| 1. |
- Albentosa, Ezequiel, et al.
(författare)
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Current Status of the EU-VGOS Project
- 2023
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Ingår i: International VLBI Service for Geodesy and Astrometry 2022 General Meeting Proceedings. ; NASA/ CP–20220018789, s. 85-89
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Konferensbidrag (refereegranskat)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.
(författare)
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Geodetic data analysis of VGOS experiments
- 2021
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Ingår i: 2021 34th General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2021.
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Konferensbidrag (refereegranskat)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. |
- Xu, Minghui, et al.
(författare)
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Baseline Vector Repeatability at the Sub-Millimeter Level Enabled by Radio Interferometer Phase Delays of Intra-Site Baselines
- 2023
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Ingår i: Journal of Geophysical Research: Solid Earth. - 2169-9356 .- 2169-9313. ; 128:3
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Tidskriftsartikel (refereegranskat)abstract
- We report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observations rather than dedicated short-baseline experiments. An analysis of phase delay observables at X band from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hr VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. Position ties are also obtained for other short baselines between legacy antennas and nearby, newly built antennas. They are critical for maintaining a consistent continuation of the realization of the terrestrial reference frame, especially when including the new VGOS network. The phase delays of the baseline WETTZ13N–WETTZELL enable an investigation of sources of error at the sub-millimeter level. We found that a systematic variation of larger than 1 mm can be introduced to the Up estimates of this baseline vector when atmospheric delays were estimated. Although the sub-millimeter repeatability has been achieved for the baseline vector WETTZ13N–WETTZELL, we conclude that long term monitoring should be conducted for more short baselines to assess the instrumental effects, in particular the systematic differences between phase delays and group delays, and to find common solutions for reducing them. This will be an important step toward the goal of global geodesy at the 1 mm level.
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| 8. |
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| 9. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Automated ambiguity estimation for VLBI Intensive sessions using L1-norm
- 2016
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Ingår i: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 102, s. 39-46
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Tidskriftsartikel (refereegranskat)abstract
- Very Long Baseline Interferometry (VLBI) is a space-geodetic technique that is uniquely capable of direct observation of the angle of the Earth's rotation about the Celestial Intermediate Pole (CIP) axis, namely UT1. The daily estimates of the difference between UT1 and Coordinated Universal Time (UTC) provided by the 1-h long VLBI Intensive sessions are essential in providing timely UT1 estimates for satellite navigation systems and orbit determination. In order to produce timely UT1 estimates, efforts have been made to completely automate the analysis of VLBI Intensive sessions. This involves the automatic processing of X- and S-band group delays. These data contain an unknown number of integer ambiguities in the observed group delays. They are introduced as a side-effect of the bandwidth synthesis technique, which is used to combine correlator results from the narrow channels that span the individual bands. In an automated analysis with the c5++ software the standard approach in resolving the ambiguities is to perform a simplified parameter estimation using a least-squares adjustment (L2-norm minimisation). We implement L1-norm as an alternative estimation method in c5++. The implemented method is used to automatically estimate the ambiguities in VLBI Intensive sessions on the Kokee–Wettzell baseline. The results are compared to an analysis set-up where the ambiguity estimation is computed using the L2-norm. For both methods three different weighting strategies for the ambiguity estimation are assessed. The results show that the L1-norm is better at automatically resolving the ambiguities than the L2-norm. The use of the L1-norm leads to a significantly higher number of good quality UT1-UTC estimates with each of the three weighting strategies. The increase in the number of sessions is approximately 5% for each weighting strategy. This is accompanied by smaller post-fit residuals in the final UT1-UTC estimation step.
