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Improving and homogenizing time and space reference systems on Earth and, more specifically, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1 mm and a long-term stability of 0.1 mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, such as those located at tide gauges, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation, contributing to a better understanding of natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities, including the International Association of Geodesy (IAG), which has enunciated geodesy requirements for Earth sciences. Moreover, the United Nations Resolution 69/266 states that the full societal benefits in developing satellite missions for positioning and Remote Sensing of the Earth are realized only if they are referenced to a common global geodetic reference frame at the national, regional and global levels. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology. This paper has been written and supported by a large community of scientists from many countries and working in several different fields of science, ranging from geophysics and geodesy to time and frequency metrology, navigation and positioning. As it is explained throughout this paper, there is a very high scientific consensus that the GENESIS mission would deliver exemplary science and societal benefits across a multidisciplinary range of Navigation and Earth sciences applications, constituting a global infrastructure that is internationally agreed to be strongly desirable. [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]

Earth Orientation Parameters (EOPs) are derived from various space geodetic observation techniques such as GNSS, SLR, DORIS and VLBI. The combination of the different techniques leads to a complete EOP series which is officially released by the IERS after a processing time of approximately 30 days in the case of the final IERS series (C04). In order to reduce the latency for near-real time applications, the so-called Intensive and Rapid products are utilized for VLBI and GNSS, respectively. In this context, the data processing can be accelerated to a delay of 1-2 days. However, neither VLBI nor GNSS solely deliver a complete set of EOPs, and the combination is not done yet in a fully rigorous way, i.e., by taking into account all correlations, especially between polar motion and Universal Time (dUT). Therefore, the present study focuses on a combination of VLBI Intensive and GNSS Rapid solutions on the basis of Normal Equations (NEQ) containing all common parameters, i.e., station coordinates, polar motion and dUT. Regarding different constraint scenarios a daily stacking of the specific NEQs is carried out. The combined solution leads then to an improvement of the numerical NEQ stability and a fully consistent estimation of polar motion and dUT. This contribution focuses on the combination of both VLBI Intensive and GNSS Rapid NEQs for a consistent estimation of a full set of EOPs. In this context, investigations of the obtained accuracies have been carried out. The combined estimates of polar motion, dUT and their associated rates are therefore discussed and compared with respect to the official reference series and the specific single-technique solutions. Regarding the time interval of the last three years, a significant improvement of the accuracy as well as of the reliability could already be shown. [ABSTRACT FROM AUTHOR]

Conventional long-term Terrestrial Reference Frames (TRF) based on station positions and velocities suffer from the fact that they are outdated from the moment of their realisation (publication). Especially in regions with high seismic activities, their usability is often limited to a very short period of time. Moreover, the geocentricity of the TRF, i.e. its origin defined to coincide with the Earth's centre-of-mass as well as its scale defined to be equal to the common SLR and VLBI scale, is not ensured over time. Regional reference frames like SIRGAS (Sistema de Referencia Geocéntrico para las Américas), on the other hand, are usually realised solely by densifying the global conventional TRF via fiducial points; thus, also the geocentricity of the densified TRF is only given at a certain point of time. Within the DFG-funded project DIrect GEocentric Realisation of the American reference frame by combination of geodetic observation TechnIques (DIGERATI), investigations with respect to the realisation of an Epoch Reference Frame (ERF) are being performed. The ERF shall be realised directly and geocentrically, i.e. from a global station network, being densified for the South American subcontinent. The investigations are focussed on the optimum combination strategies of the geodetic techniques as well as on the optimum "sampling" of realisations (e.g. monthly) in order to obtain a sufficient amount of observations in each ERF realisation. The results are compared to the current conventional realisation. [ABSTRACT FROM AUTHOR]

SLR is one of the four geodetic techniques contributing to the realization of terrestrial reference frames (TRFs). It is the unique technique enabling a highly accurate determination of the origin, and it contributes to the determination of the TRF scale. The current SLR tracking network suffers from an insufficient network geometry due to a lack of stations especially in the southern hemisphere. Previous simulation studies have shown that the extension of the global SLR tracking network is indispensable for reaching the target accuracy of future TRFs and the ambitious GGOS goals. The simulation studies have put focus on a determination of the locations where additional SLR stations are most valuable for an improved estimation of the geodetic parameters.Within the present study, we perform a simulation of a set of stations distributed equally over the globe and comparing different solutions, always adding one of these simulated stations to the real SLR station network. This approach has been chosen in order to be able to investigate the deficiencies of the existing SLR network and to judge in which regions on the globe an additional SLR station could be most valuable for the improvement of certain geodetic parameters of SLR-derived reference frames. [ABSTRACT FROM AUTHOR]