On the impact of local ties on the datum realization of global terrestrial reference frames

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.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&#160; PM, commonly treated together as HAM/CAM.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.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 C20 coefficient correction.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).

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Assessment on atmospheric parameters at co-location sites

10.5194/egusphere-egu2020-6517 ◽

2020 ◽

Author(s):

Chaiyaporn Kitpracha ◽

Kyriakos Balidakis ◽

Robert Heinkelmann ◽

Harald Schuh

Keyword(s):

Reference Frames ◽

Meteorological Data ◽

Tropospheric Delay ◽

Atmospheric Parameters ◽

Local Ties ◽

Zenith Delay ◽

Terrestrial Reference Frames ◽

Weather Model ◽

Space Geodetic Techniques ◽

Seasonal Signals

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&#8211;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.

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Determination of precise Galileo orbits using combined GNSS and SLR observations

GPS Solutions ◽

10.1007/s10291-020-01045-3 ◽

2020 ◽

Vol 25 (1) ◽

Author(s):

Grzegorz Bury ◽

Krzysztof Sośnica ◽

Radosław Zajdel ◽

Dariusz Strugarek ◽

Urs Hugentobler

Keyword(s):

Orbit Determination ◽

Reference Frames ◽

Semimajor Axis ◽

Precise Orbit Determination ◽

Orbital Plane ◽

Normal Equation ◽

Terrestrial Reference Frames ◽

Formal Error ◽

Combined Solution ◽

The Impact

Abstract 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.

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Precise Galileo orbit determination using combined GNSS and SLR observations

10.5194/egusphere-egu2020-340 ◽

2020 ◽

Author(s):

Grzegorz Bury ◽

Krzysztof Sośnica ◽

Radosław Zajdel ◽

Dariusz Strugarek

Keyword(s):

Orbit Determination ◽

Reference Frames ◽

European Space Agency ◽

Pilot Project ◽

Laser Ranging ◽

Navigation Satellite ◽

International Gnss Service ◽

Terrestrial Reference Frames ◽

Gnss Data ◽

The Impact

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.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.&#160;&#160; &#160;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 &#160;and GNSS data. However, in such a manner, we deteriorate the internal consistency of the solution, i.e., the orbit misclosures. &#160;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.&#160;

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On the prospects of a future GNSS constellation on the global terrestrial reference frame

10.5194/egusphere-egu2020-3620 ◽

2020 ◽

Author(s):

Susanne Glaser ◽

Grzegorz Michalak ◽

Rolf Koenig ◽

Benjamin Maennel ◽

Harald Schuh

Keyword(s):

Reference Frames ◽

Time Synchronization ◽

Orbital Plane ◽

Satellite System ◽

Low Earth Orbit ◽

Earth Orbit ◽

Terrestrial Reference Frames ◽

Earth Rotation Parameters ◽

The Impact

Global terrestrial reference frames (TRFs), as one of the most important geodetic products, currently miss the imperative requirements of 1 mm accuracy and 1mm/decade long-term stability. In this study, the prospects of a future Global Navigation Satellite System (GNSS) to improve global TRFs is assessed by simulations. The future constellation, named &#8220;Kepler&#8221;, is proposed by the German Aerospace Center DLR in view of the next generation Galileo system. In addition to a contemporary Medium Earth Orbit (MEO) segment with 24 satellites in three orbital planes, Kepler consists of six Low Earth Orbit (LEO) satellites in two near polar planes, all carrying long-term stable optical clocks. The MEO satellites in one orbital plane and the LEO and MEO satellites in different planes are connected with optical two-way inter-satellite links (ISLs) as the innovative key feature. The ISLs allow very precise range measurements and time synchronization (at the picosecond-level) between the satellites. Different simulation scenarios are set up to evaluate the impact of the Kepler features on the TRF-defining parameters origin and scale as well as on the Earth rotation parameters (ERPs). The origin of a Kepler-only TRF improves considerably by factors of 8, 8, and 43 in X, Y, and Z direction, respectively, w.r.t. a Galileo-only solution. The scale realized by a Kepler-TRF shows improvements of 34% w.r.t. Galileo-only. In a combination with simulated observations of Very Long Baseline Interferometry the impact on multi-technique TRFs is assessed as well. The ERPs of both techniques are combined as global ties and benefits especially on the determination of UT1-UTC are expected.

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Earth rotation variations observed by VLBI and the Wettzell “G” ring laser during the CONT17 campaign

Advances in Geosciences ◽

10.5194/adgeo-50-9-2019 ◽

2019 ◽

Vol 50 ◽

pp. 9-15 ◽

Cited By ~ 1

Author(s):

Sigrid Böhm ◽

Matthias Schartner ◽

André Gebauer ◽

Thomas Klügel ◽

Ulrich Schreiber ◽

...

Keyword(s):

Earth Rotation ◽

Ring Laser ◽

Polar Motion ◽

Reference Frames ◽

Reference Value ◽

Terrestrial Reference Frames ◽

Earth Rotation Parameters ◽

Sparse Networks ◽

The Impact ◽

Earth Rotation Variations

Abstract. The VLBI (Very Long Baseline Interferometry) technique can provide the full set of parameters needed for the transformation between celestial and terrestrial reference frames with high accuracy. Yet it has some limitations regarding temporal resolution and continuity, and the accuracy of the resulting Earth Orientation Parameters (EOP) varies depending on the network geometry. In this work we explore the benefit of combining VLBI observations with the measurements of the large ring laser gyroscope “G” in Wettzell for deriving highly resolved ERP (Earth Rotation Parameters, i.e. polar motion and universal time variations, δUT1). We examine the observations collected by two simultaneously operating VLBI networks during the 15 d of the CONT17 campaign. These two networks, of 14 globally distributed telescopes each, were designed for the estimation of Earth rotation variations, for which reason the resulting hourly ERP are appointed as benchmark in this investigation. To evaluate the advantage of a VLBI and ring laser combined solution, we create degraded versions of the original networks, containing only six stations. The ERP derived from those sparse networks and from the VLBI sparse plus ring laser solutions are then compared in terms of differences to the reference values. It should certainly be considered that these are relative numbers, since they are also determined by the number and selection of the stations remaining in the sparse networks. The root mean square of the difference to the benchmark is reduced by 24 % in case of δUT1 from one network. The polar motion yp component from the same network moves 14 % closer to the reference value due to the inclusion of the ring laser data. The impact on xp and on all ERP from the other network ranges between 2 % and 9 %. The research again confirms the feasibility and also the potential gain of a combined evaluation of VLBI and ring laser observations, but the full capacity of such a sensor fusion will emerge once the ring laser gyroscopes reach a level of accuracy similar to VLBI.

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