| 1. |
- Diamantidis, Periklis, 1988, et al.
(författare)
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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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Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 73:1
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Tidskriftsartikel (refereegranskat)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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| 3. |
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| 4. |
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| 5. |
- Haas, Rüdiger, 1966, et al.
(författare)
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VLBI with GNSS signals on intercontinental baselines
- 2018
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Ingår i: - 9789188041142
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Konferensbidrag (refereegranskat)abstract
- The International Terrestrial Reference Frame (ITRF) is constructed based on analysis results of several space geodetic techniques, among them geodetic Very Long Baseline Interferometry (VLBI) and Global Navigation Satellite Systems (GNSS). The meaningful combination of the different techniques requires possibilities to link the various instruments and their reference points. So-called co-location stations that are equipped with instrumentation for several techniques play an important role for the ITRF combination since so-called local-tie vectors on the ground enable the connection between the various instruments. Since several years, ideas have been discussed to include additional possibilities to link the different techniques, with the main goal to improve the ITRF. One of these ideas is to use GNSS signals for VLBI observations and by this improve the link between VLBI and GNSS. In our presentations we describe so-called GNSS-VLBI experiments performed in 2017 with VLBI stations at intercontinental distances. The observations, data correlation and data analysis is described and initial results are presented.
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| 6. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Identifying optimal tag-along station locations for improving VLBI Intensive sessions
- 2017
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Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 69:1, s. 16-
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Tidskriftsartikel (refereegranskat)abstract
- Very Long Baseline Interferometry (VLBI) is a unique space-geodetic technique capable of direct observation of the Earth’s phase of rotation, namely Universal Time (UT1). The International VLBI Service for Geodesy and Astrometry (IVS) conducts daily 1-h Intensive VLBI sessions to determine rapid variations in the difference between UT1 and Coordinated Universal Time (UTC). The main objective of the Intensive sessions is to provide timely UT1–UTC estimates. These estimates are especially crucial for Global Navigation Satellite Systems (GNSS). The monitoring of rapid variations in Earth rotation also provides insight into various geophysical phenomena. There is an ongoing effort to improve the quality of the UT1–UTC estimates from single-baseline Intensive sessions to realise the expected accuracy and to bring them to a better agreement with the 24-h VLBI sessions. In this paper, we investigate the possibility to improve the Intensives by including a third station in tag-along mode to these regularly observed sessions. The impact of the additional station is studied via extensive simulations using the c5++ analysis software. The location of the station is varied within a predetermined grid. Based on actual Intensive session schedules, a set of simulated observations are generated for the two original stations and each grid point. These simulated data are used to estimate UT1–UTC for every Intensive session scheduled during the year 2014 on the Kokee–Wettzell and Tsukuba–Wettzell baselines, with the addition of a third station. We find that in tag-along mode when a third station is added to the schedule we can identify areas where the UT1–UTC estimates are improved up to 67% w.r.t. the original single-baseline network. There are multiple operational VLBI stations in these areas, which could with little effort be included in a tag-along mode to the currently scheduled Intensive sessions, thus providing the possibility to improve the UT1–UTC estimates by extending the observation network.
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| 7. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Geodetic VLBI for precise orbit determination of Earth satellites: a simulation study
- 2020
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Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 94:6
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Tidskriftsartikel (refereegranskat)abstract
- Recent efforts of tracking low Earth orbit and medium Earth orbit (MEO) satellites using geodetic very long baseline interferometry (VLBI) raise questions on the potential of this novel observation concept for space geodesy. Therefore, we carry out extensive Monte Carlo simulations in order to investigate the feasibility of geodetic VLBI for precise orbit determination (POD) of MEO satellites and assess the impact of quality and quantity of satellite observations on the derived geodetic parameters. The MEO satellites are represented in our study by LAGEOS-1/-2 and a set of Galileo satellites. The concept is studied on the basis of 3-day solutions in which satellite observations are included into real schedules of the continuous geodetic VLBI campaign 2017 (CONT17) as well as simulated schedules concerning the next-generation VLBI system, known as the VLBI Global Observing System (VGOS). Our results indicate that geodetic VLBI can perform on a comparable level as other space-geodetic techniques concerning POD of MEO satellites. For an assumed satellite observation precision better than 14.1 mm (47 ps), an average 3D orbit precision of 2.0 cm and 6.3 cm is found for schedules including LAGEOS-1/-2 and Galileo satellites, respectively. Moreover, geocenter offsets, which were so far out of scope for the geodetic VLBI analysis, are close to the detection limit for the simulations concerning VGOS observations of Galileo satellites, with the potential to further enhance the results. Concerning the estimated satellite orbits, VGOS leads to an average precision improvement of 80% with respect to legacy VLBI. In absolute terms and for satellite observation precision of 14.1 mm (47 ps), this corresponds to an average value of 17 mm and 7 mm concerning the 3D orbit scatter and precision of geocenter components, respectively. As shown in this study, a poor satellite geometry can degrade the derived Earth rotation parameters and VLBI station positions, compared to the quasar-only reference schedules. Therefore, careful scheduling of both quasar and satellite observations should be performed in order to fully benefit from this novel observation concept.
