| 1. |
- Albentosa, Ezequiel, et al.
(author)
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Current Status of the EU-VGOS Project
- 2023
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In: International VLBI Service for Geodesy and Astrometry 2022 General Meeting Proceedings. ; NASA/ CP–20220018789, s. 85-89
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Conference paper (peer-reviewed)abstract
- The EU-VGOS project began in 2018 with the aim of using the VGOS infrastructure in Europe to investigate methods for VGOS data processing. The project is now structured into Working Groups dealing with operations (stations), e-transfer, correlation and post-processing, and analysis. This is a report on the status of the project.
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| 2. |
- Alef, Walter, et al.
(author)
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Geodetic data analysis of VGOS experiments
- 2021
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In: 2021 34th General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2021.
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Conference paper (peer-reviewed)abstract
- Very Long Baseline Interferometry (VLBI) serves as one of the common geodetic methods to define the global reference frames and monitor Earth's orientation variations. The technical upgrade of the VLBI method known as the VLBI Global Observing System (VGOS) includes a critical re-design of the observed frequencies from the dual band mode (S and X band, i.e. 2 GHz and 8 GHz) to observations in a broadband (2-14 GHz). Since 2019 the first VGOS experiments are available for the geodetic analysis in free access at the International VLBI service for Geodesy and Astrometry (IVS). Also regional-only subnetworks such as European VLBI stations have succeeded already in VGOS mode. Based on these brand-new observations we review the current geodetic data analysis workflow to build a bridge between geodetic observed delays derived from different bands.
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| 3. |
- Alef, Walter, et al.
(author)
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The EUropean-VGOS Project
- 2019
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In: Proceedings of the 24th European VLBI Group for Geodesy and Astrometry Working Meeting. - 9788441656345 ; , s. 107-111
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Conference paper (peer-reviewed)abstract
- In Spring 2018 the Bonn correlation centre started a collaboration with the three European stations of Wettzell, Onsala and Yebes, equipped with both S/X- and broadband systems, to perform VGOS-like test sessions. The aim is to verify and develop further the processing chain for VGOS experiments end-to-end, from the scheduling to the analysis of the derived observables. We will present the current status of the project.
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| 4. |
- Behrend, Dirk, et al.
(author)
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Reports of IERS Components: International VLBI Service (IVS)
- 2011
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In: IERS Annual Report 2008-09. Edited by Wolfgang R. Dick and Bernd Richter. International Earth Rotation and Reference Systems Service, Central Bureau. Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie, 2011. 237 p.. - 1029-0060. ; , s. 72-82
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Journal article (other academic/artistic)
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| 5. |
- Corbin, A., et al.
(author)
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Combinatorial optimization applied to VLBI scheduling
- 2020
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In: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 94:2
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Journal article (peer-reviewed)abstract
- Due to the advent of powerful solvers, today linear programming has seen many applications in production and routing. In this publication, we present mixed-integer linear programming as applied to scheduling geodetic very-long-baseline interferometry (VLBI) observations. The approach uses combinatorial optimization and formulates the scheduling task as a mixed-integer linear program. Within this new method, the schedule is considered as an entity containing all possible observations of an observing session at the same time, leading to a global optimum. In our example, the optimum is found by maximizing the sky coverage score. The sky coverage score is computed by a hierarchical partitioning of the local sky above each telescope into a number of cells. Each cell including at least one observation adds a certain gain to the score. The method is computationally expensive and this publication may be ahead of its time for large networks and large numbers of VLBI observations. However, considering that developments of solvers for combinatorial optimization are progressing rapidly and that computers increase in performance, the usefulness of this approach may come up again in some distant future. Nevertheless, readers may be prompted to look into these optimization methods already today seeing that they are available also in the geodetic literature. The validity of the concept and the applicability of the logic are demonstrated by evaluating test schedules for five 1-h, single-baseline Intensive VLBI sessions. Compared to schedules that were produced with the scheduling software sked, the number of observations per session is increased on average by three observations and the simulated precision of UT1-UTC is improved in four out of five cases (6μs average improvement in quadrature). Moreover, a simplified and thus much faster version of the mixed-integer linear program has been developed for modern VLBI Global Observing System telescopes.
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| 6. |
- Delva, Pacôme, et al.
