| 1. |
- Bååth, L.B. 1948-, et al.
(author)
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Time-Dependent Radio Fine Structure of the Compact Sources NRAO 150 and 4C 39.25
- 1980
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In: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 86:3, s. 364-372
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Journal article (peer-reviewed)abstract
- Very long baseline interferometer observations at 7.85 GHz have been used to probe the milliarcsecond structure of the unidentified, very compact radio source NRAO 150 and QSO 4 C 39.25. NRAO 150 exhibited no structural variations from 1972 to the end of 1974. A model with two circular Gaussian components fits the data well. NRAO 150 had a flux density of 7.6 plus or minus 0.5 Jy in the compact component; 4 C 39.25 showed a two-component structure, the components having a separation of (2.02 plus or minus 0.05 arc sec) x 10 to the -3rd power. The upper bound on the speed of transverse separation is 0.0001 arc sec per year or less than 2.7 c. From the spectrum there are also indications of a third, larger component.
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| 2. |
- Behrend, Dirk, et al.
(author)
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Recent Progress in the VLBI2010 Development
- 2008
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In: in Proceedings of the 2007 IAG General Assembly, Perugia, Italy, July 2-13, 2007, ed. M. Sideris, Springer. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 0939-9585. - 9783540854258 ; 133:Part 5, s. 833-840
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Conference paper (peer-reviewed)abstract
- From October 2003 to September 2005, the International VLBI Service for Geodesy and Astrometry (IVS) examined current and future requirements for geodetic VLBI, including all components from antennas to analysis. IVS Working Group 3 "VLBI 2010", which was tasked with this effort, concluded with recommendations for a new generation of VLBI systems. These recommendations were based on the goals of achieving 1mm measurement accuracy on global baselines, performing continuous measurements for time series of station positions and Earth orientation parameters, and reaching a turnaround time from measurement to initial geodetic results of less than 24 h. To realize these recommendations and goals, along with the need for low cost of construction and operation, requires a complete examination of all aspects of geodetic VLBI including equipment, processes, and observational strategies. Hence, in October 2005, the IVS VLBI2010 Committee (V2C) commenced work on defining the VLBI2010 system specifications. In this paper we give a summary of the recent progress of the VLBI2010 project. © Springer-Verlag Berlin Heidelberg 2009.
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| 3. |
- Davis, J.L., et al.
(author)
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Ground-based measurement of gradients in the “wet” radio refractivity of air
- 1993
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In: Radio Science. - 0048-6604 .- 1944-799X. ; 28:6, s. 1003-1018
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Journal article (peer-reviewed)abstract
- We have used a ground-based microwave radiometer, known as a water vapor radiometer, to investigate the local spatial and temporal variation of the wet propagation delay for a site on the west coast of Sweden. The data were obtained from a wide range of azimuths and from elevation angles greater than 23.6-degrees (air mass 2.5). Visual inspection of the data suggested a simple ‘'cosine azimuth” variation, implying that a first-order gradient model was required. This model was adequate for short time spans up to approximately 15 min, but significant temporal variations in the gradient suggested to us that we include gradient rate terms. The resulting six-parameter model has proven adequate (rms delay residual approximately 1 mm) for up to 30 min of data. Assuming a simple exponential profile for the wet refractivity gradient, the estimated gradient parameters imply average surface wet-refractivity horizontal gradients of order of 0.1-1 N km-1. These gradients are larger, by 1-2 orders of magnitude, than gradients determined by others by averaging over long (approximately 100-km) distances. This result implies that for applications that are sensitive to local gradients, such as wet propagation-delay models for radio-interferometric geodetic studies, the use of meteorological data from widely spread stations may be inadequate. The gradient model presented here is inadequate for times longer than about 30 min. even if no gradients are present, because of the complicated stochastic like temporal behavior of the wet atmosphere. When gradients are present, they can change magnitude by approximately 50% over 10-15 min. Nevertheless, our ability to fit the radiometer data implies that on timescales 23.6-degrees, the local structure of the wet atmosphere can be described with a simple model. (The model is not limited to this range of elevation angles in principle.) The estimated gradient and gradient rate vectors have preferred directions, which indicates a prevailing structure in the three-dimensional temperature and humidity fields, possibly related to systematic behavior in large-scale weather systems and/or the local air-land-sea interaction at this site.
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