Transformations between the Slovenian and international terrestrial reference frames

The current Slovenian terrestrial reference frame (D96-17) is a static frame based on GNSS technology. An additional transformation connecting it with the new realisation of ETRS89 accepted by EUREF (D17) gives the D96-17 a specific character. In order to ensure a high-quality national terrestrial reference frame, connection to the current realisation of ITRS is needed. This change is particularly important in the light of the intended transition to the semi-kinematic terrestrial reference frame, supported by a national geo-kinematic model. Transformations between the current national and international terrestrial reference frames are discussed in detail in the present paper. Processes, equations, and parameters of datum transformations are given in both directions (forward and inverse), step-by-step and direct ones, rigorous and simplified (approximate). Furthermore, an analysis of coordinate differences between current Slovenian and international terrestrial reference frames and an analysis of coordinate errors for various simplifications of transformation between both reference frames are given. This allows users to choose an optimal transformation solution to meet their requirements. The role and importance of transformations under consideration in the positioning procedures and the precise navigation are also addressed.

The Improvement in the Positioning on the Nubian and Somalia Plates by Updating Terrestrial Reference Frames

International Terrestrial Reference Frames (ITRF) is an accurate and standard frame for referencing positions at different times and in different locations around the world. The International Global Navigation Satellite System (GNSS) Service, (IGS) enable global positioning and timing at the highest possible accuracy through modernized datum’s aligned with the ITRF. In addition, ITRF site velocities for any location within Africa are between 24 and 31 mm/yr due to rigid motion of the African plate over the underlying mantle. The African plate can be divided into two Nubian and Somalia sub-plates. In the present work, the rotation rates about Euler poles and position improvement in Nubian and Somalia plates with updated ITRFs are investigated. Among the results in this study, when using a rigid plate movement and instantaneous ITRF coordinates to transform a fixed reference epoch, in case of Nubian plate, the residual in positioning and Root Mean Square Error (RMS) improved with the updating of the frame and the best results of residual and RMS appear in frame 2008 by values (0.149,0.011) m. respectively but in Somali plate, residual and RMS increased with the updating of the frame and the best results appear in frame2014 by values (i.,e., 0.096, 0.012) m, respectively.

Assessing the role of corrections included in the GRACE/GRACE-FO data in determination of hydrological and cryospheric signal in polar motion excitation

<p>Assessing the impact of continental hydrosphere and cryosphere on polar motion (PM) variations is one of the crucial tasks in contemporary geodesy. The pole coordinates, as one of the Earth Orientation Parameters, are needed to define the relationship between the celestial and terrestrial reference frames. Therefore, the variations in PM should be monitored and interpreted in order to assess the role of geophysical processes in this phenomenon.</p><p>The role of hydrological and cryospheric signals in PM is usually examined by computing hydrological excitation (hydrological angular momentum, HAM) and cryospheric excitation (cryospheric angular momentum, CAM) of  PM, commonly treated together as HAM/CAM.</p><p>The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions deliver temporal variations of the gravity field resulting from changes in global mass redistribution. The so-called GRACE/GRACE-FO Level-3 (L3) data delivers changes in terrestrial water storage (TWS) that can be used for computation of HAM/CAM.</p><p>For best possible representation of TWS, a number of corrections are introduced in the L3 data by computing centres. Such corrections are, among others, glacial isostatic adjustment (GIA) correction, geocenter correction and C<sub>20</sub> coefficient correction.</p><p>The main goal of this study is to examine the impact of corrections included in GRACE/GRACE-FO data on HAM/CAM determined. More specifically, we test their influence on HAM/CAM trends, seasonal changes and non-seasonal variations. We also examine the change in compliance between HAM/CAM and hydrological plus cryospheric signal in geodetically observed excitation when the corrections are used. To achieve our goals, we use GRACE and GRACE-FO L3 datasets provided by Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC).</p>

Validation of tropospheric ties at the test setup GNSS co-location site in Potsdam