(author)
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GENESIS: co-location of geodetic techniques in space
- 2023
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In: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 75:1
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Journal article (peer-reviewed)abstract
- Improving and homogenizing time and space reference systems on Earth and, more specifically, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1 mm and a long-term stability of 0.1 mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, such as those located at tide gauges, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation, contributing to a better understanding of natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities, including the International Association of Geodesy (IAG), which has enunciated geodesy requirements for Earth sciences. Moreover, the United Nations Resolution 69/266 states that the full societal benefits in developing satellite missions for positioning and Remote Sensing of the Earth are realized only if they are referenced to a common global geodetic reference frame at the national, regional and global levels. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology. This paper has been written and supported by a large community of scientists from many countries and working in several different fields of science, ranging from geophysics and geodesy to time and frequency metrology, navigation and positioning. As it is explained throughout this paper, there is a very high scientific consensus that the GENESIS mission would deliver exemplary science and societal benefits across a multidisciplinary range of Navigation and Earth sciences applications, constituting a global infrastructure that is internationally agreed to be strongly desirable. Graphical Abstract: [Figure not available: see fulltext.]
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| 7. |
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| 8. |
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| 9. |
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| 10. |
- Han, SongTao, et al.
(author)
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Fringe fitting and group delay determination for geodetic VLBI observations of DOR tones
- 2019
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In: Advances in Space Research. - : Elsevier BV. - 1879-1948 .- 0273-1177. ; 63:5, s. 1754-1767
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Journal article (peer-reviewed)abstract
- Extracting the group and phase delays of interferometric observations produced in the Very Long Baseline Interferometry (VLBI) measurement concept requires a special fringe fitting and delay search algorithm for the recorded bandwidth. While fringe fitting is in use routinely for several megahertz wide channels in geodetic and astrometric VLBI with quasar observations, fringe fitting for artificial tones of very small bandwidth of artificial signals for Differential One-way Ranging (DOR) requires a different way of handling. In a project called Observing the Chang’E-3 Lander with VLBI (OCEL), the DOR tones emitted by the Chang’E-3 lander were observed in a standard geodetic VLBI mode with 8 or 4 MHz wide channels to maintain compatibility with the corresponding quasar observations. For these observations, we modified the existing fringe fitting program of the Haystack Observatory Processing Software (HOPS), fourfit, to properly handle narrow band DOR tones. The main motivations are that through this modification, the data of quasars and artificial radio sources can be processed in the existing geodetic analysis pipeline, and that the algorithm can be used for similar projects as well. In this paper, we describe the algorithm and show that the new algorithm produces much more reliable group delay results than using the standard fourfitalgorithm. This is done by a simulation test and in particular by processing of real observations. It is shown that in many cases, systematic deviations of several nanoseconds, which are seen with the standard fourfit algorithm, can be avoided. The ultimate benefit of the new procedure is demonstrated by reducing the errors in delay triangle closures by at least a factor of 3, which, in the OCEL case, is from ∼300 to ∼100 ps.
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| 11. |
- Holst, Christoph, et al.
(author)
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Investigating the gravitational stability of a radio telescope’s reference point using a terrestrial laser scanner: Case study at the Onsala Space Observatory 20-m radio telescope
- 2019
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In: ISPRS Journal of Photogrammetry and Remote Sensing. - : Elsevier BV. - 0924-2716. ; 149, s. 67-76
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Journal article (peer-reviewed)abstract
- In geodetic very long baseline interferometry, pairs of radio telescopes simultaneously observe signals from quasi stellar objects to estimate the baseline between their reference points. Gravity-dependent variations of the radio telescopes’ reference points deteriorate the estimated baseline's accuracy since they lead to signal path variations in the radio telescopes. This study investigates a new concept for determining the stability of a radio telescope's reference point. Differing to previously used strategies, this concept is able to reveal instabilities due to an elevation-dependent tumbling of the telescope independent from the part of the axis offset that is constant for all elevation angles. The new concept is exclusively based on terrestrial laser scanning that at the same time is used for analyzing the main reflector's shape deformation. We applied this concept to the Onsala Space Observatory (OSO) 20-m radio telescope: The results show that we cannot disprove the reference point's stability. In general, our new strategy can be transferred to also investigate the stability of other radio telescopes’ reference points. A prerequisite for this strategy is that the laser scanner – that moves between the elevation angles – observes identical objects from different stations. These objects need to be stable during all measurements. In the case of the OSO 20-m radio telescope, the radome is used for this purpose.
