| 1. |
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| 2. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Results from the VLBI Analysis Software Comparison Campaign 2015
- 2016
-
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
-
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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| 3. |
- Petrachenko, Bill, et al.
(författare)
-
Design Aspects of the VLBI2010 System
- 2009
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This report summarizes the progress made in developing the next generation VLBI, dubberd the VLBI2010 system. The VLBI2010 Committee of the International Service for Geodesy and Astrometry (IVS) worked on the design aspects of the new system.…
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| 4. |
- Petrachenko, Bill, et al.
(författare)
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Progress Report of the IVS VLBI2010 Committee
- 2009
-
Ingår i: In: International VLBI Service for Geodesy and Astrometry 2008 Annual Report, edited by D. Behrend and K. Baver. ; NASA/TP-2009-214183, 2009, s. 13-67
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 5. |
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| 6. |
- Elgered, Gunnar, 1955, et al.
(författare)
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The Onsala Twin Telescopes Project
- 2017
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Ingår i: The 23rd Working Meeting of the European VLBI Group for Geodesy and Astrometry (EVGA), Gothenburg, May 14–18, 2017.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Onsala Space Observatory is the European site in the International VLBI Service for Geodesy and Astrometry (IVS) that has the longest history in VLBI. First geodetic VLBI measurements were performed already in 1968 with the 25 m radio telescope. Onsala is today one of the sites with thelongest time series in the IVS data base. The observatory is one of the unique fundamental space geodetic sites that have a direct access to the coast line and co-locate VLBI, GNSS, gravimetry, and sea-level monitoring. Onsala is thus an important co-location site for the GlobalGeodetic Observing System (GGOS). Being well aware of the VGOS standard it was clear around 2010 that Onsala was in need for a telescope with significantly faster slew rates than the existing radome-enclosed 20 m telescope.
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| 7. |
- Elgered, Gunnar, 1955, et al.
(författare)
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The Onsala Twin Telescopes: the Status at the Time for the Inauguration
- 2017
-
Ingår i: Proceedings of the 23rd European VLBI Group for Geodesy and Astrometry Working Meeting. - 9789188041098 ; , s. 136-139
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We summarise the activities related to the Onsala Twin Telescopes (OTT), from the time when the decision was taken to fund the proposal, to the inauguration on 18 May 2017.
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| 8. |
- Geremia-Nievinski, F., et al.
(författare)
-
SNR-based GNSS reflectometry for coastal sea-level altimetry: results from the first IAG inter-comparison campaign
- 2020
-
Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 94:8
-
Tidskriftsartikel (refereegranskat)abstract
- Ground-based Global Navigation Satellite System Reflectometry (GNSS-R) is quickly maturing toward the objective of becoming a viable alternative for operational coastal sea-level (SL) altimetry in a geocentric reference frame. SL has immense societal implications related to climate change. Of particular interest is the exploitation of existing coastal GNSS sites for reflectometry by means of signal-to-noise ratio (SNR) observables. We report results from the first inter-comparison campaign on SNR-based GNSS-R. The goal was to cross-validate retrieval solutions from independent research groups under comparable conditions. This action was an initiative of the International Association of Geodesy working group 4.3.9 (2015–2019 term). Data collected at the Onsala Space Observatory for a 1-year period (2015–2016) were compared to a co-located tide gauge (TG). SNR data for the GPS L1-C/A signal were processed by four groups, in Sweden, Luxembourg/Brazil, Germany, and the UK. Semidiurnal tidal constituents showed good agreement between TG and all GNSS-R groups. SL variations at diurnal and longer periods were also well captured by all series. Most GNSS-R solutions exhibited spurious tones at integer fractions of one sidereal day, the satellite revisit time of the particular GNSS constellation employed (GPS). Band-pass filtering between 3 h and 30 h confirmed that the dominant tidal components were well captured by most GNSS-R solutions. Higher-frequency SL variations (periods < 3 h) are poorly represented by GNSS-R as a consequence of its low temporal resolution. The solution with the worst agreement neglects a correction associated with the rate of change in sea level and uses narrower satellite elevation ranges per retrieval. Overall, there was excellent agreement, with correlation coefficients exceeding 0.9 and RMSE smaller than 5 cm.
