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The Deutsches Geodätisches Forschungsinstitut der Technischen Universität München (DGFI-TUM) is one of the three Combination Centres of the International Earth Rotation and Reference Systems Service for the International Terrestrial Reference System (ITRS). In its upcoming realization of the ITRS, the DTRF2020, DGFI-TUM will again correct for non-tidal loading (NTL) effects at the normal equation level. Next to the dedicated NTL data set for the ITRS 2020 realization provided by the Global Geophysical Fluid Center (GGFC), we also considered the data provided by the Earth System Modelling group of the Deutsches GeoForschungsZentrum (ESMGFZ). Besides also comprising all NTL components (atmospheric, oceanic, hydrological) and being mass conserving, the ESMGFZ data has the advantage of daily availability and is already in use at DGFI-TUM. The decision for one or the other data set depends on their suitability for a secular terrestrial reference frame like the DTRF2020, which will be assessed in this work. Although we also compare the site displacements induced by NTL to the residuals of station positions of the Global Navigation Satellite Systems, we will not evaluate the quality of the underlying geophysical models per se. The two data sets differ w.r.t. the underlying hydrological models and the treatment of non-tidal oceanic loading, but the most relevant difference is given in terms of trends in the displacement time-series. After a close investigation of the latter, we finally decided to apply the GGFC contribution to the ITRS 2020 realization in the DTRF2020. [ABSTRACT FROM AUTHOR]

The DTRF2014 solution is the latest ITRS realization of DGFI-TUM computed in the framework of ITRF2014. It is mainly characterized by two innovations: it is the first secular ITRS realization considering non-tidal loading corrections derived from geophysical models. Secondly, the DTRF2014 release includes all information necessary to approximate the observed instantaneous station positions at any epoch of the observation time span. Therefore, besides the classical SINEX and EOP files, the applied non-tidal loading corrections, the residual time series of station positions as well as the translation time series of the DTRF2014 origin are provided. The DTRF2014 is computed from the same input as ITRF2014 comprising the full history of observation data of the four space geodetic techniques Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite System (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). The DTRF2014 solution is computed using the DGFI-TUM approach based on the combination of normal equation systems. With reference to the IERS Conventions, the origin of DTRF2014 is realized from SLR input data only. The scale of the DTRF2014 solution is assumed to be statistically equal for SLR and VLBI. This assumption is based on tests performed in the framework of the DTRF2014 computation showing a range for the scale offset at epoch 2000.0 of ± 3.3 mm. For the GNSS and the DORIS subnetwork of the DTRF2014 solution, a weighted mean scale of SLR and VLBI is realized. Cross-validations performed between DTRF2014 and ITRF2014 show the high consistency of present ITRS realizations. For GNSS, VLBI and SLR the obtained transformation parameters range within ± 1.7 mm for positions (except for the VLBI and SLR scale and the VLBI z-translation) and ± 0.3 mm/yr for the velocities, the parameters for DORIS are all below 4.2 mm and 0.25 mm/yr (except for the scale rate). The RMS of the transformations, reflecting the agreement of network geometry, range between 0.55 mm for VLBI and 2.25 mm for DORIS. The RMS of station velocities is between 0.08 and 0.62 mm/yr. The DTRF2014 solution provides a high precision and reliability confirmed by various validations performed internally and by external groups. For instance, applications of DTRF2014 for a precise orbit determination (POD) of the altimetry satellite TOPEX and Jason-2 show small radial crossfit residuals between SLR and DORIS and indicate a good performance of DTRF2014. [ABSTRACT FROM AUTHOR]

The International DORIS Service (IDS) submitted input data for the most recent realization of the International Terrestrial Reference System (ITRS), the International Terrestrial Reference Frame 2014 (ITRF2014). As one of the ITRS Combination Centers, DGFI-TUM is in charge to analyze and assess the quality of the submitted data and to compute a combined global TRF solution (called DTRF2014) using observations of the four geodetic space techniques GNSS, VLBI, SLR and DORIS. The combination methodology used at DGFI-TUM is based on the combination of datum-free normal equations. Together with station coordinates and velocities, terrestrial pole coordinates are estimated in one common adjustment. The paper presents the analysis results of the most recent DORIS submission IDS-d09 and evaluates its quality w.r.t. the DTRF2008 (IDS-only) solution. In the most recent version of the analysis, we introduce in total 55 station discontinuities and reduce 15 stations due to a too short time span or too few observations. Time series of weekly IDS solutions are computed and validated w.r.t. DTRF2008. The transformation parameter time series and the station residuals are discussed in detail. Especially the scale parameter time series shows a significant improvement compared to the DTRF2008 input data. The scatter of the x - and y -translation is significantly reduced to 5.7 mm and 7.1 mm compared to 6.6 mm and 8.1 mm for the DTRF2008 (IDS-only) solution. The z -translation time series still shows a high correlation with solar activity. 10% of all station residuals are significantly affected by spectral peaks at draconitic period harmonics of the altimetry satellites Jason and TOPEX/Poseidon and up to 48% of all station residual time series contain significantly determined frequencies with a 14 day period. The multi-year IDS solution is validated w.r.t. DTRF2008 and the consistently estimated terrestrial pole coordinates are analyzed and compared to IERS 08 C04. The x-pole spectra comprises prominent peaks at various draconitic frequencies. [ABSTRACT FROM AUTHOR]