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| 12. |
- Holst, Christoph, et al.
(author)
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Terrestrial Laser Scanner Two-Face Measurements for Analyzing the Elevation-Dependent Deformation of the Onsala Space Observatory 20-m Radio Telescope’s Main Reflector in a Bundle Adjustment
- 2017
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In: Sensors. - : MDPI AG. - 1424-8220. ; 17:1833, s. 1-21
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Journal article (peer-reviewed)abstract
- © 2017 by the authors. Licensee MDPI, Basel, Switzerland. For accurate astronomic and geodetic observations based on radio telescopes, the elevation-dependent deformation of the radio telescopes’ main reflectors should be known. Terrestrial laser scanning has been used for determining the corresponding changes of focal lengths and areal reflector deformations at several occasions before. New in this publication is the situation in which we minimize systematic measurement errors by an improved measurement and data-processing concept: Sampling the main reflector in both faces of the laser scanner and calibrating the laser scanner in situ in a bundle adjustment. This concept is applied to the Onsala Space Observatory 20-m radio telescope: The focal length of the main reflector decreases by 9.6 mm from 85° to 5° elevation angle. Further local deformations of the main r eflector are not detected.
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| 13. |
- Jaradat, Ahmad, et al.
(author)
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Considerations of VLBI transmitters on Galileo satellites
- 2021
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In: Advances in Space Research. - : Elsevier BV. - 1879-1948 .- 0273-1177. ; 68:3, s. 1281-1300
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Journal article (peer-reviewed)abstract
- For directly linking the dynamical reference frame of satellite orbits to the quasi-inertial reference frame of extra-galactic radio sources, observations of satellites with the Very Long Baseline Interferometry (VLBI) technique are the only conceivable method. Hence, the satellite observations should be embedded in VLBI network sessions during which also natural radio sources are observed. For this reason, it would be most practical if the artificial signal generated at the satellite for VLBI observations covers the same frequency bands as regularly observed by VLBI radio telescopes and should have a similar flux density across the observed bandwidth as these natural sources. The use of satellites of Global Navigation Satellite Systems (GNSS) such as the Galileo system is advisable because they are well monitored in terms of precise orbit determination and the altitude allows common visibilities of many VLBI telescopes. So far, signal generation on a GNSS satellite dedicated to VLBI observations has not been realized yet, partly because suitable signal generation equipment has not been considered in depth. In addition, many aspects, such as legal implications and technical complications, have not yet been addressed. In this publication, we compiled various aspects of generating an artificial VLBI signal on a GNSS satellite. We describe the legal and technical aspects of generating and emitting an artificial signal on a Galileo satellite suitable for VLBI observations including a design study for the necessary equipment on the satellite. Since geodetic VLBI is currently in a transition period from traditional observations at S and X band to the broadband VLBI Global Observing System (VGOS), the proposed equipment generates a signal suitable for both frequency setups. We have also considered the restrictions for installation on a satellite, such as power consumption, weight, and size. The equipment mainly consists of three devices: noise source, amplifier, and antenna. A diode is used as the noise source. This noise is amplified by a set of low noise amplifiers and then radiated by a spiral antenna. The diode and the amplifiers were chosen from the market, but the antenna was newly designed and simulated. The output signal of this chain was tested using a VLBI baseband data simulator, then correlated and fringe-fitted for validation. The instrumentation proposed here is easy to be constructed, but will still have to be tested in the laboratory together with the instruments on the actual satellite.
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| 14. |
- Jaron, Frédéric, et al.
