu24-4333, 2024.

X2.7
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EGU24-8143
Hakan Sert, Urs Hugentobler, Ozgur Karatekin, and Veronique Dehant

Having a Very Long Baseline Interferometry (VLBI) transmitter (VT) onboard Galileo satellite allows us to determine the misorientation between GNSS and VLBI frames. To exploit the maximum performance, we study the operational strategies for VLBI ground segment. We simulate VLBI observations of a VT onboard a Galileo satellite to evaluate the rotation transformation between the VLBI and GNSS frames. The contribution of a VT as space tie is assessed by the evaluation of the formal precision of the orientation parameters between the VLBI and GNSS frames using different ground stations/baselines, aiming to find the optimal observation geometry for the best precision on the rotation transformation.

How to cite: Sert, H., Hugentobler, U., Karatekin, O., and Dehant, V.: Effect of network geometry on determination of VLBI-GNSS frame orientation using a VLBI transmitter onboard Galileo satellites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8143, https://doi.org/10.5194/egusphere-egu24-8143, 2024.

X2.8
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EGU24-8188
Dariusz Tomaszewski, Renata Pelc-Mieczkowska, and Jacek Rapiński

The multipath phenomenon is one of the factors affecting the accuracy of GNSS positioning. It results from reflections of the satellite signal from objects in the vicinity of the GNSS antenna. There are groups of techniques that allow minimizing the impact of this error on positioning results. These include: antenna placement, the use of the appropriate type of antenna, the use of a professional receiver as well as proper post-processing of observations. However, it is impossible to completely eliminate the influence of multipath on the measurement results. In the case of carrier phase differential positioning, this error has two main effects. First of all, the multipath increases the initial search space for correct ambiguities. Consequently, the accuracy of the vector solution between the reference station and the rover receiver is affected. The authors of this article examined how the characteristics of the multipath error changed at the stations of the Polish network of ASG-EUPOS reference stations in 2010-2021. Two computational strategies were adopted to determine the multipath: Code Minus Carrier linear combination (CMC) and pseudorange multipath observable (MP). Based on the research, it was found how the multipath values changed depending on the change of the receiver and the terrain around the reference stations. It was determined which stations had high multipath values in 2010 and what changes occurred over the 11 years. Based on the carried out analyses, it was also recommended to perform periodic tests that would allow it to detect incorrect configuration or incorrect operation of receivers.

How to cite: Tomaszewski, D., Pelc-Mieczkowska, R., and Rapiński, J.: Changes in the multipath value at ASG-EUPOS GNSS reference network stations in 2010-2021, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8188, https://doi.org/10.5194/egusphere-egu24-8188, 2024.

X2.9
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EGU24-11608
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ECS
Peter Vollmair, Anja Schlicht, and Urs Hugentobler

The future Atomic Clock Ensemble in Space (ACES) mission of the European Space Agency (ESA) will address the development of a time transfer concepts for tomorrow's technologies. The ACES configuration includes a new generation of high-precision atomic clocks, a microwave link terminal (MWL) on the ground and on the satellite, and an optical detector and reflector also on the satellite. Due to the fact, that the official launch date is in 2025, there is a lack of real observation data. For that, a full-scale simulation software has been implemented. The simulator produces MWL code and phase observations in downlink and uplink, as well as one- and two-way laser observations. To analyse the efficiency of a time transfer concept before launch, we used the simulator to generate a data set of 100 passes during July 2021.
Investigations based on this data set showed that the colocation of the high-precision geodetic observation techniques of the ACES mission could better separate the individual error contributions of a measurement. Due to the colocation of optical and microwave-based geodetic observation techniques, also error parameters like orbit and troposphere correction can be estimated together. Estimation of a common troposphere for all observation techniques, improves the accuracy of the determination of the offset between ground and ACES clocks. Our further investigations focus on the common troposphere estimation of multi-color optical observations, together with microwave-based observations and the effects of different weighting methods. An extension to a network of ground stations will demonstrate the advantages of the ACES mission for synchronizing multiple ground clocks. The colocation of different high-precision geodetic observation techniques and estimation of common parameters will benefit timing and ranging applications and fundamental physics studies.

How to cite: Vollmair, P., Schlicht, A., and Hugentobler, U.: Simulating the Colocation of High-Precision Microwave and Optical Techniques for Tropospheric Parameter Determination in Context of the ACES Mission., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11608, https://doi.org/10.5194/egusphere-egu24-11608, 2024.

X2.10
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EGU24-17616
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ECS
Klarissa Emma Lachmann and Jürgen Müller

We present a project of the research unit (RU) 'TIME' (Clock Metrology: A Novel Approach to TIME in Geodesy) which aims to determine gravity potential or height differences between remote sites by comparing optical clocks. A strontium optical lattice clock at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig will be connected with the German Research Centre for Geosciences (GFZ) in Potsdam through a delay-compensated optical fiber. Then, optical time transfer is carried out between the geodetic observatories in Potsdam and Wettzell (where a second optical clock will be operated) via the Atomic Clock Ensemble in Space (ACES) using the Satellite Laser Ranging (SLR) telescopes. The key innovation is using time transfer, not frequency and optical free-space links over an extended period to determine physical height differences. Challenges include clock/link variations, atmospheric effects, visibility constraints and data gaps, etc. We investigate the major error sources and apply corrections like tidal effects. This approach showcases accurately transferring physical heights via time transfer and demonstrates the RU’s time concept for integrating geometric and physical heights in future height systems, especially for Global Geodetic Observing System (GGOS) core stations like the Geodetic Observatory Wettzell (GOW).