<p>Atmospheric ties are induced by differences between the set-up of observing geodetic systems at co-location sites, are mainly attributed to frequency and position, and are usually quantified by zenith delay and gradient component offsets derived by weather models or in situ instuments.. Similar to local ties, they could be applied to combine datasets from several space geodetic techniques, thus contributing to the improvement of the realization of terrestrial reference frames (TRF). Theoretically, atmospheric ties are affected only by the height differences between antennas at the same site and meteorological conditions. Therefore, atmospheric ties could be determined analytically based on meteorological information from in situ measurements or weather models. However, there is often a discrepancy between the expected zenith delay differences and those estimated from geodetic analysis, potentially degrading a combined atmospheric ties solution should tight constraints be used. In this study, we set up a GNSS experiment campaign on the rooftop of a building in Telegrafernberg that offers unobscured data coverage for one month. We compared the estimated zenith delay and gradients from GNSS stations in this experiment, applying atmospheric ties from (1) meteorological data from the Global Pressure and Temperature model 3 (GPT3), (2) ERA5 reanalysis, and (3) in-situ measurements, as well as corrections derived from ray tracing (Potsdam Mapping Functions, PMF). The results show that atmospheric ties employing GPT3, ERA5, in-situ measurements, and ray tracing has an excellent and comparable performance in term of bias mitigation, but not in term of standard deviation, for zenith delay. Moreover, the unexpected bias in zenith delay was identified in the antenna with radome installation. A significantly large bias was identified in estimated gradients; the source of this discrepancy has been traced back to unmitigated multipath effects in this experiment.</p>

BKG’s new series of 24-hour and 1-hour VLBI experiments

<p>With the VLBI technique radio sources are observed in dedicated time intervals. The most usual length of these observing sessions are 24 and 1-hour long. 24-hour long experiments usually incorporate a global network of stations, and, thus, are the prominent source of a consistent determination of all Earth Orientation Parameters (EOPs), celestial and terrestrial reference frames. The shorter experiments are designed to determine dUT1 parameter only. The number of short or intensive sessions is growing every year. Also some of them involve 3-4 stations in observation programs instead of standard 2-station mode. This leads to a larger number of observations per session, a better coverage of the Earth, and, consequently more accurate dUT1 estimates.</p><p>All 24-hour and 1-hour sessions since 1984 up to now were re-processed by BKG using the most up-to-date modelling within the parameter estimation. This results in new series of consistently estimated EOPs, station coordinates and troposphere parameters.</p><p>In this contribution we present our new series and investigate the quality of the obtained geodetic products, especially the EOPs. The work is focused on the consistency between dUT1 parameters derived from 24-hour and 1-hour sessions, respectively. In this study we pinpoint challenges and prospects of the inclusion of 1-hour experiments into the standard analysis of the 24-hour experiments.</p>

Analysis of differential residuals of SLR observations to GNSS: Methodology and results from analyzing 25 years of data

<p>The SLR observations to GNSS play a significant role as space tie, and allow investigations of many quantities related to the global Terrestrial Reference Frames (TRF), e.g., satellite orbits, scale, station coordinates, local ties. The differences between the observed ranges (via SLR observations) minus the computed spatial distances (via GNSS orbits based on GNSS observations) form the so-called “SLR residuals”. The analysis of these SLR residuals offers the opportunity to investigate the biases of the SLR measurements, the quality of the GNSS orbits and the quality and consistency of station coordinates. However, the absolute residuals contain a various number of inconsistencies and systematics which are not straightforward to be identified and separated, and, therefore to be further investigated. The present study focuses on the derivation of three alternative scenarios/cases through the usage of differential residuals between epochs, satellites and stations. These differential SLR residuals are derived from the processing of 25 years of SLR observations to GNSS (using GPS and GLONASS). The advantage of using the differential residuals is the elimination of one or more sources of systematic errors, according to each scenario. The comparison between the absolute and the differential residuals, respectively, is proven to stand as a useful diagnostic tool for the assessment of the systematic effects.</p>

Determination of precise Galileo orbits using combined GNSS and SLR observations

Galileo satellites are equipped with laser retroreflector arrays for satellite laser ranging (SLR). In this study, we develop a methodology for the GNSS-SLR combination at the normal equation level with three different weighting strategies and evaluate the impact of laser observations on the determined Galileo orbits. We provide the optimum weighting scheme for precise orbit determination employing the co-location onboard Galileo. The combined GNSS-SLR solution diminishes the semimajor axis formal error by up to 62%, as well as reduces the dependency between values of formal errors and the elevation of the Sun above the orbital plane—the β angle. In the combined solution, the standard deviation of the SLR residuals decreases from 36.1 to 29.6 mm for Galileo-IOV satellites and |β|> 60°, when compared to GNSS-only solutions. Moreover, the bias of the Length-of-Day parameter is 20% lower for the combined solution when compared to the microwave one. As a result, the combination of GNSS and SLR observations provides promising results for future co-locations onboard the Galileo satellites for the orbit determination, realization of the terrestrial reference frames, and deriving geodetic parameters.

