G2.2

The member ACs of the ILRS Analysis Standing Committee—ASC, evaluated the preliminary release of ITRF2020—ITRF2020P.  For the most part, this evaluation is based on the reanalysis of part or all of the SLR data from geodetic spherical targets in the model; in particular, we focused on the two LAGEOS and two Etalons from 1993 to the end of 2020, extended by one year of data NOT included in the model: all of 2021. The evaluation report was submitted to ITRS for consideration in the finalization of the ITRF2020 model. Some ACs used additional data that do not contribute to ITRF development for testing. The reanalysis used the same improved modeling that was used for the development of the ILRS contribution to ITRF2020.

We will focus on the implementation of the new approach in handling systematic errors at the stations and how users will need to adapt their data analysis procedures to benefit the most from the new model. The 2021 ILRS contribution to ITRF2020 minimized the scale difference between SLR and VLBI below 2 mm (ITRF2014 ~9 mm). The reanalysis incorporates an improved “target signature” model (CoG) for better separation of true systematic errors from errors in describing the target’s signature. This model will be periodically updated from now on, so that it represents accurately the state of operations at all sites in the ILRS network of tracking stations. SLR data users should make sure from now on to use each ITRF model with the appropriate (consistent) Data Handling file and “target signature” model.

The presentation will provide an overview of the analysis procedures and models, and it will demonstrate the level of improvement with respect to the previous ILRS product series, focusing especially on the Core ILRS sites.

How to cite: Pavlis, E. C., Luceri, V., Basoni, A., Sarrocco, D., Kuzmicz-Cieslak, M., Evans, K., and Bianco, G.: The ILRS Analysis Centers’ Report on the Evaluation of ITRF2020P, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5212, https://doi.org/10.5194/egusphere-egu22-5212, 2022.

In this contribution we highlight the challenges of the combination of space geodetic techniques through local ties, during the rigorous estimation of geodetic parameters, for the realisation of a global Terrestrial Reference Frame (TRF). Local ties at geodetic fundamental sites play an important role in the determination of a TRF, providing the necessary links to connect the different geodetic techniques. Moreover, the growing demands on the accuracy and stability of the ITRF, have turned the analysis of the quality of these ties into a crucial element to achieve a highly consistent frame, providing additionally the opportunity to identify technique-specific systematic biases  when comparing the space geodetic results with local measurements.

To study the impact of the local ties at fundamental sites, we use the Very Long Baseline Interferometry (VLBI) observations collected during the CONT17 campaign, together with simultaneous Global Navigation Satellite System (GNSS) observations of a subset of the International GNSS Service (IGS) global network. To this end, our approach performs the rigorous estimation of all parameter types common to the two techniques, namely station coordinates, troposphere zenith delays and gradients, and the full set of five Earth Orientation Parameters (EOPs) and their rates, and we include their full variance-covariance information during the combination process. In the central step of this processing scheme, we realise the GNSS-VLBI combination via the "official" ITRF coordinate ties, using and evaluating different weighting schemes, to obtain a unique set of consistent parameters. Moreover, we study the impact of tropospheric ties between the collocated VLBI and GNSS stations, which are essential for the height estimates. Thus, based on the analysis of the station coordinate repeatabilities and the characteristics and behaviour of the EOPs, we discuss the impact of the accuracy and weighting of the local coordinate and troposphere ties on the estimation of the different geodetic parameters.

How to cite: Herrera Pinzón, I. and Rothacher, M.: Local Tie Analysis at Fundamental Sites in the CONT17 Campaign, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9240, https://doi.org/10.5194/egusphere-egu22-9240, 2022.

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

How to cite: Wang, J., Ge, M., Glaser, S., Balidakis, K., Heinkelmann, R., and Schuh, H.: On the contribution of global, local, and tropospheric ties to TRF and CRF in GNSS and VLBI integrated solution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6387, https://doi.org/10.5194/egusphere-egu22-6387, 2022.

The deviations of phase center offsets (PCOs) of GPS satellites were and still are significant bias sources for GPS-based terrestrial reference frames (TRF). Because of the strong correlation between the scale of the TRF and the satellite PCOs in the z-direction (z-PCOs), a no-net-scale (NNT) condition relative to, for instance, the International Terrestrial Reference Frame (ITRF) is commonly applied. Based on the released Galileo metadata, the GPS z-PCOs have been calibrated without introducing a scale determined by other techniques in the third re-processing of the International GNSS Service (IGS). Another approach purely based on GNSS is by integrating low Earth orbiters (LEOs) into the estimation of the GPS z-PCOs and the realization of the scale. Within this study, we estimated the GPS z-PCOs based on zero-difference ionosphere-free observations from six low LEOs and ground networks with different numbers of stations in 2019 and 2020. Besides the study based on six LEOs in two years, a twelve-year-based estimation of GPS z-PCOs and scale realization is done by using the two satellites of the GRACE mission.

