JPL has developed a terrestrial reference frame (TRF) based on combining space geodetic techniques at the observation level. The current version includes observations from the Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI); and it has been shown to be competitive with the international standard (ITRF2020) in terms of fundamental frame parameters (origin and scale) and their temporal evolution, both linear and seasonal. In this work, we build on the current processing infrastructure and add Doppler Orbitography by Radiopositioning Integrated on Satellite (DORIS) observations to the combination. The four-technique solution strategy uses space ties in low-Earth orbit to connect SLR, GPS and DORIS and ground ties to connect VLBI to GPS. This solution can be viewed as a pathfinder for the upcoming GENESIS mission which will provide a space tie for all 4 techniques. Additionally, we revise parametrization of the SLR range biases to improve retrieval of the scale. Finally, we discuss future enhancements to the TRF realization process. These enhancements include recovering the time-variable gravity field by jointly using observations from the frame realization and GRACE / GRACE-Follow On missions.

How to cite: Peidou, A., Haines, B., Bertiger, W., Conrad, A., Desai, S., Murphy, D., Naudet, C., and Ries, P.: Geodetic Technique Combinations at the Observation Level: Advancements in Terrestrial Reference Frame Recovery, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11071, https://doi.org/10.5194/egusphere-egu25-11071, 2025.

To update the ITRS 2020 realizations which are based on observations from the geodetic space techniques VLBI, SLR, GNSS and DORIS until the end of 2020, the ITRS Product Center asked the Technique Centers (TCs) for an extension of these series by as consistent as possible operational series. However, the GNSS input series which formerly realized an own GNSS scale, the full history of data was recomputed by the IGS TC by adopting the ITRF2020 scale based on the VLBI and SLR scale contributions.

DTRF2020, calculated by DGFI-TUM, realizes the scale from VLBI and GNSS contributions and thus needs for its update IGS/GNSS series realizing the GNSS scale. Therefore, the IGS TC provided, in addition, an extension series consistent to the input series for the ITRS 2020 realization. Moreover, the GGFC provided extensions of the previous non-tidal loading (NTL) displacement time series as well as time series for new stations to be used in the DTRF2020 update to reduce non-linear station motions.

We extended the DTRF2020 by using the provided extension series and the NTL input data until 2023. We analyzed the extended station position, datum parameter (origin and scale) and Earth Orientation Parameter time series, introduced additional discontinuities and accounted for post-seismic deformation of stations affected by earthquakes.

We present the DTRF2020 update solution (i.e. DTRF2020-u2023) and discuss the consistency of the former and the extension series. In addition, we discuss the results of comparisons of the updates computed by the three ITRS Combination Centers, which reflect the internal accuracy achieved by today's ITRS realizations.

How to cite: Seitz, M., Bloßfeld, M., Rudenko, S., Zeitlhoefler, J., and Angermann, D.: DTRF2020_u2023: The Update of DTRF2020, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4041, https://doi.org/10.5194/egusphere-egu25-4041, 2025.

We report on JTRF2020-u2022, the first update of JTRF2020, the most recent terrestrial reference frame (TRF) solution computed at Jet Propulsion Laboratory (JPL). Determined with SREF -a square-root information filter and Dyer-McReynolds smoother algorithm, JPL frame products lend themselves to being easily updated as long as frame inputs from the four space-geodetic technique consistent with the frame-defining data set are available. 

Adopting the conceptual framework of Bayesian update inherent to any Kalman filter formulation, SREF uploads the latest estimate of JTRF2020 state and its covariance and updates it by assimilating at daily intervals the extended frame inputs made available by IGS (IGSR3-igr3.atx), IVS (ivs2023b), ILRS (ilrsav86), and IDS (idswd23) from 2021 through the end of 2022 (DOY 330, GPS Week 2237). Afterwards, with SREF running in forecast mode, station position predictions are generated through the end of 2027 (DOY 365, GPS Week 2503).    

Discussions will focus on the peculiarities of the extended frame inputs used to compute JTRF2020-u2022 in relation to those adopted to generate JTRF2020, on the data pre-processing itself, and on the way in which the updated frame is constructed . We outline properties of  the updated TRF in terms of its frame-defining transformation parameters and present a quality assessment of  JTRF2020-u2022.

How to cite: Abbondanza, C., Chin, T. M., Gross, R. S., and Heflin, M. B.: JTRF2020-u2022: The First Update of JTRF2020, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6631, https://doi.org/10.5194/egusphere-egu25-6631, 2025.