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| 9. |
- Haas, Rüdiger, 1966, et al.
(författare)
-
Contributions of Onsala Space Observatory to GGOS
- 2015
-
Ingår i: Proceedings of the 22nd European VLBI Group for Geodesy and Astrometry Working Meeting. - 9789892061917 ; , s. 125-129
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 10. |
- Haas, Rüdiger, 1966, et al.
(författare)
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GLONASS-VLBI: Onsala-Wettzell test observations
- 2015
-
Ingår i: Proceedings of the 22nd European VLBI Group for Geodesy and Astrometry Working Meeting. - 9789892061917 ; , s. 107-111
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 11. |
- Haas, Rüdiger, 1966, et al.
(författare)
-
Near real-time monitoring of UT1 with geodetic VLBI
- 2011
-
Ingår i: Proceedings of the 20th Meeting of the European VLBI Group for Geodesy and Astrometry. - 1864-1113. ; , s. 64-66
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We give a short overview on the current status of near real-time monitoring of UT1 with geodetic VLBI. The use ofreal-time data transfer together with automated correlation and data analysis makes it possible to derive final dUT1-results with very low latency. The agreement with IERS C04 results is on the level of 30 msec. It is even possible to determine time series of dUT1 during ongoing 24 h IVS-sessions. The concept is highly relevant for future VLBI2010 operations.
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| 12. |
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| 13. |
- Haas, Rüdiger, 1966, et al.
(författare)
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Onsala Space Observatory – IVS Network Station Activities during 2014
- 2015
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We participated in 39 IVS sessions, includ-ing CONT14. Eleven out of the initially planned ses-sions could not be observed due to the installation ofa new radome for the 20-m radio telescope. As in theprevious six years, we used several of the sessions thatinvolved both Onsala and Tsukuba to perform ultra-rapid UT1-UTC sessions together with our colleaguesin Tsukuba. This included the complete CONT14 cam-paign where near real-time UT1-UTC on the baselineOnsala—Tsukuba could be determined. The procure-ment of the telescopes for the Onsala Twin Telescopeswas started in the summer of 2014. The contract to buytwo VGOS radio telescopes was signed at the end of the year.
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| 14. |
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| 15. |
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| 16. |
- Haas, Rüdiger, 1966, et al.
(författare)
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Status of the Onsala Twin Telescopes – One Year After the Inauguration
- 2019
-
Ingår i: IVS 2018 General Meeting Proceedings "Global Geodesy and the Role of VGOS – Fundamental to Sustainable Development". ; NASA/CP-2019-219039, s. 17-19
-
Konferensbidrag (refereegranskat)abstract
- We briefly describe the status of the Onsala twin telescopes and the experience gained since the official inauguration in May 2017.
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| 17. |
- Haas, Rüdiger, 1966, et al.
(författare)
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Ultra-Rapid DUT1-Observations with E-VLBI
- 2010
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Ingår i: Artificial Satellites. - : Walter de Gruyter GmbH. - 0208-841X .- 2083-6104. ; 45:2 / 2010, s. 75-79
-
Konferensbidrag (refereegranskat)abstract
- We give a short overview about the achievements of the Fennoscandian- Japanese ultra-rapid dUT1-project that was initiated in early 2007. The combination of real-time data transfer, near real-time data conversion and correlation, together with near-real time data analysis allows to determine dUT1 with a very low latency of less than 5 minutes after the end of a VLBI-session. The accuracy of these ultra-rapid dUT1-results is on the same order than the results of the standard rapid-service of the International Earth Rotation and Reference Frame Service (IERS). The ultra-rapid approach is currently extended to 24 hour sessions and is expected to become an important contribution for the future next generation VLBI system called VLBI2010.
