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In 2022 and at the beginning of 2023, the three latest realizations of the International Terrestrial Reference System (ITRS) became available: ITRF2020, JTRF2020, and DTRF2020. Among others, the data contribution by the International VLBI Service for Geodesy and Astrometry (IVS) to these reference frames contains new models for the gravitational deformation of six VLBI antennas. In particular, these models affect the estimated heights of the corresponding antennas. In 2023, the IVS Analysis Centers (AC) reprocessed their session series from 1979 to the present. The corresponding series of the AC at DGFI-TUM is "dgf2023a". The main changes w.r.t. to the previous series "dgf2020a", which served as input data for the ITRS 2020 realizations, are a) the usage of ITRF2020 as a priori reference frame, and b) the corrections for the gravitational deformation of another seven antennas. Thereby, the choice of stations used for the no-net-translation and no-net-rotation conditions is a crucial issue. Furthermore, the additional deformation models will likely influence the scale parameter of similarity transformations between VLBI single-session solutions and the ITRS realizations. Three of the respective seven antennas belong to the next generation VLBI Global Observing System (VGOS). In this presentation, we examine the effects of the above mentioned novelties in solution "dgf2023a". In particular, we take a look at the similarity transformations, i.e., the time series of datum parameters, and we put special emphasis on the scale parameter. Above all, a drift in the VLBI scale was observed for sessions after about 2013.75 during the computation of the ITRF2020. Not least to investigate this finding, we also apply the other two ITRS 2020 realizations, JTRF2020 and DTRF2020, as a priori reference frames.

The IVS Combination Centre generates and releases the combined VLBI products of the International VLBI Service for Geodesy and Astrometry (IVS). These solutions are generated session-wise by applying an intra-technique combination of the individual contributions delivered by multiple IVS Analysis Centres (AC). For the twice-per-week R1/R4 combination, there are eleven different ACs contributing pre-analysed 24-h VLBI observations consisting of datum free normal equations (NEQ) in SINEX file format. Station coordinates, source positions and full sets of Earth Orientation Parameters (EOP) are parameterized. As the same software packages are used by various ACs (e.g. CalcSolve by five ACs), the combined solution is in danger of being dominated by these contributions. The estimates of EOP and station coordinates could be affected then mainly by the software specific modelling. In this contribution we study the impact of such software dependencies on the final combined parameters. We investigate an enhanced weighting strategy for the individual contributions in order to better handle potential dependencies due to identical software packages. Thereby, software specific sub-combinations are carried out in the first step. Finally, the ultimate combination consists of the weighted individual contributions of each applied software. In order to assess the quality of the individual components of the combination the internal as well as the external comparisons of the estimated EOP are carried out. In this context, the combined solution as well as external time series (e.g., IERS Bulletin A) serve as reference.

At the beginning of the year 2023, the DTRF2020, the ITRS 2020 realization of DGFI-TUM was completed. In this presentation, we closely look at the results of the analysis of the DTRF2020 Earth Orientation Parameter (EOP) series. First, we examine the impact of accounting for non-tidal loading on the EOP time series of each observation technique. In particular, changes in the annual and semi-annual signals of the EOP series can be expected, as both frequencies are dominant in the non-tidal loading displacements of the observing stations. We analyze the combined DTRF2020 EOP time series in terms of their temporal changes and their sensitivity to the contributing techniques, and we compare them to reference series such as IERS Bulletin A, IERS 20 C04, and SPACE 2018 (or a later version). Furthermore, we investigate the achieved consistency with the ITRF2020 and JTRF2020 in terms of the EOP, since these solutions have been computed from the same input data. A comparison with the DTRF2014 EOP reflects the consistency of successive DTRF solutions., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The International Laser Ranging Service contributed to the development of the ITRF2020 by providing combined 15-daily and weekly solutions to the IERS ITRS CCs (International Terrestrial Reference System Combination Centers of the International Earth Rotation and Reference Systems Service). These products were generated by combining the individual ILRS Analysis Center time series contributions (1983-2020) using a new approach in handling the systematic errors,that ILRS recently implemented.A set of mean range biases was estimated and pre-applied to the station observations for the LAGEOS and Etalon satellites used in generating the solutions. The new approach has proven to give an enhanced ILRS contribution to ITRF reflected,in particular, in the scale of the resulting reference frame.The extended Station Systematic Errors Monitoring (SSEM-X) is now a standard and routine activity of the ILRS Analysis Standing Committee, as an extension of the SSEM model used in ITRF2020 and will contribute to its maintenance over the coming years. The model is applied to all the operational ILRS products implementing ITRF2020/SLRF2020.A complete re-analysis of data, spanning 1993-2022, using the ITRF2020/SLRF2020 model and the new IERS EOP20C04 series (consistent with ITRF2020) will be generated in early 2023 to make available to the community a complete and consistent series of weekly TRF solutions. A further enhancement of the SLR products is expected with the introduction of LARES-2 as the fifth satellite contributing to the standard ILRS operational products.An overview of the ILRS contribution to ITRF2020,the current status of the ILRS operational products and the future perspectives will be presented., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The presentation gives an overview over the DTRF2020 combination and its challenges.