In mid-2024, the IAG-IERS Joint Working Group 1.2.4 was established to complement the evaluation of the International Terrestrial Reference System (ITRS) realizations by the three ITRS combination centres (CC) of IERS with a special focus on the intercomparison of different global Terrestrial Reference Frame (TRF) solutions. The assessment aims at investigating conceptual differences of the three global ITRS realizations based on today’s user and application requirements. The main objectives of the JWG 1.2.4 include (i) the comparison of the combination strategies followed by the IERS ITRS CC; (ii) the assessment/quantification of station position time series differences between the ITRS realizations and w.r.t. geophysical models (e.g., loading displacements): (iii) the development and promotion of alternative rigorous (and independent) methods for the intercomparison of global TRF solutions (e.g., POD of low-, medium- and high-Earth-orbiting satellites); (iv) the development of methods and procedures for the quality control of TRF solutions.

Following a brief presentation of the structure of the JWG, we will give a status of its current activities. Then, we will introduce some of the benchmarks the JWG has identified to assess the quality of both global and regional TRF solutions. Finally, these benchmarks will be illustrated by their use on the latest ITRS 2020 realizations.

How to cite: Susnik, A., Moreaux, G., Ampatzidis, D., Krasna, H., Geisser, L., and Seitz, M.: IAG-IERS Joint Working Group 1.2.4 on the evaluation of the Terrestrial Reference Frames, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16930, https://doi.org/10.5194/egusphere-egu25-16930, 2025.

New realizations of the International Terrestrial Reference System (ITRS) were recently released by the three ITRS combination centers: ITRF2020 by IGN (France), DTRF2020 by DGFI-TUM (Germany) and JTRF2020 by JPL (USA). The three solutions are based on the same reprocessed data from the four space geodetic techniques (DORIS, GNSS, SLR, VLBI). However, the strategies used by the three ITRS centers to combine these input data into their respective solutions differ in several key aspects, such as the modeling of time-varying station trajectories, the connection of the station networks of the four techniques into a unique common frame, and the choices made to define the origin and scale of that frame.

This presentation compares the ITRF2020, DTRF2020 and JTRF2020 solutions and discusses how different aspects of the combination strategies employed by the three ITRS centers affect their solutions in terms of origin, scale and geometry. This comparison and discussion covers two main aspects: (1) individual station trajectories in the three solutions, and their ability to accurately describe the Earth’s time-varying shape, and (2) origins, scales (and orientations) of the station coordinates of each technique in the three solutions.

How to cite: Barnéoud, J., Rebischung, P., Gobron, K., de la Serve, M., Collilieux, X., and Altamimi, Z.: A comparison of ITRF2020, DTRF2020 and JTRF2020, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11660, https://doi.org/10.5194/egusphere-egu25-11660, 2025.

Genesis is an ESA mission dedicated to Global Navigation Satellite System (GNSS) Science, conducted by the ESA Navigation Directorate. Its primary objective is the contribution to the improvement of the International Terrestrial Reference Frame (ITRF) accuracy (1mm) and long-term stability (0.1mm/year). Secondary objectives include the contribution to a high number of other scientific disciplines (geodesy, geodynamics, earth rotation, geophysics, earth gravity field, atmosphere and ionosphere sciences, metrology, relativity…) [1].

On the industrial side, the company OHB Italia has been contracted by ESA as prime for the development, qualification, launch and 2 years operation of the mission, with a launch date in 2028 [2]. Antwerp Space (B), as the major sub-contractor of OHB-I, oversees the payload and geodetic instruments. Industrial activities were kicked-off in April 2024, the System Requirements Review was successfully closed-out in Q4 2024, and work is on-going towards a Preliminary Design Review in Q4 2025.

In parallel, on the scientific side, after a first successful Genesis Workshop held in February 2024 [3], a Genesis Science Exploitation Team was set-up and members appointed. The Genesis Science Exploitation Team are actively supporting the mission development (in particular consolidation of requirements) and will play a key role in its future exploitation.

The presentation will provide a high-level description of the mission, and provide the  programmatic status of Genesis.

[1]: Delva et al. Earth, Planets and Space 75, 5 (2023)

[2]: https://www.esa.int/Applications/Satellite_navigation/ESA_kicks_off_two_new_navigation_missions

[3]: https://www.esa.int/Applications/Satellite_navigation/The_geodetic_community_meets_Genesis

How to cite: Gidlund, S., Fusco, G., Waller, P., Morlet, C., Perez Lissi, F., Sakalauskaite, E., Enderle, W., Schoenemann, E., Berton, J.-C., Gini, F., and Navarro, V.: Overview of Genesis - an ESA mission at the Foundation of Navigation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15776, https://doi.org/10.5194/egusphere-egu25-15776, 2025.