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| 18. |
- Haas, Rüdiger, 1966, et al.
(författare)
-
Ultra-rapid earth rotation determination with VLBI during CONT11 and CONT14
- 2017
-
Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 91:7, s. 831-837
-
Tidskriftsartikel (refereegranskat)abstract
- We present earth rotation results from the ultra-rapid operations during the continuous VLBI campaigns CONT11 and CONT14. The baseline Onsala--Tsukuba, i.e., using two out of the 13 and 17 stations contributing to CONT11 and CONT14, respectively, was used to derive UT1-UTC in ultra-rapid mode during the ongoing campaigns. The latency between a new observation and a new UT1-UTC result was less than 10 min for more than 95{\%} of the observations. The accuracy of the derived ultra-rapid UT1-UTC results is approximately a factor of three worse than results from optimized one-baseline sessions and/or complete analysis of large VLBI networks. This is, however, due to that the one-baseline picked from the CONT campaigns is not optimized for earth rotation determination. Our results prove that the 24/7 operation mode planned for VGOS, the next-generation VLBI system, is possible already today. However, further improvements in data connectivity of stations and correlators as well in the automated analysis are necessary to realize the ambitious VGOS plans.
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| 19. |
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| 20. |
- Haas, Rüdiger, 1966, et al.
(författare)
-
VLBI with GNSS signals on intercontinental baselines
- 2018
-
Ingår i: - 9789188041142
-
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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| 21. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Atmospheric Signal Propagation
- 2017
-
Ingår i: Springer Handbooks. - Cham : Springer International Publishing. - 2522-8692 .- 2522-8706. ; , s. 165-193
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Global navigation satellite system (GNSS ) satellites emit signals that propagate as electromagnetic waves through space to the receivers which are located on or near the Earth’s surface or on other satellites. Thereby, electromagnetic waves travel through the ionosphere and the neutral atmosphere (troposphere) which causes signals to be delayed, damped, and refracted as the refractivity index of the propagation media is not equal to one. In this chapter, the nature and effects of GNSS signal propagation in both the troposphere and the ionosphere, aref examined. After a brief review of the fundamentals of electromagnetic waves their propagation in refractive media, the effects of the neutral atmosphere are discussed. In addition, empirical correction models as well as the state-of-the-art atmosphere delay estimation approaches are presented. Effects related to signal propagation through the ionosphere are dealt in a dedicated section by describing the error contribution of the first up to third-order terms in the refractive index and ray path bending. After discussing diffraction and scattering phenomena due to ionospheric irregularities, mitigation techniques for different types of applications are presented.
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| 22. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Combination of GPS and VLBI on the observation level during CONT11—common parameters, ties and inter-technique biases
- 2014
-
Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 88:11, s. 1017-1028
-
Tidskriftsartikel (refereegranskat)abstract
- Multi-technique space geodetic analysis software has been developed which allows to combine data on the observation level. In addition to local tie information, site-wise common parameters, i.e., troposphere and clocks, can be estimated with this software. Thus, it will be discussed how common parameters have to be estimated and where biases/offsets need to be taken into account. To test such a novel concept, Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) data from the CONT11 campaign are being utilized. Since the VLBI baselines of this campaign extend over several thousands of kilometers, GPS data are processed in precise-point positioning mode and satellite orbits and clocks are kept fixed to the IGS final products. From the obtained results, it can be shown that the combination of space geodetic data on the observation level leads to a consistent improvement of station position repeatability as well as nuisance parameters like troposphere estimates. Furthermore, estimation of common parameters (troposphere or clocks) at co-located sites helps to improve the solution further and derive an utmost physically consistent model of the concerned parameters.