In the conventions of the International Earth Rotation and Reference Systems Service (e.g. IERS Conventions 2010), it is recommended that the instantaneous station position, which is fixed to the Earth's crust, is described by a regularized station position and conventional correction models. Current realizations of the International Terrestrial Reference Frame use a station position at a reference epoch and a constant velocity to describe the motion of the regularized station position in time. An advantage of this parameterization is the possibility to provide station coordinates of high accuracy over a long time span. Various publications have shown that residual non-linear station motions can reach a magnitude of a few centimeters due to not considered loading effects. Consistently estimated parameters like the Earth Orientation Parameters (EOP) may be affected if these non-linear station motions are neglected. In this paper, we investigate a new approach, which is based on a frequent (e.g. weekly) estimation of station positions and EOP from a combination of epoch normal equations of the space geodetic techniques Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI). The resulting time series of epoch reference frames are studied in detail and are compared with the conventional secular approach. It is shown that both approaches have specific advantages and disadvantages, which are discussed in the paper. A major advantage of the frequently estimated epoch reference frames is that the non-linear station motions are implicitly taken into account, which is a major limiting factor for the accuracy of the secular frames. Various test computations and comparisons between the epoch and secular approach are performed. The authors found that the consistently estimated EOP are systematically affected by the two different combination approaches. The differences between the epoch and secular frames reach magnitudes of $$23.6~\upmu \hbox {as}$$ (0.73 mm) and $$39.8~\upmu \hbox {as}$$ (1.23 mm) for the x-pole and y-pole, respectively, in case of the combined solutions. For the SLR-only solutions, significant differences with amplitudes of $$77.3~\upmu \hbox {as}$$ (2.39 mm) can be found. [ABSTRACT FROM AUTHOR]

A new realization of the International Terrestrial System was computed at the ITRS Combination Centre at DGFI as a contribution to ITRF2008. The solution is labelled DTRF2008. In the same way as in the DGFI computation for ITRF2005 it is based on either normal equation systems or estimated parameters derived from VLBI, SLR, GPS and DORIS observations by weekly or session-wise processing. The parameter space of the ITRS realization comprises station positions and velocities and daily resolved Earth Orientation Parameters (EOP), whereby for the first time also nutation parameters are included. The advantage of starting from time series of input data is that the temporal behaviour of geophysical parameters can be investigated to decide whether the parameters can contribute to the datum realization of the ITRF. In the same way, a standardized analysis of station position time series can be performed to detect and remove discontinuities. The advantage of including EOP in the ITRS realization is twofold: (1) the combination of the coordinates of the terrestrial pole-estimated from all contributing techniques-links the technique networks in two components of the orientation, leading to an improvement of consistency of the Terrestrial Reference Frame (TRF) and (2) in their capacity as parameters common to all techniques, the terrestrial pole coordinates enhance the selection of local ties as they provide a measure for the consistency of the combined frame. The computation strategy of DGFI is based on the combination of normal equation systems while at the ITRS Combination Centre at IGN solutions are combined. The two independent ITRS realizations provide the possibility to assess the accuracy of ITRF by comparison of the two frames. The accuracy evaluation was done separately for the datum parameters (origin, orientation and scale) and the network geometry. The accuracy of the datum parameters, assessed from the comparison of DTRF2008 and ITRF2008, is between 2-5 mm and 0.1-0.8 mm/year depending on the technique. The network geometry (station positions and velocities) agrees within 3.2 mm and 1.0 mm/year. A comparison of DTRF2008 and ITRF2005 provides similar results for the datum parameters, but there are larger differences for the network geometry. The internal accuracy of DTRF2008-that means the level of conservation of datum information and network geometry within the combination-was derived from comparisons with the technique-only multi-year solutions. From this an internal accuracy of 0.32 mm for the VLBI up to 3.3 mm for the DORIS part of the network is found. The internal accuracy of velocities ranges from 0.05 mm/year for VLBI to 0.83 mm/year for DORIS. The internal consistency of DTRF2008 for orientation can be derived from the analysis of the terrestrial pole coordinates. It is estimated at 1.5-2.5 mm for the GPS, VLBI and SLR parts of the network. The consistency of these three and the DORIS network part is within 6.5 mm. [ABSTRACT FROM AUTHOR]

Precise orbits of altimetry satellites are a prerequisite for the investigation of global, regional, and coastal sea levels together with their changes, since accurate satellite positions in the radial direction are required for the reliable determination of the water surface height (distance between the altimeter position in space and the water surface). Significant progress in the improvement of altimetry satellite orbit quality has been achieved in the last 30 years increasing the orbit accuracy in the radial direction from decimeter to centimeter and even sub-centimeter level. That was possible due to the improvements in the modeling of Earth's time variable gravity field, ocean tides, terrestrial and celestial reference frames, but also due to the accomplishments reached in the observation methods used for altimetry satellites, namely Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and Global Positioning System (GPS—used for some satellites). In this paper, we review the main improvements in the models used for the determination of orbits of altimetry satellites, namely, in so called Geophysical Data Records (GDR) orbit standards from GDR-C to Precise Orbit Ephemeris-F (POE-F), illustrate the impact of the improvements in precise orbit determination of these satellites on the orbit accuracy in the radial direction. Additionally we investigate orbit differences in the radial direction, single-satellite crossover differences, radial, and geographically correlated orbit errors of contemporary orbits of various altimetry satellites namely Cryosat-2, Envisat, ERS-1, ERS-2, Jason-1, Jason-2, Jason-3, SARAL, Sentinel-3A, Sentinel-3B, and TOPEX/Poseidon derived by different institutions. [ABSTRACT FROM AUTHOR]