We jointly estimate orbits (GPS and LEOs), station coordinates, z-PCOs of GPS satellites, and some other parameters in an integrated processing. The NNT condition on the ground network is not applied in the processing. By adding six LEOs, the correlation coefficients between the GPS z-PCOs and the scale is reduced significantly (from about 0.85 to 0.30). It means that the GPS z-PCOs and the scale have been decorrelated efficiently, and consequently the precision of the estimation is improved. For GPS satellites operated in 2019 and 2020, excluding GPS III, their estimated z-PCOs have an average difference of -231 mm compared to the values in igs14_2134.atx and the corresponding scale to the IGS14 reference frame is +1.89 part per billion. These results agree well with the solutions based on the metadata of Galileo. The improvement due to different numbers of LEOs and the impact of LEO z-PCO errors on the estimation is studied, where more LEOs decorrelate the GPS z-PCOs and the scale more efficiently. The accuracy of the LEO z-PCOs is critical to the solution. A one-millimeter accuracy of the z-PCOs of the LEOs is required to achieve a one-millimeter scale on the surface of the Earth. Thanks to the long-term available data of LEO missions in the last decade and even longer, the LEO-based method has an advantage on the real-data-based estimation of PCOs of former GPS satellites over the Galileo-based method. The z-PCOs of satellites of GPS blocks IIA, IIR, IIRM, and IIF are estimated by integrating the two GRACE satellites from 2004 to 2015. A twelve-year scale relative to the ITRF is realized simultaneously. The performance of the LEO-based method is shown by the long-time series.  

How to cite: Huang, W., Männel, B., Brack, A., and Schuh, H.: LEO-based solution of GPS PCOs and impact on terrestrial scale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4436, https://doi.org/10.5194/egusphere-egu22-4436, 2022.

The current international and regional reference frame research is mainly realized by single GPS (Global Position System) technology. With the full deployment of BDS (Beidou Navigation System), it is urgent to study and establish the corresponding terrestrial reference frame. Since 2019, the global and regional BDS service performance has been evaluated and tested, and the long-term BDS observations of MGEX (Multi GNSS Experiment) sites distributed worldwide provide the possibility for the preliminary construction of the BDS terrestrial reference frame. We aim to preliminarily realize and evaluate the CTRF2020 (COMPASS/BDS Terrestrial Reference Frame at the epoch of 2020.0) that can be expressed with the coordinates and velocities of a series of reference sites at the epoch of 2020.0. Firstly, the actual BDS global service performance evaluation reflects BDS satellite's high visibility and change trend in recent three years, which provides primary input data for the frame. Then, the BDS observations of about 100 global sites in the recent three years are calculated by PPP (Precise Point Positioning) and NET solution, to obtain the global high-precision BDS coordinate time series. Then, the BDS time series of the two solutions are fitted and compared with the IGS14 velocity field. The results show that the series accuracy of PPP-BDS and NET-BDS solutions is equivalent, and there is an mm-level systematic deviation with IGS14 solutions. The horizontal series fitting accuracy of PPP-BDS and NET-BDS solutions is better than that of the vertical direction, the accuracy of NET-BDS solution is slightly better than PPP-BDS, and the difference of fitting accuracy is 0.12, 0.13, and 0.50 mm in the NEU direction. The velocity field accuracy of PPP-BDS and NET-BDS solution is the same, and the overall three-dimensional velocity difference is less than 0.2 mm/a. The velocity fields of PPP-BDS and NET-BDS solution have little difference from IGS14, and the overall difference is less than 0.5 mm/a. Finally, we give the limitations and improvement points of CTRF2020. The preliminary realization and evaluation of CTRF2020 may be expected to provide a reference for the future realization of a comprehensive terrestrial reference framework dominated by BDS technology and supplemented by multi-source space geodetic technology.

How to cite: ren, Y., wang, H., and wang, J.: The preliminary realization and evaluation of BDS3 global terrestrial reference framework (CTRF2020), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1394, https://doi.org/10.5194/egusphere-egu22-1394, 2022.

The Hermert-like constrain condition is commonly used in various space geodetic reference frame alignment and reference frame definition, this often demands discussions on the proper constrain strength, selection of fiducial network, and more. Thus, the published and constrained reference frame products/solutions are often demanded the transformation to alternative constrain condition due to the area of interest change, reduction of the coverage, or other reasons.