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| 10. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Automated analysis of Kokee–Wettzell intensive sessions
- 2015
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Ingår i: Proceedings of the 22nd European VLBI Group for Geodesy and Astrometry Working Meeting. - 9789892061917 ; , s. 235-239
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 11. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Automated analysis of Kokee–Wettzell Intensive VLBI sessions—algorithms, results, and recommendations
- 2015
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Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 67:181
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Tidskriftsartikel (refereegranskat)abstract
- The time-dependent variations in the rotation and orientation of the Earth are represented by a set of Earth Orientation Parameters (EOP). Currently, Very Long Baseline Interferometry (VLBI) is the only technique able to measure all EOP simultaneously and to provide direct observation of universal time, usually expressed as UT1-UTC. To produce estimates for UT1-UTC on a daily basis, 1-h VLBI experiments involving two or three stations are organised by the International VLBI Service for Geodesy and Astrometry (IVS), the IVS Intensive (INT) series. There is an ongoing effort to minimise the turn-around time for the INT sessions in order to achieve near real-time and high quality UT1-UTC estimates. As a step further towards true fully automated real-time analysis of UT1-UTC, we carry out an extensive investigation with INT sessions on the Kokee–Wettzell baseline. Our analysis starts with the first versions of the observational files in S- and X-band and includes an automatic group delay ambiguity resolution and ionospheric calibration. Several different analysis strategies are investigated. In particular, we focus on the impact of external information, such as meteorological and cable delay data provided in the station log-files, and a priori EOP information. The latter is studied by extensive Monte Carlo simulations.Our main findings are that it is easily possible to analyse the INT sessions in a fully automated mode to provide UT1-UTC with very low latency. The information found in the station log-files is important for the accuracy of the UT1-UTC results, provided that the data in the station log-files are reliable. Furthermore, to guarantee UT1-UTC with an accuracy of less than 20 μs, it is necessary to use predicted a priori polar motion data in the analysis that are not older than 12 h.
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| 12. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Experience from geodetic very long baseline interferometry observations at Onsala using a digital backend
- 2015
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Ingår i: Journal of Geodetic Science. - : Walter de Gruyter GmbH. - 2081-9943. ; 5:1, s. 26-34
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Tidskriftsartikel (refereegranskat)abstract
- The Onsala Space Observatory has installed amodern digital backend for geodetic and astronomicalVery Long Baseline Interferometry (VLBI). This systemconsists of a Digital Base-Band Converter (DBBC) and aMark 5B+ recorder. From 2011 until late 2014 this newsystem was run for geodetic VLBI observations in paral-lel with the old system consisting of a Mark 4 rack andMark 5A recording system. Several of these observed ses-sions were correlated at the correlator in Bonn includingboth data sets. We present results from the analysis andcomparison of these sessions. Both the original observeddelays and corresponding geodetic parameters are com-pared. No significant differences are detected, for eitherthe raw observations or for the geodetic parameters. Thisshows that the digital backend can be used operationallyfor geodetic VLBI observations.
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| 13. |
- Kareinen, Niko Petteri, 1986
(författare)
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Geodetic analysis for the Very Long Baseline Interferometry Global Observing System
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Very Long Baseline Interferometry (VLBI) is an essential technique forspace-geodesy. It realizes the International Celestial Reference Frame(ICRF) and provides a link between the Earth- and space-fixed coordinatesystems by directly observing all Earth Orientation Parameters (EOP)simultaneously. In particular, it is the only technique available that candirectly measure UT1-UTC and nutation. This of special importance tosatellite-based techniques, which need regular input from VLBIobservations to account for drifts in their derived UT1-UTC estimates.Currently, daily UT1-UTC estimates from VLBI are provided by 1-hourIntensive sessions with three regular baseline configurations, whichprovide UT1-UTC with an appropriate accuracy of 20 μs. Increased UT1-UTC accuracy is given by bi-weekly 24-hour Rapid turnaround sessions forEOP determination, which employ a network of at least 8 stations.However, the typical delay for the results obtained from these sessions isclose to the specified upper limit of 15 days.The VLBI Global Observing System (VGOS) is the upcoming VLBIcomponent of the Global Geodetic Observing System (GGOS) of theInternational Association of Geodesy (IAG). It represents a completeredesign of the current VLBI system to meet the requirements for a systemcapable of observing phenomena with a magnitude of a few millimetres.For VGOS the main goals are a global accuracy of 1 mm for positionsand 1 mm/y for velocities and continuous monitoring of EOP and stationpositions. Major effort in hardware and software across the whole signalchain are needed to accomplish these goals. This includes investments in,to name a few, new telescopes, front- and backends, recording systems,correlation, and data analysis. Most of the related systems need to beautomated to ensure reliable continuous operations. In this thesis theaspects of geodetic VLBI data analysis related to the transition to VGOSare investigated through two practical cases.The VGOS requirements necessitate upgrades in the stationhardware. In 2011 Onsala Space Observatory installed a digital backend(Digital Base-Band Converter (DBBC) system) alongside the operationalanalogue Mark IV system. The effect of this hardware change on the VLBIobservables and estimated geodeticparameters is investigated through analysing a series of sessions recordedin parallel on both the old and the new systems.Automated near-real time VLBI analysis is studied using the Intensivesessions on the Kokee—Wettzell baseline. The impacts in terms ofavailability of a priori data for the analysis are investigated to determinethe most crucial factors for high-accuracy UT1-UTC production.