Genesis is an upcoming satellite mission by the European Space Agency (ESA) that will integrate multiple space geodetic techniques on a single satellite at an altitude of 6000 km, including a dedicated Very Long Baseline Interferometry (VLBI) transmitter. This innovative, dynamic space geodetic observatory aims to establish highly accurate and continuous space ties, significantly enhancing the accuracy and stability of Terrestrial Reference Frames (TRFs). Including a VLBI transmitter will enable observations to Genesis using existing VGOS antennas and infrastructure, but several technical challenges must be addressed to ensure the success of VLBI observations.

A key open question is the choice of the satellite’s orbit, whether polar (97°) or inclined (60°). Additionally, the allocation of observation time between satellite and quasar observations must be carefully optimized. While sufficient VLBI observations of Genesis are needed to estimate accurate station positions, maintaining a good sky coverage of quasar observations is essential for precise tropospheric delay modeling, the major error source in VLBI measurements.

This simulation study addresses these points by evaluating various observation scenarios for two VGOS station networks, considering different orbital configurations and observation time distributions. Based on the determination of TRFs from weekly VLBI sessions over a two-year investigation period, we aim to identify the optimal setup for VLBI observations of Genesis.

How to cite: Kern, L., Wolf, H., Steinmetz, S., and Böhm, J.: Evaluating VLBI Scenarios for Genesis: Orbital and Observational Configurations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15312, https://doi.org/10.5194/egusphere-egu25-15312, 2025.

Set for launch in 2028, geodesy’s flagship mission GENESIS is equipped with the instruments of the four space geodetic techniques. The co-location satellite aims to improve the accuracy and stability of future realizations of the ITRF. However, VLBI observations to satellites are not standard and are not performed routinely yet. In the past, observations were limited to specific telescopes and setups. At the moment, most VLBI antennas are not capable of observing currently available satellite signals. This makes testing difficult, although further research on the VLBI component of GENESIS is urgently needed to achieve the accuracy for the mission goals. We present the newly accessible capability of the Australian VGOS telescopes to observe GNSS satellites in L-band.  With this new discovery, we are able to generate geodetic delay observables with a full continental-wide telescope array on a routine basis in the style of IVS observations. In particular, we report on a series of successful observations to satellites of the GPS, Galileo and BeiDou constellations with the AuScope 12-m antennas in Hobart, Katherine and Yarragadee using the standard VGOS equipment. We describe the experimental setup, signal chain and key developments enabling these observations. Furthermore, we report on the ongoing efforts in the fringe fitting phase with critical effects on the delay. This crucial development is a valuable opportunity to further develop VLBI observations to satellites in preparation for the GENESIS mission. In addition, such observations could realize the first-ever inter-technique ties between VLBI and GNSS in the Australian region.

How to cite: Schunck, D., McCallum, L., McCallum, J., and McCarthy, T.: Observing GNSS Satellites with the AuScope VLBI Array: A Testbed for the VLBI Component of the GENESIS Mission, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18387, https://doi.org/10.5194/egusphere-egu25-18387, 2025.

The global high-precision Terrestrial Reference Frame (TRF) is determined through the combination of the four space geodetic techniques: VLBI, SLR, GNSS, and DORIS. The multi-technique combination heavily relies on the common objects or parameters, i.e., the ties, which link the different techniques. Current realizations of the International Terrestrial Reference Frame (ITRF) employ local ties and Earth Orientation Parameter (EOP). Additional ties such as tropospheric ties and satellite orbit ties are being explored. Another promising approach is using clock ties, where co-located instruments connected to a common clock enable intra- and inter-technique clock tie modeling.

Clock ties between GNSS and VLBI are of particular interest as (1) GNSS provides dense observations at every epoch, allowing the epoch-wise estimation of clock parameters, and (2) VLBI typically relies on clock modeling due to the insufficient number of observations per epoch. Using the GNSS and VLBI co-location stations during VLBI CONT campaigns, we demonstrate inter-technique clock agreements within 0.1 and 0.2 ns for short (24-hour) and long (15-day) periods, respectively. We further explore the application of clock ties in the GNSS and VLBI integrated solution, focusing on the impact on station coordinates and tropospheric delays. We also address the challenges of modeling deterministic and stochastic components of the clock ties, highlighting their potential to improve the precision of TRF realizations.