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| 23. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Combination of space geodetic techniques on the observation level with c5++: Common nuisance parameters and dataweighting
- 2017
-
Ingår i: International Association of Geodesy Symposia. - Cham : Springer International Publishing. - 2197-9359 .- 0939-9585. - 9783319456287 ; 146, s. 31-37
-
Konferensbidrag (refereegranskat)abstract
- The multi-technique space geodetic analysis software c5++ can be used to combine several techniques on the observation level. A variance component estimation (VCE) feature has been implemented recently and is now being tested concerning the impact on the combination of observational data fromdifferent space geodetic techniques. This goes along with studying which nuisance parameters can be estimated as common unknowns at colocated instruments and how offsets between the different techniques need to be considered in order to enable such an estimation strategy. It can be shown that the combination on the observation level is a sophisticated method to merge data from different space geodetic techniques with the goal of improving the quality of the target parameters. Moreover, it can be shown that formal errors of local ties between GPS and VLBI are at least 6 times too small on average.
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| 24. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Combining GPS and VLBI for inter-continental frequency transfer
- 2015
-
Ingår i: Metrologia. - : IOP Publishing. - 0026-1394 .- 1681-7575. ; 52:2, s. 251-261
-
Tidskriftsartikel (refereegranskat)abstract
- For decades the global positioning system (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past very long baseline interferometry (VLBI) has also been considered for this purpose and the method's capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining GPS and VLBI on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses, both with individual software packages and with the multi-technique software c5++. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 10−14 to 10−15 (MDEV) on inter-continental baselines for averaging times of one day. Then we use the c5++ software for a combined analysis of GPS and VLBI data on the observation level. We demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10%, in particular for averaging periods longer than 3000 s.
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| 25. |
|
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| 26. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Combining VLBI and GPS for inter-continental frequency transfer
- 2015
-
Ingår i: 22nd European VLBI for Geodesy and Astrometry (EVGA) Working Meeting.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- For decades the Global Positioning System (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past VLB) has also been considered for this purpose and the method's capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining VLBI and GPS on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 1e-14 to 1e-15 (MDEV) on inter-continental baselines for averaging times of one day. We also perform a combined analysis of VLBI and GPS data on the observation level and demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10%, in particular for averaging periods longer than 3000 s. We discuss the implications of these findings and present our ideas about how VLBI can contribute to international frequency transfer tasks.
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| 27. |
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| 28. |
|
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| 29. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
GLONASS-R: GNSS reflectometry with an FDMA based satellite navigation system
- 2014
-
Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 49:4, s. 271-282
-
Tidskriftsartikel (refereegranskat)abstract
- The information from reflected Global Navigation Satellite System (GNSS) signals can become a valuable data source, from which geophysical properties can be deduced. This approach, called GNSS Reflectometry (GNSS-R), can be used to develop instruments that act like an altimeter when arrival times of direct and reflected signals are compared. Current GNSS-R systems usually entirely rely on signals from the Global Positioning Service (GPS), and field experiments could demonstrate that information from such systems can measure sea level with an accuracy of a few centimeters. However, the usage of the Russian GLONASS system has the potential to simplify the processing scheme and to allow handling of direct and reflected signals like a bistatic radar. Thus, such a system has been developed and deployed for test purposes at the Onsala Space Observatory, Sweden, that has an operational GPS-based GNSS-R system. Over a period of 2 weeks in October 2013, GPS-based GNSS-R sea level monitoring and measurements with the newly developed GLONASS-R system were carried out in parallel. In addition, data from colocated tide gauge measurements were available for comparison. It can be shown that precision and accuracy of the GLONASS-based GNSS-R system is comparable to, or even better than, conventional GPS-based GNSS-R solutions. Moreover, the simplicity of the newly developed GLONASS-R system allows to make it a cheap and valuable tool for various remote sensing applications.
