ALBERT

Description: The real-time estimation of polar motion (PM) is needed for the navigation of Earth satellite and interplanetary spacecraft. However, it is impossible to have real-time information due to the complexity of the measurement model and data processing. Various prediction methods have been developed. However, the accuracy of PM prediction is still not satisfactory even for a few days in the future. Therefore, new techniques or a combination of the existing methods need to be investigated for improving the accuracy of the predicted PM. There is a well-introduced method called Copula, and we want to combine it with singular spectrum analysis (SSA) method for PM prediction. In this study, first, we model the predominant trend of PM time series using SSA. Then, the difference between PM time series and its SSA estimation is modeled using Copula-based analysis. Multiple sets of PM predictions which range between 1 and 365 days have been performed based on an IERS 08 C04 time series to assess the capability of our hybrid model. Our results illustrate that the proposed method can efficiently predict PM. The improvement in PM prediction accuracy up to 365 days in the future is found to be around 40% on average and up to 65 and 46% in terms of success rate for the PMx and PMy, respectively.

Description: The tropospheric horizontal gradients with high spatiotemporal resolutions provide important information to describe the azimuthally asymmetric delays and significantly increase the ability of ground-based GNSS (Global Navigation Satellite Systems) within the field of meteorological studies, like the nowcasting of severe rainfall events. The recent rapid development of multi-GNSS constellations has potential to provide such high-resolution gradients with a significant degree of accuracy. In this study, we develop a multi-GNSS process for the precise retrieval of high-resolution tropospheric gradients. The tropospheric gradients with different temporal resolutions, retrieved from both single-system and multi-GNSS solutions, are validated using independent numerical weather models (NWM) data and water vapor radiometer (WVR) observations. The benefits of multi-GNSS processing for the retrieval of tropospheric gradients, as well as for the improvement of precise positioning, are demonstrated. The multi-GNSS high-resolution gradients agree well with those derived from the NWM and WVR, especially for the fast-changing peaks, which are mostly associated with synoptic fronts. The multi-GNSS gradients behave in a much more stable manner than the single-system estimates, especially in cases of high temporal resolution, benefiting from the increased number of observed satellites and improved observation geometry. The high-resolution multi-GNSS gradients show higher correlation with the NWM and WVR gradients than the low-resolution gradients. Furthermore, the precision of station positions can also be noticeably improved by multi-GNSS fusion, and enhanced results can be achieved if the high-resolution gradient estimation is performed, instead of the commonly used daily gradient estimation in the multi-GNSS data processing.

Description: In this study, we present a global terrestrial reference frame (TRF) from simulated very long baseline interferometry (VLBI) observations. In the time span from 2008 until 2014, 695 standard VLBI rapid turnaround (R1, R4) 24 h-sessions were simulated using a network of 28 globally distributed stations. Within the software VieVS@GFZ, we apply different measurement noise at the observation level and investigate the impact on the TRF and on the Earth rotation parameters. We find that the effect of varying only the noise applied within the simulation is not proportional to the changes in the estimates and their uncertainties. For instance, increasing the noise level from 15 ps to 300 ps increases the uncertainty of the station positions by a factor of 3.5, of station velocities by 5, of polar motion by 3.4, and of UT1-UTC by 1.5. A comparison with the VLBI-TRF derived from real observations within the same time span shows that the solution simulated with a noise level based on the formal errors of real observations is still too optimistic.

Description: In recent years, Kalman filtering has emerged as a suitable technique to determine terrestrial reference frames (TRFs), a prime example being JTRF2014. The time series approach allows variations of station coordinates that are neither reduced by observational corrections nor considered in the functional model to be taken into account. These variations are primarily due to non-tidal geophysical loading effects that are not reduced according to the current IERS Conventions (2010). It is standard practice that the process noise models applied in Kalman filter TRF solutions are derived from time series of loading displacements and account for station dependent differences. So far, it has been assumed that the parameters of these process noise models are constant over time. However, due to the presence of seasonal and irregular variations, this assumption does not truly reflect reality. In this study, we derive a station coordinate process noise model allowing for such temporal variations. This process noise model and one that is a parameterized version of the former are applied in the computation of TRF solutions based on very long baseline interferometry data. In comparison with a solution based on a constant process noise model, we find that the station coordinates are affected at the millimeter level.