How to cite: Wang, J., Balidakis, K., Heinkelmann, R., Ge, M., and Schuh, H.: Applying Clock Ties in GNSS and VLBI Integrated Solution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7460, https://doi.org/10.5194/egusphere-egu25-7460, 2025.

This effort as part of the research unit "Clock Metrology" funded by the German Research Foundation (DFG) focuses on innovative strategies for combining all four space geodetic techniques—DORIS, GNSS, SLR, and VLBI—to significantly enhance the global Terrestrial Reference Frames (TRF) with the goal to achieve the GGOS (Global Geodetic Observing System) requirements of 1 mm accuracy and long-term stability of 1 mm/decade. To achieve this goal, a novel approach of combining the space geodetic techniques is being developed. A common target (CT) will be introduced at the Geodetic Observatory Wettzell (GOW) to combine all the techniques at a common reference point. Additionally, a common clock (CC) will be established at GOW to reference the observations of all techniques to a single time frame. The impact of the CT and CC on the TRF combination is evaluated through simulations involving all four space geodetic techniques. In this study, we exhibit the results of the SLR, GNSS, and VLBI simulations of global station networks as reference solutions for the following implementation of CT and CC. As basis for these simulations, we used current state-of-the-art models and standards based on real observation data to determine realistic accuracy measures for today’s technique-specific observations. We assume a white noise of the observations with an amplitude corresponding to the noise derived from actual data analysis of the real observations for each technique. Subsequently, weekly normal equation systems from the simulated data were accumulated into a combined global TRF solution over a 7-year period to assess the impact on the derived station coordinates and velocities, as well as the Earth Rotation Parameters. The estimated corrections to the station positions remain within a few millimeters, indicating a consistent solution w.r.t a priori. Additionally, in order to perform the combination via a CC, extended software modifications are carried out as in the space geodetic data analysis the clock parameters are usually pre-eliminated and no longer available for further processing. Based on these modifications, first results of the novel combination of intra- and inter-techniques at GOW will be presented.

How to cite: Reinhold, A., Glaser, S., and Seitz, M.: On simulation studies for novel combination strategies of space geodetic techniques, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1646, https://doi.org/10.5194/egusphere-egu25-1646, 2025.

Realizing the terrestrial scale from Global Navigation Satellite System (GNSS) data requires precisely calibrated and highly stable satellite antenna phase centers. The results from the third International GNSS Service (IGS) reprocessing campaign, however, have raised renewed concerns about the long-term stability of the GNSS transmit antenna phase center relative to the spacecraft center of mass. Reevaluation of the satellite radial phase center offsets (z-PCOs) based on SINEX data from multiple IGS analysis centers revealed abrupt changes over time of up to 1 decimeter. Commonly cited reasons for time-varying z-PCO estimates are “aging” of the antenna and its radiation pattern, changing composition of the satellite constellation and ground network, and movement of the center of mass as propellant is consumed for satellite momentum dumps or orbital maneuvers. In this presentation, we shed light on another, yet unknown, effect arising from the drift of the satellite clock that prompts GNSS ground segment operators to adjust the clock frequency to ensure that the timing corrections in the satellite broadcast navigation message remain within the limits specified by the signal-in-space interface. The frequency adjustments are easily detectable in satellite clock bias estimates or broadcast clock correction parameters. However, this is only half the story; changing the clock frequency results in boresight angle-dependent lengthening or shortening of the carrier phase range, which the satellite antenna z-PCO parameter can almost perfectly absorb. As we will demonstrate through simulations and real-world examples from various GNSS spacecraft, a 4 × 10^-10 correction applied to the onboard clock fundamental frequency (10.23 MHz) alters the estimated satellite z-PCOs for the standard ionosphere-free linear combination of L1 and L2/E5 by approximately 7 cm. The results indicate that satellite clock adjustments are the cause of the observed z-PCO jumps in the IGS “repro3” time series.

How to cite: Dilssner, F., Springer, T., Gonzalez, F., Schönemann, E., and Gini, F.: The Ticking Truth: How Subtle Clock Adjustments Influence GNSS Satellite Antenna Offset Estimates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19163, https://doi.org/10.5194/egusphere-egu25-19163, 2025.