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| 30. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Ground-based GNSS-R solutions by means of software defined radio
- 2016
-
Ingår i: Proceedings of the 2016 IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2016). Beijing; China; 10-15 July 2016. - 9781509033324 ; 2016-November, s. Art no 7730472, Pages 5635-5637
-
Konferensbidrag (refereegranskat)abstract
- Usually ground-based GNSS-R installations are either existing geodetic GNSS stations or they are built with dedicated components that enable the deduction and monitoring of physical and geometrical properties of the reflecting area around that particular site. In both cases, hardware components usually enable real-time operation of such instruments. However, as software-defined radio (SDR) technology has advanced in the recent years it is now possible to carry out signal processing in real-time, which makes it an ideal candidate for the realization of a flexible GNSS-R system. It is shown how SDR can help to realize GNSS-R solutions for sea-level monitoring at the Onsala Space Observatory, Sweden. Moreover, such SDR solutions can be mounted on an unmanned aerial vehicle (UAV) in order to collect data from higher altitudes and even provide Delay-Doppler information for extended GNSS-R studies.
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| 31. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Hard and Software Tools for the Education of Geodetic VLBI
- 2016
-
Ingår i: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". ; :NASA/CP-2016-219016, s. 234-238
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Onsala Space Observatory hosts two 2.3 m radio telescopes called SALSA (”Such a lovely small antenna”) which are utilised to bring front-line interactive astronomy to the classroom. Until now SALSA has been used for astronomical educational purposes solely, in particular demonstrating the concept of single dish measurements. However, it is possible to combine both SALSAs to an interferometer by making use of hardware which has been developedfor software-defined radio. In doing so, one can utilise the SALSA antenna pair as a student demonstrator for geodetic Very Long Baseline Interferometry. Itis discussed which COTS hardware components are necessary to turn the SALSA installation into an interferometer. A simple Octave-based correlator hasbeen written in order to process SALSA data. Results from a test run during which the Sun was tracked are presented and discussed here.
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| 32. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Hard- and software tools for the education of Geodetic VLBI
- 2016
-
Ingår i: 9th IVS General Meeting, March 2016, Johannesburg, South Africa.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Onsala Space Observatory hosts two 2.3 m radio telescopes called SALSA ("Such a lovely small antenna") which are utilised to bring front-line interactive astronomy to the classroom. Until now SALSA has been used for astronomical educational purposes solely, in particular demonstrating the concept of single dish measurements. However, it is possible to combine both SALSAs to an interferometer by making use of hardware which has been developed for software-defined radio. In doing so, one can utilise the SALSA antenna pair as a student demonstrator for geodetic Very Long Baseline Interferometry. We will discuss the COTS hardware components that are necessary to turn the SALSA installation into an interferometer. Moreover, we will show how a simple correlator has been used to detect fringes and provide single-band delays. Such delays were then processed with our analysis software c5++. We are going to discuss how it is possible to mimic the complete processing chain of geodetic VLBI and how this can be used for training of students and other interested parties.
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| 33. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Inverse modeling of ground-based GNSS-r - Results and new possibilities
- 2017
-
Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2017-July, s. 2671-2681
-
Konferensbidrag (refereegranskat)abstract
- Inverse modeling of ground-based GNSS reflections has the potential to improve existing GNSS-R applications and lead to novel concepts for retrieving geophysical parameters around existing or deliberately built GNSS sites. As we will show in this paper, consistent, accurate and precise determination of sea-surface heights can be realized. Moreover, we demonstrate that it is possible to detect sea-ice freeze states at coastal GNSS sites. In addition, it will be shown how the inverse modeling approach can be used for self-calibration of troposphere delays, which would otherwise bias the target parameters and need to be corrected for by external measurements or data-sets.