Description: In 2017, the astronomical and geodetic communities will celebrate the 50-year anniversary of the first Very Long Baseline Interferometry (VLBI) experiments. The Journal of Geodesy Special Issue titled “VLBI contribution to reference frames and Earth’s rotation studies” is dedicated to this great jubilee. VLBI plays a key role in astronomy and geodesy. It provides unprecedented spatial resolution and position measurements on the sky with an accuracy superior to other techniques. VLBI is a key space geodetic technique since the 1970s, which fundamentally contributes to the maintenance of terrestrial (TRF) and celestial (CRF) reference frames, including the International Terrestrial (ITRF) and Celestial (ICRF) Reference Frames, and monitoring of the Earth’s rotation with respect to the ITRF and ICRF through determination of highly accurate Earth orientation parameters (EOP). Furthermore, the VLBI technique is unique in determining the Universal Time and the precession–nutation of the Earth’s rotation axis in space. Consequently, it is the only technique capable to provide a consistent TRF–EOP–CRF solution. VLBI also essentially contributes to Solar System dynamics and space and terrestrial navigation, atmospheric studies and refining geophysical models, crustal movements and plate tectonics, time and frequency transfer, and testing physical theories. The most accurate and valuable VLBI results are obtained from global VLBI station networks involving radio telescopes located in different countries on different continents. Therefore, a successful realization of the VLBI observing programs requires both inter-institutional and international cooperation. Since 1999, this cooperation is realized through the International VLBI Service for Geodesy and Astrometry (IVS). Moreover, the IVS provides the combined IVS products, such as EOP and VLBI TRF realization (VTRF). The latter essentially contributes to the multi-technique ITRF solution providing the longest station position time series and defining, together with Satellite Laser Ranging (SLR), the ITRF scale and scale rate. Including the radio source positions in the combination is under investigation. A dedicated paper in this issue describes the current state and perspectives of the IVS operations. The previous Special Issue of Journal of Geodesy devoted to VLBI and published in June 2007 (volume 81, issue 6–8) was highly successful. Since that time, many important changes and impressive improvements in the VLBI technique and analysis happened, such as VLBI2010 technical design, the start of the next-generation global VLBI network VLBI Global Observing System (VGOS), publishing ICRF2 and preparing ICRF3, and implementing new IVS combined EOP–TRF products based on combination of datum-free normal equations. All these researches and developments aim at achieving the 1 mm accuracy in TRF and EOP. To reach this goal, the development of technology should be accompanied by improvement in the geophysical and astronomical modelling as well as in the analysis strategy. This Special Issue of Journal of Geodesy is intended to put together papers describing the latest developments in geodetic and astrometric VLBI. Taking into account the scope of the Journal and the limited size of the volume, only the papers devoted to the geodetic and astrometric data analysis were called for. After the Call for papers was sent to the community, we received many responses from our colleagues expressing their interest to contribute to the volume. We believe that the papers published here comprise a representative collection reflecting the main topics of geodetic and astrometric data analysis, which will provide a good reference source for future research in the field. It is a pleasure to express our gratitude to all the authors presenting their results in this issue. All the papers submitted to this volume have passed a peer-review process following the standard procedure of the Journal of Geodesy. We are much indebted to many reviewers whose valuable comments and suggestions helped to prepare the final papers of high quality included in this volume. Finally, we would like to acknowledge the hard and continuous work of all the IVS components over many years—stations, correlators, technology development, operational and analysis centers that regularly deliver data and products of highest quality for scientific analysis and practical applications.

Description: The differences of atmospheric delays (Atmospheric ties) are theoretically affected by the height differences between antennas at the same site and the meteorological conditions. However, there is often a discrepancy between the expected zenith delay differences and those estimated from geodetic analysis. The purpose of this experiment is to investigate the possibility effects that could caused biases on GNSS atmospheric delays at co-location site.

Description: Terrestrial reference frames (TRF), such as the ITRF2008, are primary products of geodesy. In this paper, we present TRF solutions based on Kalman filtering of very long baseline interferometry (VLBI) data, for which we estimate steady station coordinates over more than 30 years that are updated for every single VLBI session. By applying different levels of process noise, non-linear signals, such as seasonal and seismic effects, are taken into account. The corresponding stochastic model is derived site-dependent from geophysical loading deformation time series and is adapted during periods of post-seismic deformations. Our results demonstrate that the choice of stochastic process has a much smaller impact on the coordinate time series and velocities than the overall noise level. If process noise is applied, tests with and without additionally estimating seasonal signals indicate no difference between the resulting coordinate time series for periods when observational data are available. In a comparison with epoch reference frames, the Kalman filter solutions provide better short-term stability. Furthermore, we find out that the Kalman filter solutions are of similar quality when compared to a consistent least-squares solution, however, with the enhanced attribute of being easier to update as, for instance, in a post-earthquake period.

Description: The Global Geodetic Observing System requires a global terrestrial reference frame (TRF) that should have an accuracy better than 1 mm and a stability better than 0.1 mm/yr as several phenomena in geophysics and climatology such as the prediction of the global sea level rise require a most accurate and stable reference. These goals have not been met so far. Simulation studies allow to better understand the error-limiting factors in the TRF determination and hence, they can contribute to the improvement of the next ITRF. Within project GGOS-SIM we combine normal equation systems (NEQs) of simulated VLBI and SLR observations to determine a global TRF. The time span of 2008-2014 is considered and the software EPOS is employed for the combination. The NEQs include station coordinates, velocities as well as pole coordinates and dUT1. We test different combination strategies including local ties as well as global ties in terms of pole coordinates and proper datum constraints. Our results are compared to ITRF2008 and IERS C04 focusing on origin and scale, i.e. the main contributions of the considered space geodetic techniques to the ITRF.

Description: The Potsdam Open Source Radio Interferometry Tool (PORT) is the very long baseline interferometry (VLBI) analysis software developed and maintained at the GFZ German Research Centre for Geosciences. Chiefly, PORT is tasked with the timely processing of VLBI sessions and post-processing activities supporting the generation of celestial and terrestrial reference frames. In addition, it serves as a framework for research and development within the GFZ's VLBI working group and is part of the tool set employed in educating young researchers. Starting out from VLBI group delays, PORT estimates station and radio sources positions, as well as Earth orientation parameters, tropospheric parameters, and station clock offsets and drifts. The estimation procedures take into account all the necessary data analysis models that were agreed on for contributing to the ITRF2020 processing activities. The PORT code base is implemented in the MATLAB® and Python programming languages. It is licensed under the terms of the GNU General Public License and available for download at GFZ's Git server https://git.gfz-potsdam.de/vlbi-data-analysis/port.