(author)
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Cross-Polarization Gain Calibration of Linearly Polarized VLBI Antennas by Observations of 4C 39.25
- 2024
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In: Radio Science. - 0048-6604 .- 1944-799X. ; 59:4
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Journal article (peer-reviewed)abstract
- Radio telescopes with dual linearly polarized feeds regularly participate in Very Long Baseline Interferometry. One example is the VLBI Global Observing System (VGOS), which is employed for high-precision geodesy and astrometry. In order to achieve the maximum signal-to-noise ratio, the visibilities of all four polarization products are combined to Stokes I before fringe-fitting. Our aim is to improve cross-polarization bandpass calibration, which is an essential processing step in this context. Here we investigate the shapes of these station-specific quantities as a function of frequency and time. We observed the extra-galactic source 4C 39.25 for 6 hours with a VGOS network. We correlated the data with the DiFX software and analyzed the visibilities with PolConvert to determine the complex cross-bandpasses with high accuracy. Their frequency-dependent shape is to first order characterized by a group delay between the two orthogonal polarizations, in the order of several hundred picoseconds. We find that this group delay shows systematic variability in the range of a few picoseconds, but can remain stable within this range for several years, as evident from earlier sessions. On top of the linear phase-frequency relationship there are systematic deviations of several tens of degrees, which in addition are subject to smooth temporal evolution. The antenna cross-bandpasses are variable on time scales of ∼1 hr, which defines the frequency of necessary calibrator scans. The source 4C 39.25 is confirmed as an excellent cross-bandpass calibrator. Dedicated surveys are highly encouraged to search for more calibrators of similar quality.
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| 15. |
- Klopotek, Grzegorz, 1990, et al.
(author)
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Position determination of the Chang’e 3 lander with geodetic VLBI
- 2019
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In: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 71:1
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Journal article (other academic/artistic)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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| 16. |
- Nothnagel, Axel, et al.
(author)
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A VLBI delay model for gravitational deformations of the Onsala 20 m radio telescope and the impact on its global coordinates
- 2019
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In: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 93:10, s. 2019-2036
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Journal article (peer-reviewed)abstract
- Deformations of the reflectors of radio telescopes used in geodetic and astrometric VLBI observations belong to the class of systematic error sources which affect the estimated position of the telescope and which necessitates correction at the observation level. The determination of the gravitationally induced deformations requires some effort and needs specific modeling of the impact on the VLBI delay observables. This has been exercised on the Onsala 20 m radio telescope. In this publication, we present an elevation-dependent model for the contributions of the gravitational deformations to the delay observables for application in VLBI data analysis. New is that thermal expansion in some of the contributing components need to be applied also to the gravitational deformation effects. A further novelty is that we can substantiate the validity of and the need for these corrections. Concerning the validity we show that the empirical model used by astronomical colleagues for deliberately shifting the sub-reflector for gain optimization, exactly (within 0.5 mm RMS) matches the measured gravitationally induced displacement of the sub-reflector plus the change in focal length. The other evidence is the impact on the vertical component of the telescope’s coordinates of − 6.1 mm, which reduces the discrepancy determined in the computations of the ITRF2014 to 1.7 mm.
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| 17. |
- Nothnagel, Axel, et al.
(author)
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Strategic Plan of the IVS for the Period 2016-2025
- 2016
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In: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". Edited by Dirk Behrend, Karen D. Baver, and Kyla L. Armstrong, NASA/CP-2016-219016, 2016. ; :NASA/CP-2016-219016, s. 3-12
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Conference paper (other academic/artistic)
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| 18. |
- Nothnagel, Axel, et al.
(author)
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WVR calibration applied to European VLBI observing sessions
- 2007
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In: in "Dynamic Planet: Monitoring and Understanding a Dynamic Planet with Geodetic and Oceanographic Tools", IAG Symposium, Cairns, Australia, 22-26 August, 2005, Series: International Association of Geodesy Symposia , Vol. 130, Tregoning, Paul; Rizos, Chris (Eds.), Springer. - 9783540493495 ; 130, s. 152-157
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Conference paper (peer-reviewed)
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| 19. |
- Steinforth, Christoph, et al.
(author)
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Determination of Local Ties at Ny-Ålesund
- 2005
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In: Proceedings of the IERS Workshop on site co-location. Matera, Italy, 23 - 24 October 2003. Edited by Bernd Richter, Wolfgang Schwegmann, and Wolfgang R. Dick. (IERS Technical Note ; 33) Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie, 2005. 148 pp., paperback, ISBN 3-89888-793-6 (print version). - 3898887936 ; IERS-TN:33, s. 84-92
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Conference paper (other academic/artistic)
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| 20. |
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