A Global Geodetic Reference Frame (GGRF) is a fundamental requirement to relate measurements taken from anywhere on the Earth. It provides the basis on which a broad spectrum of scientific and industrial applications rely, and is crucial in any field where precise location information is necessary, including monitoring climate change, agriculture, and changes in groundwater.
The working group Satellite Geodesy at the Institute of Geodesy (IFG), Graz University of Technology (TUG) provide a wide range of of products which are processed and published to the international community such as gravity field ans mass transport solutions, Precise Orbit Data (POD) of Low Earth Orbit (LEO) satellites and Global Navigation Satellite Systems (GNSS) station networks and much more. The products are utilized by various organizations, such as the International Combination Service for Time-variable Gravity Fields (COST-G) of the International Association of Geodesy (IAG), the International GNSS Service (IGS) or the European Copernicus POD Service Quality Working Group (CPOD). A consistent and accurate GGRF is the foundation of all our products and, as such, is essential to ensure the quality of our in-house computed products.
A GGRF is established and maintained by an international community, culminating in the International Terrestrial Reference Frames (ITRF). The latest version, ITRF2020, is based on four space geodetic techniques: GNSS, VLBI, SLR, and DORIS. In ITRF2020, GNSS was ignored in the calculation of the geocenter motion and the estimation of the reference system scale because it was considered too inaccurate. 
In this work we present our approach to estimate the geocenter motion and to improve the scale estimation using only GNSS. 
We discuss the problems of combined estimation of geocenter motion, reference system scale and satellite antenna center offsets and variations in GNSS only reference frame estimation. We analyze the results and implications for GNSS products and station position time series.

How to cite: Dumitraschkewitz, P., Mayer-Gürr, T., and Süsser-Rechberger, B.: Scale and Geocenter determination by GNSS only, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8525, https://doi.org/10.5194/egusphere-egu25-8525, 2025.

The global geodetic parameters are essential for the realization of an International Terrestrial Reference Frame (ITRF), which has broad geophysical and geodetic applications. With well-distributed global tracking stations and continuous observations, GNSS is suited to providing high quality geodetic parameters, including geocenter coordinates (GCCs) and Earth rotation parameters (ERPs). However, the spurious draconitic signals have been observed in GNSS-based geodetic products. Fortunately, the fast development of multi-GNSS and LEO satellite techniques in recent years provides new opportunity to improve the determination of geodetic parameters by combining observations from constellations with different orbital characteristics. In this study, we perform the integrated precise orbit determination (IPOD) under the ITRF2020 with dual-system GPS/Galileo observations from the Sentinel-6A satellite and 61 ground stations for geodetic parameter estimation. The effects of the LEO solar radiation pressure (SRP) model are analyzed and the contribution of the multi-GNSS combination and the LEO is investigated.

Three Sentinel-6A macro models, from the manufacturer, the German Aerospace Center (DLR) and the University of Colorado and Jet Propulsion Laboratory (UoC/JPL), are used to perform GPS-based IPOD solutions to investigate the impact of LEO SRP model on geodetic parameter estimation. The relationship between the estimated constant SRP scale and sun elevation is first analyzed. The estimated SRP scale based on the UoC/JPL macro model is found to have a weaker correlation with sun elevation compared the other two macro models. By comparing the geodetic parameters obtained using different macro models, we find that the GCC-Z estimates are more sensitive to the LEO priori SRP model than the GCC-X/Y and ERPs, and the best solution with the smallest amplitude in 3 cpy frequency is achieved by UoC/JPL macro model.

Based on the UoC/JPL macro model, we further investigate the performance of the geodetic parameter estimation in IPOD with dual-system GPS and Galileo observations. The introduction of the LEO improves the ERPs estimates, with a reduction in the length of day (LoD) formal error of about 60%, and a reduction in the drift rate of the GNSS-derived dUT1 away from the IERS 20 C04 reference value of over 30%. The introduction of the LEO also improves the GCC estimates with a reduction in GCC-Z formal error and STD value by about 20% and 10% respectively. Compared to single system solution, the combination of GPS, Galileo and the LEO obtains the most stable GCC estimates with the smallest STD values, except for the GCC-Z for the period above 30 days, mainly due to the failure to eliminate the Galileo-induced 3 cpy spurious signal. However, the introduction of Sentinel-6A also induces the spurious annual signal due to its SRP model deficiency. Though this deficiency can be partly compensated by changing the parameterization of SRP scale factor from constant to piece-wise constant, the influence of residual errors of LEO SRP modelling is still visible in the Z component of GCC. These results indicate that refining the a priori macro model of LEO satellite is necessary to obtain accurate geodetic parameters in IPOD with multi-GNSS.

How to cite: Zhao, Y., Zhang, K., Li, X., Fu, Y., and Yuan, Y.: Global geodetic parameter estimation using the combination of GPS, Galileo and LEO satellites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4664, https://doi.org/10.5194/egusphere-egu25-4664, 2025.

