| 41. |
- Hunkeler, Priska A., et al.
(författare)
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Improved 1D inversions for sea ice thickness and conductivity from electromagnetic induction data : Inclusion of nonlinearities caused by passive bucking
- 2016
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Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 81:1, s. WA45-WA58
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Tidskriftsartikel (refereegranskat)abstract
- The porosity of sea ice is a fundamental physical parameter that governs the mechanical strength of sea ice and the mobility of gases and nutrients for biological processes and biogeochemical cycles in the sea ice layer. However, little is known about the spatial distribution of the sea ice porosity and its variability between different sea ice types; an efficient and nondestructive method to measure this property is currently missing. Sea ice porosity is linked to the bulk electrical conductivity of sea ice, a parameter routinely used to discriminate between sea ice and seawater by electromagnetic (EM) induction sensors. Here, we have evaluated the prospect of porosity retrieval of sea ice by means of bulk conductivity estimates using 1D multi-frequency EM inversion schemes. We have focused on two inversion algorithms, a smoothness-constrained inversion and a Marquardt-Levenberg inversion, which we modified for the nonlinear signal bias caused by a passive bucking coil operated in such a highly conductive environment. Using synthetic modeling studies, 1D inversion algorithms and multiple frequencies, we found that we can resolve the sea ice conductivity within +/- 0.01 S/m. Using standard assumptions for the conductivity-porosity relation of sea ice, we were able to estimate porosity with an uncertainty of +/- 1.2%, which enables efficient and nondestructive surveys of the internal state of the sea ice cover.
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| 42. |
- 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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| 43. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
-
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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| 44. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
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Automated analysis of Kokee–Wettzell Intensive VLBI sessions—algorithms, results, and recommendations
- 2015
-
Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 67:181
-
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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| 45. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
-
Identifying optimal tag-along station locations for improving VLBI Intensive sessions
- 2017
-
Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 69:1, s. 16-
-
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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| 46. |
- Kareinen, Niko Petteri, 1986, et al.
(författare)
-
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
-
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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| 47. |
- Kaufmann, Thomas, et al.
(författare)
-
Meshless eigenvalue analysis for resonant structures based on the radial point interpolation method
- 2009
-
Ingår i: Microwave conference. - : IEEE. - 9781424428021 - 9781424428014 ; , s. 818-821
-
Konferensbidrag (refereegranskat)abstract
- Meshless methods are a promising field of numerical methods recently introduced to computational electromagnetics. The potential of conformal and multi-scale modeling and the possibility of dynamic grid refinements are very attractive features that appear more naturally in meshless methods than in classical methods. The radial point interpolation method (RPIM) uses radial basis functions for the approximation of spatial derivatives. In this publication an eigenvalue solver is introduced for RPIM in electromagnetics. Eigenmodes are calculated on the example of a cylindrical resonant cavity. It is demonstrated that the computed resonance frequencies converge to the analytical values for increasingly fine spatial discretization. The computation of eigenmodes is an important tool to support research on a time-domain implementation of RPIM. It allows a characterization of the method's accuracy and to investigate stability issues caused by the possible occurrence of non-physical solutions
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| 48. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Geodetic VLBI for precise orbit determination of Earth satellites: a simulation study
- 2020
-
Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 94:6
-
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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| 49. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
-
Geodetic VLBI with an artificial radio source on the Moon: a simulation study
- 2018
-
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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| 50. |
- Klopotek, Grzegorz, 1990, et al.
(författare)
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Implementation of VLBI Near-Field Delay Models in the c5++ Analysis Software
- 2017
-
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
-
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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