In this presentation, we introduce the principles, special properties and challenges of this specific measurement scenario. We also provide preliminary numbers for the expected accuracies of the various components and the resulting height difference based on simulations.

We acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 490990195 – FOR 5456.

How to cite: Lachmann, K. E. and Müller, J.: Determination of physical heights via time transfer , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17616, https://doi.org/10.5194/egusphere-egu24-17616, 2024.

X2.11
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EGU24-1600
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ECS
Alexandru Mihai Lăpădat, Jan Kodet, and Thomas Pany

The growing demand for Earth science applications poses challenges in improving geodetic reference frames. Systematic errors currently restrict the accuracy of these frames because the classical geometric ties between multiple geodetic techniques fall short of sufficiency. Our objective is to identify and analyze the impact of variable GNSS receiver hardware delays (incl. antenna-hardware delays) on carrier-phase time transfer with an accuracy of picoseconds/millimeters. We propose using a ground-based GNSS pseudolite system synchronized to an optical timing system (clock tie) developed at the Geodetic Observatory Wettzell to calibrate the variable hardware delays and facilitate a closure in time between multiple geodetic techniques.

This study analyzes the requirements for developing a GNSS pseudolite and its transmission chain. We reformulate the classic iono-free Precise Point Positioning (PPP) mathematical theory to incorporate pseudolite data, separating the known receiver clock error from unknown transceiver hardware delays. The analysis suggests a preference for highly directive and mechanically stable Right Hand Circularly Polarized (RHCP) log periodic or helix transmission antennae. Calibration for Phase Center Offset (PCO), Phase Center Variations (PCVs) and careful installation to minimize multipath are crucial. This results in a carrier-phase observation model with three unknowns: transceiver hardware delays (our focus), frequency-dependent ambiguity terms, and low tropospheric delay influence.

Utilizing a USRP-based transmission procedure, we successfully tracked an E1B Galileo signal replica with an in-house developed GNSS software-defined receiver (SDR). The transmission was implemented using two approaches: over-the-air and loopback. The over-the-air transmission was carefully planned using a link budget calculation to ensure that it did not exceed the allowed free-air transmission constraints. Empirical validation ensured a carrier-to-noise ratio (C/N0) below 30dB/Hz near critical public areas. In the loopback approach, the transmitted GNSS signal was fed into the local SDR within the pseudolite, sharing the same Analog-Digital-Converter (ADC)/ Digital-Analog-Converter (DAC)ADC/DAC, clock and local oscillators. In a future stage, this signal is supposed to be compared to a reference signal derived from the optical timing system. 

In our analysis, we also assessed the stability of the USRP frequency synthesizer, known as Phase Lock Loop (PLL), in the context of high-precision applications, such as real-time kinematic (RTK) positioning. We found that tuning the synthesizer in integer-n mode is crucial in maintaining a stable carrier frequency and achieving a 100% real-time kinematic positioning fixing rate. 

How to cite: Lăpădat, A. M., Kodet, J., and Pany, T.: Towards calibration of GNSS receiver hardware delays for improving geodetic reference systems through clock ties. A requirements analysis for developing a GNSS pseudolite transmission chain, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1600, https://doi.org/10.5194/egusphere-egu24-1600, 2024.

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EGU24-20392
yantian Xu

Based on approximately 15,000 global station coordinates and velocity values, the plate model PB2002_CASM was constructed using actual measured velocity field analysis and supervised clustering analysis based on global PB2002 plate boundary divisions. For plates with more stations, mathematical interpolation algorithms were applied to calculate grid velocity values, including inverse distance weighting, Euler vector method, finite element interpolation method, least squares configuration method, kriging interpolation method, and linear interpolation based on triangulation. For plates with sparse or missing stations, such as in the ocean, singular spectrum analysis was used to extract trend components and obtain station motion speeds, and the PB2002_CASM plate motion model was used to calculate grid velocity values. There are a total of 61,560 1°×1° grids within the global longitude range of -179° to 180° and latitude range of -85° to 85°, which were mathematically interpolated to form 20,071 grid points. The PB2002_CASM plate motion model was used to calculate the velocity of 40,624 1°×1° grid points. The accuracy of the calculated velocity was validated, with a deviation within 1mm, achieving 82% and 89% accuracy in the E and N directions, respectively.

How to cite: Xu, Y.: Global digital plate model PB2002_CASM and 1°×1° grid velocity field construction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20392, https://doi.org/10.5194/egusphere-egu24-20392, 2024.