We present an efficient methodology to transform reference reframe product to a posterior selection of the constrained condition from the product which either minimum or redundant datum constraints have been imposed. This analytical methodology significantly reduces the computation effort for datum alignment, especially for the large GNSS network. By avoiding the expensive normal equation system reconstruction and the subsequent inversion thereof, we achieved computational complexity reduction with an inversion of an auxiliary matrix of up to 14X14 dimension, while the computation is validated analytically as well as numerically to the truncation error level. This Fast Constraints Transformation (FCT) method can be conveniently applied to the widely used space geodetic solution files following the Solution Independent Exchange (SINEX) format, especially with our provided software package written in Matlab. We validate and evaluated FCT with two globally distributed GNSS-derived solutions and one South America terrestrial reference frame. The results confirm the numerical equivalence of the classical method and FCT. We also present the discussion on the computation efficiency with the above networks as well as numerical simulations. For the large network of up to 5000 stations, The FCT accelerates the transformation by more than 100 times compared to the classical strategy.

FCT method could serve as a beneficial procedure to many TRF-related applications, including but not limited to:

• Deploy Over Constain condition to an existing solution
• Transforming an Over Constrained solution to a Minimal Constrained solution
• Re-computation of a specified Minimal Constrained solution from an Over Constained or loosely-constrained solution.
• For global networks with a large number of stations the FCT significantly reduces the computation effort.
• For the cases of regional and local TRFs, the methodology shows significant advantages since the final product can be considered an Over Constrain solution.
• FCT could be applied for local and regional networks removing the imposed constraints and deriving the initial GNSS network.

How to cite: Wang, L., Ampatzidis, D., Mouratidis, A., and Balidakis, K.: Solution-Level Fast Constraints Transformations with Case Studies for GNSS Networks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10229, https://doi.org/10.5194/egusphere-egu22-10229, 2022.

In historical versions of the International Terrestrial Reference Frame (ITRF), the time evolution of station positions was described by piece-wise linear models. These kinematic models have been extended with exponential and logarithmic functions in ITRF2014 to account for post-seismic displacements, then with annual and semi-annual sine waves in ITRF2020 to account for the seasonal deformation of the Earth. However, part of the Earth’s surface deformation, such as inter-annual hydrological loading deformation, or high-frequency atmospheric loading deformation, is still not captured by such deterministic functions.

A reference frame in the form of a time series could allow such aperiodic displacements to be taken into account. This would require the aperiodic motions sensed by the different space geodetic techniques to be tied in a common frame by means of co-motion constraints. However, common aperiodic displacements between co-located space geodetic stations have not been evidenced at a global scale so far, and the relevance of such constraints is thus debatable. In this study, in order to investigate the possible existence of common aperiodic displacements at ITRF co-location sites, we use the solutions provided by the technique services for ITRF2014. Those solutions are first carefully aligned to a common reference frame in order to minimize differential network effects and obtain comparable position time series across techniques. The obtained position time series are then cleaned from linear, post-seismic and periodic signals (including seasonal deformation and technique systematic errors). The remaining aperiodic displacements are finally inter-compared at co-location sites.

Modest correlations are thus evidenced between the GNSS residual position time series and the other space geodetic techniques, mostly in the vertical component. The magnitude of the common aperiodic displacements evidenced in this study is finally discussed.

How to cite: de La Serve, M., Rebischung, P., Altamimi, Z., Collilieux, X., and Métivier, L.: Study of common aperiodic displacements at ITRF co-location sites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2985, https://doi.org/10.5194/egusphere-egu22-2985, 2022.

GNSS velocities estimated by different analysts can significantly differ due to the choices made concerning the GNSS data processing (corrections applied and noise level of the series), the completeness of the series, the removed position discontinuities and the alignment to a terrestrial reference frame. The position discontinuities that populate the GNSS time series have probably the biggest impact on the error or dispersion of the velocity estimates at the same sites. Even when using exactly the same position series, different analysts may provide different velocity estimates and uncertainties mainly due to the choice of removing different position discontinuities.

The IAG Joint Working Group 3.2 (2019 – 2023) aims at providing a global combined GNSS velocity field that takes into account the repeatability, the alignment and the relative weighting of the velocity estimates by different groups. We expect that this IAG’s unified GNSS velocity field will be useful for the scientific community inside, but especially outside, the geodetic community in areas such as tectonics, sea-level change and GIA modeling among others.

In this contribution, we present the current status of the global combined velocity field, aligned to the recently released ITRF2020, and based on the velocity fields provided by several other groups.

How to cite: Santamaría, A., Rietbroek, R., Frederikse, T., Rebischung, P., and Legrand, J.: Towards an IAG combined global GNSS velocity field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1183, https://doi.org/10.5194/egusphere-egu22-1183, 2022.