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| 14. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Going digital – the transition at Onsala from Mark4 to DBBC
- 2014
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Ingår i: IVS 2014 General Meeting Proceedings "VGOS: The New VLBI Network", Edited by Dirk Behrend, Karen D. Baver, and Kyla L. Armstrong, Science Press (Beijing). - 9787030429742 ; , s. 178-182
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 15. |
- 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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| 16. |
- Kareinen, Niko Petteri, 1986
(författare)
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Observation strategies for current and future geodetic very long baseline interferometry
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Geodetic Very Long Baseline Interferometry (VLBI) is an essential technique for space geodesy. It is uniquely capable of simultaneously observing all Earth Orientation Parameters (EOP) and directly giving access to the Earth’s rotation angle (related to Universal Time, UT1). The EOP provide the link between the terrestrial and celestial reference frame. The latter is defined by positions of extra-galactic radio sources observed with geodetic VLBI whereas the terrestrial frame is realised through station positions and velocities. Ongoing phenomena such as the sea-level rise caused by global warming have magnitudes in the millimetre per year range. An accurate global reference frame is therefore crucial for reliably measuring these changes. The Global Geodetic Observing System (GGOS) and its VLBI component, the VLBI Global Observing System (VGOS), are designed to meet these challenges. The transition to the VGOS era brings challenges for all aspects of geodetic VLBI: telescope design, receiver development, recording, data transfer, correlation, observation planning, and analysis. The transition to VGOS involves gradually phasing out the legacy dual- frequency S/X telescopes, while delivering all IVS geodetic products and ensuring the continuity of the time series of geodetic parameters. The VGOS targets include continuous observations and delivery of initial geodetic products in less than 24 hours. This will require a fully automated VLBI analysis chain to make results available in near-real time. This thesis aims at contributing to the improvement of current geodetic VLBI products and supporting the transition from the legacy S/X systems to observations in the VGOS era. Broadband VGOS observations necessitate upgrades for the receiver chain and the data recording devices. The Onsala Space Observatory (OSO) operated its analogue and a new digital back-end in parallel for almost two years. We present the results from a comparison, in which the new system was found to have no biases w.r.t. the old setup. We also investigate ways to improve the current IVS Intensive sessions. This involves using approaches that have relevance for the upcoming VGOS observations. We present fully automated analysis of INT1 sessions between 2001 and 2015 to investigate different analysis strategies and the impact of mapping functions, the use of auxiliary data, and lack of recent a priori EOP on the UT1-UTC accuracy. Up-to-date a priori polar motion was recognized as a key factor for the accuracy of UT1 estimates. Results from implementation and testing of fully automated robust L1-norm based ambiguity estimation are presented. We find that the L1-norm outperforms least-squares for ambiguity estimation. Lastly, optimal locations for a third station in tag-along mode for INT sessions are determined. We conclude that UT1-UTC WRMS can be reduced to 61 % (INT1) and 67 % (INT2) of the WRMS without the tag-along station. The UT1-UTC was improved significantly even without optimised schedules.
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| 17. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Robust Ambiguity Estimation for an Automated Analysis of the Intensive Sessions
- 2016
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Ingår i: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". ; :(NASA/CP-2016-219016), s. 198-202
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Very Long Baseline Interferometry (VLBI) is a unique space-geodetictechnique that can directly access the Earth's phase of rotation, namely UT1.The daily estimates of the difference between UT1 and Coordinated UniversalTime (UTC) are computed from 1-hour long VLBI Intensive sessions. Thesesessions are essential in providing timely UT1 estimates for satellitenavigation systems. To produce timely UT1 estimates, efforts have been made tocompletely automate the analysis of VLBI Intensive sessions. This requiresautomated processing of X- and S-band group delays. These data often contain anunknown number of integer ambiguities in the observed group delays. In anautomated analysis with the c5++ software the standard approach in resolvingthe ambiguities is to perform a simplified parameter estimation using aleast-squares adjustment (L2-norm minimisation). We implement the robustL1-norm with an alternative estimation method in c5++. The implemented methodis used to automatically estimate the ambiguities in VLBI Intensive sessions onthe Kokee-Wettzell baseline. The results are compared to an analysis setupwhere the ambiguity estimation is computed using the L2-norm. Additionally, weinvestigate three alternative weighting strategies for the ambiguityestimation. The results show that in automated analysis the L1-norm resolvesambiguities better than the L2-norm. The use of the L1-norm leads to asignificantly higher number of good quality UT1-UTC estimates with each of thethree weighting strategies.
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| 18. |
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