To ensure inter-operability and consistency of various geodetic products for Solid Earth science and operational geodesy applications, a long-term global terrestrial reference frame is required, i.e., the International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). As one of the key parameters in the determination of the ITRF, geocenter motion must be precisely estimated. Space geodetic techniques such as Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and Global Navigation Satellite Systems (GNSS), which connect satellites and ground stations, are capable of determining geocenter motion. However, only the SLR technique is currently included in the realization of the ITRF, despite GNSS being widely regarded as a promising technology. Compared to SLR-based solutions, GNSS exhibits lower accuracy due to various modeling issues, including deficiencies in non-gravitational force models. In recent years, numerous studies have demonstrated the significant potential for improving geocenter coordinate estimation through the integration of ground-based observations and space-based observations from low Earth orbiters (LEOs). Nevertheless, the full contribution of LEOs to geocenter motion estimation remains underexplored, largely due to the limited temporal coverage and the small number of LEOs available..

To evaluate the extent of associated improvements, a network of 150 ground tracking stations and up to 13 LEOs (GRACE-A/B/C/D, SWARM-A/B/C, JASON-1/2/3, SENTINEL-3A/3B, and SENTINEL-6A) was processed over a 20-year period (2004–2024). Three solutions were investigated: (1) a reference solution utilizing only ground-based GNSS observations; (2) a combined ground- and space-based solution employing a reduced-dynamic (DYN) strategy for LEO orbit determination; and (3) a combined ground- and space-based solution employing a kinematic (KIN) strategy for LEO orbit determination. All solutions were implemented in accordance with the processing strategy of the IGS's 3rd reprocessing campaign.

In this contribution, we evaluate contribution of space-based observations to geocenter motion estimation through spectral analysis and external comparisons. Significant improvements are observed in the combined ground- and space-based solutions, particularly in enhancing geocenter motion stability and mitigating certain artificial signals. Among the three solutions, the GNSS/LEO integrated solution employing the KIN strategy for LEOs demonstrates a substantial reduction in draconitic errors compared to the others.

How to cite: Tang, L., Männel, B., Wang, J., Nie, L., and Wickert, J.: The contribution of multi-LEOs to geocenter motion estimation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12120, https://doi.org/10.5194/egusphere-egu25-12120, 2025.

Global Navigation Satellite Systems (GNSS) are based on measurements of signal propagation time, so that clock information is required for both the transmitting satellite and the receiving ground station. In undifferenced GNSS network analyses, synchronization errors of the satellite and station clocks are usually estimated by epoch-wise biases. If, as is common in practice, white noise is assumed as stochastic behavior for the clocks, there are high correlations between the clock estimates and other geodetic parameters. While the satellite clock is correlated with the radial orbit component and geocenter parameters, the station clock shows high correlations with the station height coordinate and the tropospheric zenith delay. These effects can be reduced by introducing proper models for describing the clock characteristics. Especially for global network solutions, deterministic clock modeling is suitable to reduce a substantial number of clock estimates. However, adequate modeling requires a high degree of stability for the corresponding clocks.

In this contribution, we show preliminary results of modeling highly stable GNSS ground and space clocks using piece-wise linear representations. For the satellites, we select the rubidium clocks of the GPS IIF and IIIA blocks as well as the passive hydrogen maser clocks of the Galileo FOC spacecraft for modeling. In addition, a selection of hydrogen maser stations of the International GNSS Service (IGS) is also modeled. Over a period of several weeks, daily network solutions with and without piece-wise linear clock modeling are processed and then compared. We discuss different strategies and show the effects of various modeling options (length of split interval, parameter weighting) on the correlations of the estimated parameters.

How to cite: Widczisk, J. S., Männel, B., and Wickert, J.: Modeling of highly stable GNSS ground and space clocks using piece-wise linear representations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2545, https://doi.org/10.5194/egusphere-egu25-2545, 2025.

Global Navigation Satellite Systems (GNSS) rely on one-way signal travel time measurements from satellites to receivers. To ensure accurate ranging, GNSS must estimate and compensate for clock offsets on at least one end of the transmitter-receiver system. Consequently, clock offsets are highly correlated with GNSS-derived parameters such as station coordinates, tropospheric delay estimates, satellite radial orbit parameters, and apparent geocenter coordinates. Improper handling of clock offsets can lead to their absorption into these parameters, degrading geodetic product accuracy. Traditionally, clocks have been handled within individual techniques without being specially addressed. However, some research efforts and experimental trials have explored using VLBI to synchronize clocks for GNSS and refining clock models to reduce correlations with other estimated parameters in GNSS, thereby improving overall precision.