Assessment on atmospheric parameters at co-location sites

<p>Atmospheric ties are affected by the differences of atmospheric parameters of space geodetic techniques at co-location sites. Similar to local ties, they could be applied along with local ties for a combination of space geodetic techniques to improve the realization of terrestrial reference frames (TRF). Theoretically, atmospheric ties are affected by the height differences between antennas at the same site and meteorological conditions. Therefore, atmospheric ties could be determined by analytical equation based on meteorological information from in situ measurements or weather model. However, there is often a discrepancy between the expected zenith delay differences and those estimated from geodetic analysis, thus potentially degrading a combined atmospheric ties solution. In this study, we analyse the time series of zenith delays from co-located GNSS antennas at Wettzell (height differences below 3 meters), for 11 years (2008–2018). GNSS observations were analyzed with Bernese GNSS software version 5.2 with double-differencing technique and relative tropospheric delay and gradients were estimated with L1, L2, and the ionosphere-free (L3) linear combination thereof. Atmospheric ties were derived analytically employing meteorological data from Global Pressure and Temperature model 3 (GPT3) and ERA5 reanalysis, as well as corrections derived from ray tracing (Potsdam Mapping Functions, PMF). The comparison shows that zenith delay differences are dominated by equipment changes. The discrepancies between atmospheric ties and estimated zenith delay differences are frequency dependent, with the L1 solutions being the least biased. For these small vertical differences, seasonal signals are not significant for all frequencies.</p>

Precise Galileo orbit determination using combined GNSS and SLR observations

<p>The European navigation system Galileo is on its final stretch to become a fully operational capability (FOC) Global Navigation Satellite System (GNSS). The current constellation consists of 24 healthy satellites decomposed into three Medium Earth Orbits and since late 2016 is considered as an operational system. So far, the official Galileo orbits are provided by the European Space Agency and in the frame of the International GNSS Service (IGS) Multi-GNSS pilot project (MGEX) whose one of the goals is to develop orbit determination strategies for all new emerging navigation satellite systems.</p><p>All the Galileo satellites are equipped with Laser Retroreflector Arrays (LRA) for Satellite Laser Ranging (SLR). As a result, a number of Galileo satellites is tracked by laser stations of the International Laser Ranging Service (ILRS). SLR measurements to GNSS, such as Galileo, comprise a valuable tool for the validation of the orbit products as well as for an independent orbit solution based solely on laser ranging data. However, the SLR data may be used together along with the GNSS observations for the determination of the combined GNSS orbit using the two independent space techniques co-located onboard the Galileo satellites. The Galileo orbit determination strategies, as well as the usage of laser ranging to the navigation satellites, is crucial, especially in the light of the discussion concerning possible usability of the Galileo observation in the future realizations of the International Terrestrial Reference Frames.    </p><p>In this study, we present results from the precise Galileo orbit determination using the combined GNSS data transmitted by the Galileo satellites and the range measurements performed by the ILRS stations. We test different weighting strategies for GNSS and SLR observations. We test the formal errors of the Keplerian elements which significantly decrease when we apply the same weights for SLR  and GNSS data. However, in such a manner, we deteriorate the internal consistency of the solution, i.e., the orbit misclosures.  </p><p>For the solution with optimal weighting strategy, we present results of the quality of Galileo orbit predictions based on the combined solutions, as well as the SLR residuals. The combined GNSS+SLR solution seems to be especially favorable for the Galileo In-Orbit Validation (IOV) satellites, for which the standard deviation (STD) of the SLR residuals decreases by 13% as compared to the microwave solutions, whereas for the Galileo-FOC satellite the improvement of the STD of SLR residuals is at the level of 9%. Finally, we test the impact of adding SLR observations to the LAGEOS satellites which stabilizes the GNSS solutions, especially in terms of the realization of terrestrial reference frame origin. </p>