Very Long Baseline Interferometry Research Papers (original) (raw)
Radio astronomy is a very young research field in South East Asia. There has not been a research-grade radio telescope built in this part of the world yet. A plan has been proposed by the University of Malaya's Radio Cosmology Research... more
Radio astronomy is a very young research field in South East Asia. There has not been a research-grade radio telescope built in this part of the world yet. A plan has been proposed by the University of Malaya's Radio Cosmology Research Laboratory to build a medium-sized radio telescope in order to eventually join the global projects of the Very Long Baseline Interferometry (VLBI) Network and Square Kilometer Array (SKA). Main parameters taken into consideration in finding the main prime candidate sites involves features that produce Radio Frequency Interference (RFI). These features are mainly telecommunication and satellite navigation signals and population density. Other important features considered are rainfall level, land contour and availability for future collaboration with institutions at the chosen sites. In this paper we described the experimental procedure and the RFI measurement on our five prime candidate's sites in Malaysia, covering frequency band from 1 MHz to 2000 MHz. The levels and sources of RFI on these sites were monitored and analyzed. The RFI level in Langkawi showed the lowest average of À100:33 dB m ð4:4 Â 10 6 JyÞ. These RFI have been found to fluctuate relatively lowly (between 1 dB m and 2 dB m). This site is also ideally located close to the Langkawi National Observatory and we recommend that this site as the best site to build the first research-grade radio telescope in this region.
Global Positioning System (GPS) has been very useful tool for the last two decades in the area of geodynamics because or the validation of the GPS results by the Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR)... more
Global Positioning System (GPS) has been very useful tool for the last two decades in the area of geodynamics because or the validation of the GPS results by the Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) observations. The modest budget requirement and the high accuracy relative positioning availability of GPS increased the use of it in determination of crustal and/or regional deformations. Since the civilian use the GPS began in 1980, the development on the receiver and antenna technology with the ease of use software packages reached to a well known state, which may be named as a revolution in the Earth Sciences among other application fields. Analysis of a GPS network can also give unknown information about the fault lines that can not be seen from the ground surface. Having information about the strain accumulation along the fault line may allow us to evaluate future probabilities of regional earthquake hazards and develop earthquake scenarios for specific faults. In this study, the use of GPS in geodynamical studies will be outlined throughout the instrumentation, the measurements, and the methods utilized. The preliminary results of three projects, sponsored by the Scientific & Technical Research Council of Turkey (TUBITAK) and Istanbul Technical University (ITU) which have been carried out in Turkey using GPS will be summarized. The projects are mainly aimed to determine the movements along the fault zones. Two of the projects have been implemented along the North Anatolian Fault Zone. (NAFZ), one is in Mid-Anatolia region, and the ther is in Western Marmara region. The third project has been carried out in the Fethiye-Burdur region. The collected GPS data were processed by the GAMIT/GLOBK software The results are represented as velocity vectors obtained using the yearly combinations of the daily measured GPS data.
