| 1. |
- Diamantidis, Periklis, 1988
(author)
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Combination of space-geodetic techniques in the era of VGOS and multi-GNSS
- 2023
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Doctoral thesis (other academic/artistic)abstract
- The definition and maintenance of accurate and stable Earth-fixed and inertial reference frames is crucial in studying, among else, geophysical and geodynamical phenomena, precise positioning, and space navigation. The tools used to this end are space-geodetic techniques, like e.g., Very Long Baseline Interferometry (VLBI) and Global Navigation Satellite Systems (GNSS) which utilize signals from extraterrestrial radio sources to determine parameters of interest. An important concept in this regard is that of co-location. Stations of different space-geodetic techniques that are in close vicinity can be linked to the same frequency standards, are subject to similar loading effects, and share common atmosphere. In practice, they are also linked with observations that determine the vector between their respective reference points. These common features and inter-technique observations allow for the stacking of the heterogeneous space-geodetic observables in an estimation process that has been shown to suppress technique-specific biases and improve the accuracy and precision of the inferred parameters. The combination of space-geodetic techniques at co-location sites is delivering high-quality geodetic products. Recent developments in GNSS and VLBI include the incorporation of state-of-the-art instrumentation, in the form of satellite constellations in so-called multi-GNSS, or next-generation radio telescopes in VLBI that comprise the VLBI Global Observing System (VGOS). This thesis presents the principles of VLBI and GNSS, and then studies the untapped potential of novel concepts for the combination of space-geodetic techniques in the era of VGOS and multi-GNSS.
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| 2. |
- Diamantidis, Periklis, 1988, et al.
(author)
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Combining VGOS, legacy S/X and GNSS for the determination of UT1
- 2022
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In: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 96:8
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Journal article (peer-reviewed)abstract
- We perform a combination on the observation level (COL) between VLBI and co-located GNSS in the context of VLBI intensive sessions. Our approach revolves around an estimation procedure which uses 3 h of GNSS data that uniformly encapsulate the 1-h VLBI data, in order to provide consistent troposphere information. We test this approach on both VGOS and Legacy S/X using the VGOS-B and concurrently observed INT1 sessions. The COL strategy is found to increase the precision by 15 % over both session types and leads to an increase of 65 % in the agreement between the sessions when estimating tropospheric gradients every 3 h. A more frequent estimation of the gradients every 1 h, which can be rigorously pursued with the utilization of multi-GNSS, results in a further convergence of the two session types by 30 %. The COL-aided length-of-day (LOD) products also show a 55 % better agreement to external GNSS-derived LOD. In the light of the increasing precision of broadcast GNSS orbits and clocks, this COL strategy can be used to derive rapid UT1-UTC products.
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| 3. |
- Diamantidis, Periklis, 1988, et al.
(author)
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Evaluation of the first three years of VGOS 24 h sessions using a Kalman filter with C5+
- 2023
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In: Earth, Planets and Space. - 1880-5981 .- 1343-8832. ; 75:1
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Journal article (peer-reviewed)abstract
- We analyzed the first three years of VGOS 24 h sessions, and evaluated the performance in terms of Earth orientation parameters (EOP) and station positions. We estimated radio source coordinates which might be of particular importance in VGOS due to the different observing frequencies compared to legacy S/X VLBI. We investigated the impact of this procedure in the determination of the celestial reference, and detected possible increased crosstalk between radio source positions and nutation. We, thus, created an updated source coordinate catalogue that contains information from the VGOS observations and utilized that as our a priori for further analysis. We found that this procedure significantly attenuates mean biases in the estimated EOP time series. We then utilized a kalman filter with an empirically tuned stochastic modelling for nutation and polar motion and estimated repeatabilities on the 60–80 as level for nutation, and 100–130 as level for polar motion. The station position repeatabilities were evaluated to be 1.80 and 2.12 mm for the east and north components and 3.98 mm for the vertical, while the baseline length repeatabilities were estimated to be 4.22 mm at 6000 km. These results are promising with respect to the expected future VGOS performance, while increased attention should be paid to the celestial frame determination in the VGOS observing bands.