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| 8. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Geodetic VLBI with an artificial radio source on the Moon: a simulation study
- 2018
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Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 92:5, s. 457-469
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Tidskriftsartikel (refereegranskat)abstract
- We perform extensive simulations in order to assess the accuracy with which the position of a radio transmitter on the surface of the Moon can be determined by geodetic VLBI. We study how the quality and quantity of geodetic VLBI observations influence these position estimates and investigate how observations of such near-field objects affect classical geodetic parameters like VLBI station coordinates and Earth rotation parameters. Our studies are based on today's global geodetic VLBI schedules as well as on those designed for the next-generation geodetic VLBI system. We use Monte Carlo simulations including realistic stochastic models of troposphere, station clocks, and observational noise. Our results indicate that it is possible to position a radio transmitter on the Moon using today's geodetic VLBI with a two-dimensional horizontal accuracy of better than one meter. Moreover, we show that the next-generation geodetic VLBI has the potential to improve the two-dimensional accuracy to better than 5 cm. Thus, our results lay the base for novel observing concepts to improve both lunar research and geodetic VLBI.
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| 9. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Implementation of VLBI Near-Field Delay Models in the c5++ Analysis Software
- 2017
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Ingår i: Proceedings of the First International Workshop on VLBI Observations of Near-field Targets, October 5 - 6, 2016, A. Nothnagel and F. Jaron (eds.). - 1864-1113. ; 54, s. 29-33
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We describe the implementation of two near-field delay models in the c5++ analysis software. The motivation for this work is to allow the calculation of a priori delay information for the correlation of VLBI raw observations of near-field targets and to prepare for the analysis of VLBI data of near-field objects. The software is tested by correlating VLBI observations of the Chinese Chang’E lunar lander on the Onsala– Wettzell baseline.
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| 10. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Lunar Observations and Geodetic VLBI – A Simulation Study
- 2017
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Ingår i: Proceedings of the 23rd European VLBI Group for Geodesy and Astrometry Working Meeting. ; , s. 122-126
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The recent OCEL (Observing the Chang’E Lander with VLBI) sessions allow the geodetic VLBI community to gain new experience concerning observations of an artificial lunar radio source. Although the analysis of obtained data is still ongoing, the performance of the OCEL sessions, in terms of lunar-based parameters, is still rather unclear. In order to address this and related questions, we carried out Monte Carlo simulations using the c5++ analysis software and OCEL schedules with the purpose to evaluate the accuracy with which the position of an artificial radio source on the surface of the Moon can be determined with geodetic VLBI. We present the results of our study and discuss the limiting factors of this concept. Our simulation results can provide valuable insights concerning global observations of lunar radio transmitters and stimulate new observing ideas for space geodesy.