With recent advancements, high-precision optical clocks and fiber-optic time transfer technologies provide new possibilities for enhancing clock synchronization in GNSS. Optical fiber offers a stable, interference-free medium for time and frequency transfer, eliminating atmospheric effects such as ionospheric and tropospheric delays that impact GNSS-based synchronization. Moreover, fiber-optic time transfer is inherently two-way, removing the need for external clock offset estimation. Thus, fiber-optic clock synchronization represents the most precise timing technique currently available. Establishing a common clock by connecting multiple GNSS receivers via optical fiber could significantly reduce the number of estimated clock parameters in GNSS solutions, leading to improved geodetic measurement precision and a more accurate geodetic reference frame.

Despite its potential, the approach of implementing a common clock with optical fiber for GNSS receivers remains largely untested. Although optical fiber links have been successfully used in clock synchronization, their integration into GNSS receiver clocking is still in an experimental stage. Notable existing efforts include: (1) the Geodetic Observatory Wettzell, where multiple receivers are connected to a single clock via optical fiber, representing a local baseline setup, and (2) the GFZ experiment, where a receiver is linked to the ultra-stable clock of PTB, representing a regional baseline approach.

This study characterizes existing fiber-optic connections among GNSS receivers to identify practical challenges and evaluate their impact on GNSS parameter estimation. We process data from receivers sharing a common clock using Bernese GNSS Software. Initially, PPP will be performed to compare GNSS-derived clock parameters across receivers sharing a common clock, verifying the effectiveness of fiber-optic clock synchronization. Following this validation, we will leverage GNSS-derived clock parameters to evaluate inter-receiver clock differences measured via optical fiber. Subsequently, we will implement a strategy in which only one receiver’s clock parameters are estimated, using fiber-measured clock differences as a priori constraints for other receivers in the PPP solution. This approach reduces the number of estimated clock parameters and serves as a preliminary test of the feasibility of a GNSS common clock framework.

Future developments will focus on refining our analysis framework to enable simultaneous estimation of GNSS receivers operating under a common clock. This research contributes to the broader goal of integrating ultra-stable clocks into global GNSS networks, ultimately enhancing the stability of geodetic reference frames and improving GNSS-derived geodetic products.

How to cite: Wang, Z., Hugentobler, U., Männel, B., and Rodríguez-Villamizar, J.: Common Clock for GNSS Receivers: Characterization of Parameters Estimated with GNSS Receivers Connected to the Same External High-Precision Clock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7197, https://doi.org/10.5194/egusphere-egu25-7197, 2025.

GNSS station operators and network analysers face the challenge of achieving consistent phase center corrections (PCCs), particularly since multi-GNSS calibrations are not available for all antennas. To address this issue, a global initiative involving nine calibration organizations has launched a comprehensive ring calibration campaign. This collaboration aims to enhance the consistency of calibration methods and establish a robust quality assessment framework by sharing six structurally diverse antenna samples for calibration. The recommended methods by the International GNSS Service (IGS), including anechoic chamber and field robot calibration methods, are participating with the aim of defining a global benchmark for the accuracy and reliability of receiver antenna PCCs.

Over the past year, the campaign has made significant progress. Measures and methods to easily compare the PCC patterns and define a benchmark value for consistent PCCs have been established. In this contribution, we present the latest results of this campaign. We demonstrate in the pattern domain that the consistency among the system/frequencies per receiver antenna design varies around an uncertainty level of ±1 mm, with an elevation-dependent effect also needing consideration. In the position domain, we present system-specific PPP results per antenna and facility. For example, the variation among different antennas and facilities for classical GPS ionosphere free linear combination is generally below 2-3 mm for the horizontal component and below 5 mm for the vertical component when comparing calibration sets among the various facilities. We also present comprehensive and effective comparison methods for the pattern domain and evaluation tools for the position domain that are essential for establishing a globally fair and transparent benchmark for receiver antenna PCC calibration and verification processes.

How to cite: Kersten, T., Billich, A., Sutyagin, I., Kröger, J., and Schön, S.: Progress Update on a Global Collaboration for GNSS Receiver Antenna Calibration: Results from the IGS Ring Calibration Campaign (IGS ringCalVal), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9053, https://doi.org/10.5194/egusphere-egu25-9053, 2025.

The International Laser Ranging Service (ILRS) regularly contributes to the maintenance of the reference frame by providing daily and weekly combined time series of station coordinates and Earth Rotation Parameters (ILRSA from the ILRS primary ASI Combination Center) derived from individual Analysis Centers (ACs) solutions, using LAGEOS-1/-2 and Etalon-1/-2 satellite data.