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| 4. |
- Diamantidis, Periklis, 1988
(author)
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Exploring Strategies for the Combination of Multiple Space-geodetic Techniques
- 2020
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Licentiate thesis (other academic/artistic)abstract
- Space-geodetic techniques are based on signal acquisition from extraterrestrial radio sources that can be used to infer geodetic positioning and define Earth-fixed and inertial reference systems. These techniques, which include Very Long Baseline Interferometry (VLBI) and Global Navigation Satellite Systems (GNSS) among others, exhibit different strengths and weaknesses in recovering parameters of interest. VLBI, for example, has access to all parameters linking Earth-fixed and inertial reference systems, so-called Earth Orientation Parameters (EOP), while GNSS is superior at determining in one of the EOP, Polar Motion (PM), due to the ubiquity and global distribution of the GNSS network of permanent receivers. The combination of different space-geodetic techniques shows promise in suppressing technique-specific biases and determining parameters with greater precision. This thesis presents the principles of VLBI and GNSS, and then explores the different combination strategies that can be used in the aim of generating of high-quality space-geodetic products.
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| 5. |
- Diamantidis, Periklis, 1988, et al.
(author)
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VLBI and GPS inter- and intra-technique combinations on the observation level for evaluation of TRF and EOP
- 2021
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In: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 73:1
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Journal article (peer-reviewed)abstract
- We study the effects of combination on the observation level (COL) of different space-geodetic techniques and of networks of the same technique and present the corresponding improvement for the determination of station positions and earth orientation parameters. Data from the continuous geodetic very long baseline interferometry (VLBI) campaign CONT17 are used in a batch least-squares (LSQ) estimator. This campaign includes 15 days of observations with two legacy S/X networks, namely Legacy-1 (L1) and Legacy-2 (L2). For this study the VLBI L1 network is used as the base and reference solution. Data from the L1 network are combined first with data from co-located Global Positioning System (GPS) stations by estimating common tropospheric parameters. The derived station positions repeatabilities of the VLBI and GPS networks are evaluated with respect to single-technique solutions. In terms of precision, we find a 25% improvement for the vertical repeatability of the L1 network, and a 10% improvement for the horizontal one. The GPS network also benefits by 20% and 10% in the horizontal and vertical components, respectively. Furthermore, a combined solution using data of the L1 and L2 network is performed by estimating common earth orientation parameters. The combined L1&GPS and L1&L2 solutions are compared to the reference solution by investigating UT1 and polar motion estimates. UT1 is evaluated in terms of mean bias and formal errors with respect to the International Earth Rotation Service (IERS) C04 products which were used as a priori values. The L1&GPS solution has the lowest formal error and mean bias for UT1 with a 30% improvement. The weighted root mean square (WRMS) and weighted mean offset (WMO) differences between the obtained polar motion estimates and the ones derived by the International GNSS Service (IGS) are also compared. We find that the L1&GPS solution gives the lowest WRMS and WMO, exhibiting an average 40% improvement with respect to the reference solution. The presented results highlight the potential of COL for ongoing transition to multi-space geodetic analysis, e.g., Global Navigation Satellite Systems (GNSS) with the next-generation VLBI system.
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| 6. |
- Elgered, Gunnar, 1955, et al.
(author)
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Assessment of GNSS stations using atmospheric horizontal gradients and microwave radiometry
- 2023
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In: Advances in Space Research. - 1879-1948 .- 0273-1177. ; In Press
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Journal article (peer-reviewed)abstract
- We have assessed the quality of four co-located GNSS stations by studying time series of estimated linear horizontal gradients in the signal delay. The stations have different electromagnetic environments. We also examine the consistency of the results by using two different GNSS softwares, GipsyX and c5++, and applying three different elevation cutoff angles: 5°, 10°, and 20°. The estimated gradients are compared with the corresponding ones estimated from microwave radiometer observations acquired during six months (April–September 2021). For all four stations and using both softwares we find that is is possible to track gradient variations over time scales from less than one hour using GPS observations only. We have indications that it is an advantage to equip the area below the GNSS antenna with microwave absorbing material. However, the differences are small, a reduction in rms differences in the gradients compared to those from the microwave radiometer of less than 2 %. More studies are needed to decide if such an investment is reasonable in terms of cost and maintenance.