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| 34. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Software-Defined Radio Direct Correlation GNSS Reflectometry by Means of GLONASS
- 2016
-
Ingår i: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. - 2151-1535 .- 1939-1404. ; 9:10, s. 4834-4842
-
Tidskriftsartikel (refereegranskat)abstract
- Ground-based GNSS reflectometry (GNSS-R)systems can be realized by different means. The concept ofcorrelation between direct and reflected GNSS signals is basicallypossible with all GNSS systems. However, using signals fromthe Russian GLONASS system simplifies the signal processingso that software-defined radio (SDR) components can be usedat replace expensive hardware solutions. This paper discusseshow such a solution, called GLONASS-R, can be realized usingentirely off-the-shelf components. Field tests with such a systemdemonstrate the capability to monitor sea surface heights with aprecision of 3 cm or better even with a sampling rate of 1.5 Hz.The flexibility of a SDR and the simple concept of GLONASS-Rallow build such a system with low costs and adapt it to the needsof any ground-based GNSS-R problem.
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| 35. |
- Hobiger, Thomas, 1978, et al.
(författare)
-
Software defined radio for ground and airborne GNSS reflectometry
- 2018
-
Ingår i: Proceedings of the European Navigation Conference 2018 (ENC 2018) Abstracts and Technical Papers. - 9789188041142 ; , s. 49-50
-
Konferensbidrag (refereegranskat)abstract
- Software defined radio (SDR) appears as a suitable solution for dedicated GNSS reflectometry (GNSS-R) applications. Not only does the flexibility of SDR allow for easy and rapid prototyping, but also do recent technological developments of SDR front-ends support real-time operation of GNSS-R. Our presentation includes a discussion about the technical aspects of SDR for GNSS-R and we show results from a ground-based GNSS-R SDR receiver which was operated continuously over a more than a month at the Onsala Space Observatory. A summary of our current activities in relation to airborne GNSS-R solutions and initial results in the form of Delay-Doppler Maps (DDMs) will conclude the presentation.
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| 36. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
-
Automated ambiguity estimation for VLBI Intensive sessions using L1-norm
- 2016
-
Ingår i: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 102, s. 39-46
-
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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| 37. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Automated analysis of Kokee–Wettzell intensive sessions
- 2015
-
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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| 38. |
- 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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| 39. |
- 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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| 40. |
- 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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| 41. |
- 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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| 42. |
- 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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| 43. |
- 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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| 44. |
- 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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| 45. |
- 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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| 46. |
- Koyama, Yasuhiro, et al.
(författare)
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Ultra rapid dUT1 estimations from e-VLBI sessions
- 2010
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Ingår i: Advances in Geosciences: Volume 20: Solid Earth (SE); World Scientific Publishing Company. - : World Scientific Publishing Company. - 9789812838186 ; 20, s. 197-204
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Konferensbidrag (refereegranskat)abstract
- Promptness of the data processing of the international VLBI observations has been continuously improved in the past decades. In particular, e-VLBI technique has proved that it has a capability to shorten the latency by transferring the observed data to the correlator by using high speed communication networks. The method has been introduced to routine intensive VLBI sessions to monitor dUT1. To improve the promptness even further, we started an initiative to develop automated data transfer and data processing systems using Europe-Japan baselines. On February 21, 2008, we succeeded to demonstrate the effectiveness of the developed systems and estimated the dUT1 parameter 3 minutes 45 seconds after the last scan of the one hour intensive style e-VLBI session. This achievement was realized by the developments of the K5/VSSP32 data acquisition terminal, automated data processing and analysis software.
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| 47. |
- Kurihara, Shinobu, et al.
(författare)
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4-station ultra-rapid EOP experiment with e-VLBI technique and automated correlation/analysis
- 2013
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Ingår i: Reports of the Finnish Geodetic Institute, Proceedings of the 21st Meeting of the European VLBI Group for Geodesy and Astronomy, Ed. by N. Zubko and M. Poutanen Kirkkonummi. - 0355-1962. - 9789517112970 ; 2013:1:1, s. 233-236
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Since 2007, the Geospatial Information Authorityof Japan (GSI) and the Onsala Space Observatory(OSO) have performed the ultra-rapid dUT1 experiments,which can provide us with near real-timedUT1 value. Its technical knowledge has already beenadopted for the regular series of the Tsukuba-Wettzellintensive session. Now we tried some 4-station ultrarapidEOP experiments in association with Hobart andHartRAO so that we can estimate not only dUT1 butalso the two polar motion parameters. In this experimenta new analysis software c5++ developed by theNational Institute of Information and CommunicationsTechnology (NICT) was used. We describe past developmentsand an overview of the experiment, and concludewith its results in this report.