1] For the first time in the history of the International Terrestrial Reference Frame, the ITRF2000 combines unconstrained space geodesy solutions that are free from any tectonic plate motion model. Minimum constraints are applied to... more
1] For the first time in the history of the International Terrestrial Reference Frame, the ITRF2000 combines unconstrained space geodesy solutions that are free from any tectonic plate motion model. Minimum constraints are applied to these solutions solely in order to define the underlying terrestrial reference frame (TRF). The ITRF2000 origin is defined by the Earth center of mass sensed by satellite laser ranging (SLR) and its scale by SLR and very long baseline interferometry. Its orientation is aligned to the ITRF97 at epoch 1997.0, and its orientation time evolution follows, conventionally, that of the no-net-rotation NNR-NUVEL-1A model. The ITRF2000 orientation and its rate are implemented using a consistent geodetic method, anchored over a selection of ITRF sites of high geodetic quality, ensuring a datum definition at the 1 mm level. This new frame is the most extensive and accurate one ever developed, containing about 800 stations located at about 500 sites, with better distribution over the globe compared to past ITRF versions but still with more site concentration in western Europe and North America. About 50% of station positions are determined to better than 1 cm, and about 100 sites have their velocity estimated to at (or better than) 1 mm/yr level. The ITRF2000 velocity field was used to estimate relative rotation poles for six major tectonic plates that are independent of the TRF orientation rate. A comparison to relative rotation poles of the NUVEL-1A plate motion model shows vector differences ranging between 0.03°and 0.08°/m.y. (equivalent to approximately 1-7 mm/yr over the Earth's surface). ITRF2000 angular velocities for four plates, relative to the Pacific plate, appear to be faster than those predicted by the NUVEL-1A model. The two most populated plates in terms of space geodetic sites, North America and Eurasia, exhibit a relative Euler rotation pole of about 0.056 (±0.005)°/m.y. faster than the pole predicted by NUVEL-1A and located about (10°N, 7°E) more to the northwest, compared to that model.
In the past few years, numerical weather models (NWM) have been investigated to improve mapping functions which are used for tropospheric delay modeling in VLBI and GPS data analyses. The Vienna Mapping Functions VMF are based on direct... more
In the past few years, numerical weather models (NWM) have been investigated to improve mapping functions which are used for tropospheric delay modeling in VLBI and GPS data analyses. The Vienna Mapping Functions VMF are based on direct raytracing through NWM, and so they are able to exploit the full information provided in the NWM. On the other hand, the Isobaric Mapping Functions IMF are using intermediate parameters calculated from the NWM. In this study, pressure level data from ECMWF (European Centre for Medium-Range Weather Forecasts) are applied to determine the coefficients of the VMF and the IMF. Used for the analyses of IVS-R1 and IVS-R4 VLBI sessions, both mapping functions improve the repeatability of baseline lengths (by ~10% for IVS-R1 and ~5% for IVS-R4) compared to the Niell Mapping Functions NMF.
In order to meet the requirements of the new generation of radio telescopes, we have developed a new topology called DYQSA, which stands for DYson Quad-Spiral Array. The design exhibits dual circular polarization in contrast to dual... more
In order to meet the requirements of the new generation of radio telescopes, we have developed a new topology called DYQSA, which stands for DYson Quad-Spiral Array. The design exhibits dual circular polarization in contrast to dual linear polarization of state-of-the-art feeds. It covers the required ultra-wideband (UWB) from 2 GHz to 14 GHz with an almost constant and real input impedance which facilitates the design of the feeding structure and the Low Noise Amplifiers (LNAs). Different versions are investigated for enhancing feed performance, ensuring higher aperture efficiencies and mechanical stability. Simulation results of the reflector loaded by the proposed feed show an aperture efficiency of 65 ± 5% can be achieved with a noise antenna temperature around 14 K and a System Equivalent Flux Density (SEFD) of about 1300 Jy, both averaged over the required bandwidth at zenith. Measurements of the single-element and the four-element feeds are presented. Comparisons with other state-of-the-art feeds are shown in terms of total aperture efficiencies, design adaptability to different reflectors, calibration signal injection, the required number of LNAs per feed, cost, and physical volume.