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| 11. |
- Klopotek, Grzegorz, 1990
(författare)
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Observations of Artificial Radio Sources within the Framework of Geodetic Very Long Baseline Interferometry
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Very long baseline interferometry (VLBI) is a mature and fascinating technique with unique and indisputable applications in radio astronomy, planetary sciences, and space geodesy. The latter discipline is a field of science facilitating our understanding of various global-scale phenomena connected to Earth dynamics. Space geodesy provides, in the microwave regime, accurate and long-term stable celestial and terrestrial reference frames, to which those environmental changes can be properly referenced and their spatio-temporal variability can be subsequently accurately investigated. In order to attain better knowledge on complex, and yet subtle, geodynamical phenomena of scientific and economic importance, there is a need for an improved global geodetic infrastructure and enhanced quality of space-geodetic measurements. The common effort of the geodetic community known as the Global Geodetic Observing System (GGOS) shall address that need and provide the highest possible accuracy of geodetic products and reference frames as well as the high consistency across space-geodetic techniques. The ambitious goals of GGOS necessitate appropriate changes to be made also in the area of geodetic/astrometric VLBI, realized at preset in the form of the VLBI Global Observing System (VGOS), a next-generation system aiming to meet the requirements of GGOS and deliver geodetic products with an unprecedented quality. In order to make VGOS succeed, the key components of this complex system need to be refined, including also new observing concepts and scheduling strategies, in order to fully exploit the enhanced performance that this system can bring. Thanks to its characteristics, VGOS creates also a great opportunity for extending the current VLBI research with new applications, for the benefit of the scientific community and society at large. The subject of this thesis concerns observations of artificial radio sources within the framework of geodetic VLBI, in connection to both the current VLBI system and VGOS. This includes information on the combination of observations of natural radio sources and satellite/lunar objects as well as benefits and challenges related to the observing strategy and the technical feasibility of the presented concept. The thesis is based mostly on extensive simulation studies concerning objects on the Moon and geodetic Earth-orbiting satellites, but it also includes an analysis of VLBI observations of the lunar lander performed during dedicated experiments and with a global network of radio telescopes. The information content of this thesis may be treated as a further step towards global observations of artificial radio sources with VLBI in the VGOS era and stimulate new observing concepts for space geodesy.
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| 12. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Position determination of the Chang’e 3 lander with geodetic VLBI
- 2019
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Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 71:1
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present results from the analysis of observations of the Chang’e 3 lander using geodetic Very Long Baseline Interferometry. The applied processing strategy as well as the limiting factors to our approach is discussed. We highlight the current precision of such observations and the accuracy of the estimated lunar-based parameters, i.e., the lunar lander’s Moon-fixed coordinates. Our result for the position of the lander is 44.1219 3 ∘ N , -19.51159∘E and -2637.3 m, with horizontal position uncertainties on the lunar surface of 8.9 m and 4.5 m in latitude and longitude, respectively. This result is in good agreement with the position derived from images taken by the Narrow Angle Camera of the Lunar Reconnaissance Orbiter. Finally, we discuss potential improvements to our approach, which could be used to apply the presented concept to high-precision lunar positioning and studies of the Moon.[Figure not available: see fulltext.].
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| 13. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Results from the VLBI Analysis Software Comparison Campaign 2015
- 2016
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Ingår i: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". Edited by Dirk Behrend, Karen D. Baver, and Kyla L. Armstrong, NASA/CP-2016-219016. ; , s. 203-207
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the VLBI Analysis Software Comparison Campaign 2015 (VASCC2015) was to compare differentVLBI analysis software packages on the basis of computed theoretical delays. Eleven research groups and institutes participated in this project which allowed us to compare software packages that are used in operational VLBI analyses or that are still under development. We present the first results and we show how well the individual software packages agree at this stage.
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| 14. |
- Klopotek, Grzegorz, 1990
(författare)
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Simulation studies of new observing concepts for geodetic Very Long Baseline Interferometry
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Very Long Baseline Interferometry (VLBI) is a space-geodetic technique in which observations are carried out simultaneously by radio telescopes separated by hundreds or thousands of kilometers. The time difference of signal reception between the telescopes is the basic observable used in geodetic VLBI. This technique is capable of determining all five Earth Orientation Parameters (EOP), which provide the connection between the Earth-fixed and space-fixed reference frames. Currently, there is an ongoing effort concerning the establishment of the VLBI Global Observing System (VGOS), which will significantly improve the present measurement precision and increase the total number of observations per session. This requires the key components of the infrastructure, data handling as well as observation approaches to be upgraded and refined. Thus, the focus of this thesis is set on new observing concepts for VGOS. This includes extensive simulations regarding an improved determination of the rotation of the Earth (UT1-UTC) from one-hour VLBI sessions and investigations on the potential of lunar observations in regular geodetic VLBI sessions. The studies summarized in this work address the main topic from two different aspects, providing valuable insights concerning observations in the VGOS era and stimulating new concepts for space geodesy.
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