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| 48. |
- Kwak, Younghee, et al.
(författare)
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Assessing the Impact of Global GNSS-VLBI Hybrid Observations
- 2015
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Ingår i: European Geosciences Union General Assembly 2015.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- GNSS-VLBI hybrid observations refer to an approach where GNSS signals are received by GNSS antennas and correlated with VLBI correlators. The VLBI-like GNSS single differences (two stations to one satellite) are then analyzed together with standard VLBI observations. In this work, we use GNSS observations during CONT11, a continuous VLBI campaign over 15 days in September 2011. During CONT11, GNSS and VLBI are connected to the identical clocks at seven sites, which mean clock parameters can be regarded as site common parameters. We construct GNSS single differences between the ranges from two stations to a satellite, using post-processed range measurements from a precise point positioning (PPP) GPS solution with the C5++ software. Combining VLBI and VLBI-like GNSS delays during CONT11, we estimate station coordinates, Earth orientation parameters, and site common parameters, i.e. zenith wet delays and clock parameters with the Vienna VLBI Software (VieVS). We compare combined solutions with single technique solutions and assess the impact of GNSS-VLBI hybrid observations with respect to those parameters.
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| 49. |
- Kwak, Younghee, et al.
(författare)
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Combination of Two Radio Space-Geodetic Techniques with VieVS during CONT14
- 2016
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Ingår i: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". ; :NASA/CP-2016-219016, s. 265-269
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Unlike CONT11, CONT14 does not have official information on common frequency standards for co-located sites. Nevertheless, according to Kwak et al. (2015) [1], we have a possibility to find the co-located sites, which used the same clocks, through comparing clock rates from single technique solutions. Moreover, CONT14 includes co-located VLBI radio telescopes, i.e. HOBART26 and HOBART12. Therefore, it is also a good test bed to develop the analysis strategy for future twin/sibling telescopes. In this study, we compute VLBI-like GNSS delays (GNSS single differences) between the ranges from two stations to a satellite, using phase measurements with most of the errors correctedby the c5++ software. We estimate station coordinates and site common parameters, i.e. zenith wet delays, troposphere gradients and clock parameters, with the Vienna VLBI Software. Common clock parameters are limited to the sites sharing the same frequency standard and having good performance of it during CONT14. Local tie vectors are introduced as fictitious observations for co-located instruments, GNSS-VLBI and even VLBI-VLBI, i.e. at Hobart. In this paper, we show the comparison results between the combination solutions and the single technique solutions in terms of station position repeatability during 15 days.
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| 50. |
- Kwak, Younghee, et al.
(författare)
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Observation Level Combination of GNSS and VLBI with VieVS: a simulation based on CONT11
- 2015
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Ingår i: AGU FALL MEETING.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- GNSS and VLBI antennas were connected to the identical hydrogen maser clocks at seven sites during CONT11, which means that clock parameters can be regarded as common parameters at those sites as well as troposphere parameters. We construct GNSS single differences between the ranges from two stations to a satellite, using corrected phase measurements with the c5++ software. Combining GNSS single difference and VLBI data during CONT11, we estimate station coordinates and site common parameters, i.e. zenith wet delays, troposphere gradients and clock parameters, with the Vienna VLBI Software (VieVS). Local tie vectors, which contribute to the combination of terrestrial frames between GNSS and VLBI, are introduced as fictitious observations. We compare combination solutions with single technique solutions, assess the impact of the combination at the observation level with respect to geodetic results and discuss the current limitation and potentials to be developed.
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