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| 7. |
- Elgered, Gunnar, 1955, et al.
(author)
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Comparison of four different GNSS antenna installations using estimated atmospheric gradients and microwave radiometry
- 2022
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Conference paper (other academic/artistic)abstract
- We have assessed the quality of four GNSS antenna installations by studying time series of estimated linear horizontal gradients in the signal delay. The stations have different electromagnetic environments. We also examine the consistency of the results by using two different GNSS softwares, GipsyX and c5++, and applying three different elevation cutoff angles: 5◦, 10◦, and 20◦. The estimated gradients are compared to the corresponding ones estimated from microwave radiometer observations acquired during six months (April–September 2021). We have indications that it is an advantage to equip the area below the GNSS antenna with microwave absorbing material. However, the differences are small and more studies are needed to decide if such an investment is reasonable in terms of cost and maintenance.
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| 8. |
- Haas, Rüdiger, 1966, et al.
(author)
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Assessment of Parameters Describing the Signal Delay in the Neutral Atmosphere Derived from VGOS Observations
- 2023
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In: International VLBI Service for Geodesy and Astrometry 2022 General Meeting Proceedings. ; NASA/ CP–20220018789, s. 248-252
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Conference paper (peer-reviewed)abstract
- We use the VGOS session VR2101 to assess parameters describing the signal delay in the neutral atmosphere. VGOS results are compared to corresponding results derived from co-located GNSS stations for all sites, and additionally to results from a ground-based microwave radiometer for the Onsala Space Observatory. For the first time all three independent techniques can be compared with the same high temporal resolution of 5 min. Zenith total delays from VGOS and GNSS reveal correlation coefficients larger than 0.9 for all but one of the participating sites. Corresponding offsets are between 1–8 mm and root mean square differences are between 4–14 mm. Horizontal gradients from VGOS and GNSS have correlation coefficients between 0.2–0.5 for the east components and 0.4–0.7 for the north components. Corresponding offsets are sub-millimeter, and root mean square differences are on the order of 0.5–2.2 mm. The comparisons for the Onsala site of VGOS and GNSS w.r.t. microwave radiometer show correlation coefficients larger than 0.96 for the zenith total delays with offsets on the order of 7–11 mm and root mean square differences of 9–12 mm. Horizontal gradients show correlation coefficients of about 0.2 for the east components and about 0.5 for the north components. The corresponding offsets are between 0.6–1.8 mm and the root mean square differences are 0.9–1.3 mm.
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| 9. |
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| 10. |
- Haas, Rüdiger, 1966, et al.
(author)
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Onsala Space Observatory – IVS Network Station Activities During 2021–2022, in International VLBI Service for Geodesy and Astrometry 2021+2022 Biennial Report, edited by K. L. Armstrong, D. Behrend, and K. D. Baver, NASA/TP-20230014975
- 2023
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Reports (other academic/artistic)abstract
- During 2021 and 2022 we participated in a total of 81 IVS 24-hour legacy S/X sessions with the Onsala 20-m telescope. We observed in a total of 73 VGOS 24-hour sessions with one or both of the Onsala twin telescopes (OTT) in 2021 and 2022. In total we observed 114 VGOS one-hour Intensive sessions during 2021 and 2022, the majority together with the partner station Ishioka in Japan, but also five with Kokee Park. More than 50 % of these sessions involved both Onsala twin telescopes.We also performed 22 local interferometry measurements at Onsala during 2021 and 2022 for measuring the local ties between our telescopes that are used for geodetic VLBI. In 2021 we also performed 91 short (20-minute) flux-monitoring sessions with the OTT as a standalone instrument.
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