The inclusion of additional satellites in the ILRS official products has been under discussion since years within the ILRS Analysis Standing Committee and the launch of LARES-2 in July 2022 has been considered a real opportunity to improve the product quality. LARES-2 is a LAGEOS-type satellite specifically designed for high-accuracy satellite laser ranging.

The inclusion of LARES-2 as the fifth satellites of the constellation contributing to the routine products foresees a preliminary analysis to model stations' long-term systematic errors related to LARES-2.

The approach to manage the systematic errors is the same used for the ITRF2020 contributions: ACs simultaneously estimate station coordinates and range biases to determine a combined ILRSA set of long-term mean range biases for LARES-2, to be applied in the data analysis.

The identified systematic errors are reported in the ILRS Data Handling File (DHF), which is closely linked to the target signature model.

Preliminary time series of products including LAGEOS/ETALON/LARES-2 are available and an enhanced quality in the estimated station coordinates is evident, particularly in the UP component.

This presentation will address the status of the LARES-2 inclusion to the ILRS official products, emphasizing the quality of estimated parameters.

How to cite: Sarrocco, D., Luceri, V., Basoni, A., Vespe, F., and Blossfeld, M.: LARES-2 in the ILRS official products: analysis and current status, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4085, https://doi.org/10.5194/egusphere-egu25-4085, 2025.

Further development of the Satellite Laser Ranging (SLR) technique is essential not only to achieve the goals defined by the Global Geodetic Observing System (GGOS) but also to meet the growing challenges associated with understanding the increasingly frequent and dynamic processes occurring at the Earth's surface. SLR plays a key role in the realization of the Terrestrial Reference Frames (TRFs). It is also crucial for determining the gravitational potential, as the C₂₀ and C₃₀ coefficients in solutions from dedicated GRACE Follow-On missions are replaced by those derived from laser observations.

This study investigates the optimal orbital parameters to consider for future geodetic satellites. We performed simulations for satellites with different inclination angles ranging from 0° to 180° with 1° interval, and at five different altitudes ranging from 1,500 km to 10,300 km with 1,200 km interval. The analysis was carried out under two different scenarios: (1) optimisation for TRF realisation and (2) optimisation for recovery of the Earth's gravitational potential. The results indicate that the optimal satellite altitude for TRF implementation is about 3,700 km, with inclination angles between 0°-20° or 160°-180°, which minimises formal errors in the determination of geocenter coordinates and Earth Rotation Parameters (ERP). On the other hand, geodetic satellites designed primarily to determine the parameters of the Earth's low gravity potential should operate at an altitude of 1,500 km with inclinations between 30°-40° or 135°-145°.

In particular, none of the current satellites in the SLR constellation have orbital parameters optimised for low-degree gravity recovery. Adding a single satellite with suitable inclination parameters to the existing constellation of ten geodetic satellites would reduce the error in the determination of the Earth's oblateness term, C₂₀, by order of magnitude.

How to cite: Najder, J., Sośnica, K., Zajdel, R., and Kur, T.: Simulation for SLR space segment development to improve global geodetic parameters - different inclination angles & altitudes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20752, https://doi.org/10.5194/egusphere-egu25-20752, 2025.

In the context of its contribution to the second yearly update of the 2020 realization of the International Terrestrial Reference Frame (ITRF2020), the International DORIS Service (IDS) has undertaken the estimation of DORIS station positions and velocities, as well as Earth Rotation Parameters (ERPs), derived from DORIS data. These computations are based on the latest weekly multi-satellite series from the five IDS Analysis Centers and the IDS Associated Analysis Center, covering data from 2024 and 2021 backward.

The primary objectives of this study are to evaluate the DORIS contribution to this update of the ITRF2020 in terms of: (1) geocenter motion and scale, (2) station positions and week-to-week position repeatability, (3) EOPs, and (4) a cumulative position and velocity solution. Additionally, this study examines the benefits of new models as well as the implementation of South Atlantic Anomaly (SAA) mitigation strategies on specific DORIS missions. Particular focus is placed on the enhanced stability of the IDS-derived scale since late 2012.

How to cite: Moreaux, G., Lemoine, F., Capdeville, H., Štěpánek, P., Otten, M., Nahmani, S., Pollet, A., and Schreiner, P.: IDS contribution to the second update of the ITRF2020, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12833, https://doi.org/10.5194/egusphere-egu25-12833, 2025.