ALBERT

Description: The accurate knowledge of the Earth’s orientation and rotation in space is essential for a broad variety of scientific and societal applications. Among others, these include global positioning, near-Earth and deep-space navigation, the realisation of precise reference and time systems as well as studies of geodynamics and global change phenomena. In this paper, we present a refined strategy for processing and combining Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) observations at the normal equation level and formulate recommendations for a consistent processing of the space-geodetic input data. Based on the developed strategy, we determine final and rapid Earth rotation parameter (ERP) solutions with low latency that also serve as the basis for a subsequent prediction of ERPs involving effective angular momentum data. Realising final ERPs on an accuracy level comparable to the final ERP benchmark solutions IERS 14C04 and JPL COMB2018, our strategy allows to enhance the consistency between final, rapid and predicted ERPs in terms of RMS differences by up to 50% compared to existing solutions. The findings of the study thus support the ambitious goals of the Global Geodetic Observing System (GGOS) in providing highly accurate and consistent time series of geodetic parameters for science and applications.

Description: 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.

Description: The geodetic datum describes the absolute location, orientation and scale of the observed system with respect to a target frame. Since Very Long Baseline Interferometry (VLBI) observations only provide information on the relative relationships between the participating radio telescopes, the introduction of datum conditions is necessary to ensure that the normal equation system is invertible and thus solvable. For some time, "Helmert rendering" or the "no-net-translation (NNT) and no-net-rotation (NNR)" approach are applied to datum-free VLBI normal matrices to regularize the normal matrix system. When using "Helmert rendering", the transformation parameters are added to the system and are forced to be zero. In this case, the condition matrix is used to expand the normal equation system. The results are retrieved in such a way that the sum of translation and rotations with respect to the reference frame are zero. A similar condition matrix is formed with the NNT/NNR approach, which is then weighted and added to the singular normal matrix. In both cases, the datum-free normal matrix is regularized with a datum matrix which needs to belong to the same family as that of a spectral decomposition into an Eigenvector system. A permitted alternative is the scaling of the datum matrix so that the length of each column is one. In this poster, we explain the background and discuss the implications of proper scaling and approximations.

Description: 〉Very Long Baseline Interferometry (VLBI) is the only space geodetic technique that provides the full set of Earth Orientation Parameters (EOP), which are required to transform terrestrial reference frames to celestial reference frames and vice-versa with high accuracy. The purpose of this study is to generate long EOP time series using the Vienna VLBI and Satellite Software (VieVS) with different settings and parametrizations, such as the representations of EOP with piecewise linear offsets and the time intervals of estimation or the handling of the constraints on stations and sources. A special focus will be put on VLBI Global Observing System (VGOS) sessions and the differences with respect to EOP from legacy X/S band sessions. The study will also focus on comparing and combining different space geodetic techniques at the solution level to analyse EOP and to produce single EOP time series.〈/p〉

Description: Observing satellites with VLBI radio telescopes has been topic of research by the geodetic community for decades. Recently, not least because of the approval of ESA's Genesis mission with the planned launch in 2027, the interest has gained new momentum. Genesis with an orbital height of about 6000 km will be equipped with a dedicated VLBI transmitter enabling co-location with other geodetic techniques in space. Moreover, VLBI transmitters can also be used on other satellites, e.g., with the Galileo constellation.In this presentation, we provide an overview of the opportunities with VLBI transmitters on satellites, focusing on the Genesis mission and the Galileo constellation. Based on simulations with the Vienna VLBI and Satellite Software (VieVS), we investigate the benefit for orbit determination as well as frame ties as accessed with station coordinates and Earth orientation parameters. With Galileo, the focus is on the correlation between the right ascension of the ascending node of the orbit with UT1, with Genesis on VLBI station coordinate estimates as measure for transfer of space ties to local ties.

Description: Geophysical effects such as plate tectonics and loading from solid Earth tide, atmospheric pressure, and water mass redistributions (over land and ocean) cause the deformation of Earth’s crust. The variation in the loading of Earth’s crust leads to displacement of the geodetic sites and changes the station coordinates by a few cm on annual to sub-diurnal periods. Thus, geophysical models are implemented within the space geodesy observation equation. The main objective of implementing geophysical modelling is to compute a-priori station coordinates by adding the deformations computed from the geophysical models to the terrestrial reference frame (TRF) coordinates at the observation epochs. In Very Long Baseline Interferometry (VLBI) analysis, we use geophysical models to consider the loading effects due to the global circulation of geophysical fluids (atmosphere, ocean, and continental hydrology) for precise parameter estimation. Thus, it is imperative to use highly accurate geophysical models in VLBI analysis, as VLBI is also one of the major techniques for the determination of the terrestrial reference frame. In this study, we present the comparison of elastic surface loading products, i.e., change in station coordinates retrieved from different loading services such as Uni Strasbourg, TU Wien, International Mass Loading Service, and ESMGFZ. For this study, first, the general comparison of the loading products is made to study the variation in the 3D displacement values for the VLBI stations. Then, we create different files with 3D displacement values of the stations retrieved from different loading services by interpolation. Finally, we present our analysis of variation in the repeatability values of baseline lengths and EOPs by implementing different geophysical model products in the Vienna VLBI and Satellite Software (VieVS).