Precise orbits of altimetry satellites are a basis for sea surface height estimation, since they provide the reference for the measured distance from a satellite altimeter to a water surface. Various factors affect accuracy of orbits of these satellites, such as type and quality of observations used, proper corrections of measurements, proper modelling of satellite shape, size, optical properties of its surfaces and its orientation in space, accurate modelling of forces acting on a satellite, accurate terrestrial and celestial reference frame realizations and transformation between them, proper modelling of displacements of tracking stations, a proper parameter adjustment algorithm, observation weighting and other factors. Significant progress has been reached in precise orbit determination of altimetry satellites in the latest decades by different orbit determination groups. In this presentation, we investigate the accuracy in the radial direction of contemporary orbits of such altimetry satellites as TOPEX/Poseidon, Jason-1, Jason-2, Jason-3, Sentinel-3A and some others derived in the ITRF2014 realization by various institutions using various types of observations covering totally time span from 1992 until 2021. We investigate single-satellite altimetry crossover differences, radial errors of individual orbit solutions and orbit differences in the radial direction between various orbit solutions for each satellite.

As one of the three ITRS Combination Centres of the IERS, DGFI-TUM is in charge of computing an ITRS 2020 realization. Compared to the ITRS 2014 realization, the situation regarding the input data has changed significantly as not only the observation periods have increased but also new general and technique-specific models are applied in the analysis. Many model changes have an impact on the realized intrinsic scale of the individual techniques in particular and GNSS now provides an independent scale for the first time. This changes the situation for scale realization and validation significantly. With regard to the combination strategy, post-seismic deformation and all three components of non-tidal loading are considered for the first time in DTRF2020. We will present the strategy and results of the DTRF2020 computation. Results from the analysis of the DORIS data and aspects relevant to DORIS applications will be highlighted.

A major problem in the precise orbit determination (POD) of satellites at altitudes below 1,000 km is the modeling of the aerodynamic drag which mainly depends on the thermospheric density and causes the largest non‐gravitational acceleration. Typically, empirical thermosphere models are used to calculate density values at satellite positions but current thermosphere models cannot provide the required accuracy. Thus, unaccounted variations in the thermospheric density may lead to significantly incorrect satellite positions. For the first time, we bring together thermospheric density corrections for the NRLMSISE‐00 model in terms of scale factors with a temporal resolution of 12 hr derived from satellite laser ranging (SLR) and accelerometer measurements. Whereas, the latter are in situ information given along the satellite orbit, SLR results have to be interpreted as mean values along the orbit within the underlying time interval. From their comparison, we notice a rather similar behavior with correlations of up to 80% and more depending on altitude. During high solar activity, scale factors vary around 30% at low solar activity and up to 70% at high solar activity from the value one. In addition, we found the scaled thermospheric density decreasing stronger as the modeled density of NRLMSISE‐00. To check the reliability of the SLR‐derived scale factors, we compare the POD result of two different software packages, namely DOGS‐OC from DGFI‐TUM and GROOPS from IGG Bonn. Furthermore, a validation of our estimated scale factors with respect to an external data set proofs the high quality of the obtained results., Plain Language Summary: Variations in the density of the thermosphere must be taken into account when modeling and predicting the motion of satellites in the near‐Earth environment. Typically, thermospheric densities at the position of satellites are provided by models, but their accuracy is limited. Due to the sensitivity of satellites orbiting the Earth in the altitude range of the thermosphere, they can be used to derive information about the thermospheric density. In this study, we compare for the first time thermospheric density corrections in terms of scale factors for the NRLMSISE‐00 model with a temporal resolution of 12 hr derived from two geodetic measurement techniques, namely satellite laser ranging (SLR) and accelerometry. Our results demonstrate that both measurement techniques can be used to derive comparable scale factors of the thermospheric density, which vary around the desired value one. This indicates to which extent the NRLMSISE‐00 model differs from the observed thermospheric density. On average, during high solar activity, the model underestimates the thermospheric density and can be scaled up using the estimated scale factors. We additionally discuss our estimated scale factors with respect to an external data set. Furthermore, we validate the approach of deriving scale factors from SLR measurements by using two independent software packages., Key Points: For the first time, we compare scale factors of the thermospheric density derived from satellite laser ranging (SLR) and accelerometer measurements. The estimated scale factors vary by up to 30% at low solar activity and up to 70% at high solar activity from the desired value 1. Correlations of 0.7–0.8 are obtained between the estimated scale factors from SLR and accelerometer measurements depending on the height., German Research Foundation (DFG), Technical University of Munich (TUM)