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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)