The CONT02 campaign is of great interest for studies combining very long baseline interferometry (VLBI) with other space-geodetic techniques, because of the continuously available VLBI observations over two weeks in October 2002 from a... more
The CONT02 campaign is of great interest for studies combining very long baseline interferometry (VLBI) with other space-geodetic techniques, because of the continuously available VLBI observations over two weeks in October 2002 from a homogeneous network. Especially the combination with the Global Positioning System (GPS) offers a broad spectrum of common parameters. We combined station coordinates, Earth orientation parameters (EOPs) and troposphere parameters consistently in one solution using technique-specific datum-free normal equation systems. In this paper, we focus on the analyses concerning the EOPs, whereas the comparison and combination of the troposphere parameters and station coordinates is covered in a companion paper in Journal of Geodesy. In order to demonstrate the potential of the VLBI and GPS space-geodetic techniques, we chose a sub-daily resolution for polar motion (PM) and universal time (UT). A consequence of this solution set-up is the presence of a one-to-one correlation between the nutation angles and a retrograde diurnal signal in PM. The Bernese GPS Software used for the combination provides a constraining approach to handle this singularity. Simulation studies involving both nutation offsets and rates helped to get a deeper understanding of this singularity. With a rigorous combination of UT1-UTC and length of day (LOD) from VLBI and GPS, we showed that such a combination works very well and does not suffer from the systematic effects present in the GPS-derived LOD values. By means of wavelet analyses and the formal errors of the estimates, we explain this important result. The same holds for the combination of nutation offsets and rates. The local geodetic ties between GPS and VLBI antennas play an essential role within the inter-technique combination. Several studies already revealed non-negligible discrepancies between the terrestrial measurements and the space-geodetic solutions. We demonstrate to what extent these discrepancies propagate into the combined EOP solution.
In this article we present two methods for combination of different Global Navigation Satellite Systems (GNSS) Zenith Total Delay (ZTD) time-series for the same GNSS site, but from different producers or different processing setups. One... more
In this article we present two methods for combination of different Global Navigation Satellite Systems (GNSS) Zenith Total Delay (ZTD) time-series for the same GNSS site, but from different producers or different processing setups. One method has been setup at ASI/CGS, the other at KNMI. Using Near Real-Time (NRT) ZTD data covering 1 year from the E-GVAP project, the performance of the two methods is inter-compared and validation is made against a combined ZTD solution from EUREF, based on post-processed ZTDs. Further, validation of the ASI combined solutions is made against independent ZTDs derived from radiosonde, Numerical Weather Prediction (NWP) model and Very Long Baseline Interferometry (VLBI) ZTD.
- by Henrik Vedel and +2
- •
- Mechanical Engineering, Aerospace Engineering, Time Series, Space
We describe a new method which achieves high-precision very long baseline interferometry (VLBI) astrometry in observations at millimeter (mm) wavelengths. It combines fast frequency-switching observations, to correct for the dominant... more
We describe a new method which achieves high-precision very long baseline interferometry (VLBI) astrometry in observations at millimeter (mm) wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method source-frequency phase referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration strategy, along with an error analysis to characterize its performance. Also, we provide observational demonstrations of a successful application with observations using the Very Long Baseline Array at 86 GHz of the pairs of sources 3C274 and 3C273 and 1308+326 and 1308+328 under various conditions. We conclude that this method is widely applicable to mm-VLBI observations of many target sources, and unique in providing bona fide astrometrically registered images and high-precision relative astrometric measurements in mm-VLBI using existing and newly built instruments, including space VLBI.
... area. R. Devoti a , C. Ferraro a , E. Gueguen b , Corresponding Author Contact Information , E-mail The Corresponding Author , R. Lanotte a , V. Luceri a , A. Nardi a , R. Pacione a , P. Rutigliano a , C. Sciarretta a and F. Vespe c.... more
... area. R. Devoti a , C. Ferraro a , E. Gueguen b , Corresponding Author Contact Information , E-mail The Corresponding Author , R. Lanotte a , V. Luceri a , A. Nardi a , R. Pacione a , P. Rutigliano a , C. Sciarretta a and F. Vespe c. ...
- by E. Gueguen and +1
- •
- Geology, Geochemistry, Geophysics, Data Analysis
Kashima Space R e s e a r c h C e n t e r C o m m u n i c a t i o n s R e s e a r c h L a b o r a t o r y M i n i s t r y o f P o s t s and T e l e c o m m u n i c a t i o n s 693-1 H i r a i Kashima-machi ABSTRACT C o m m u n i c a t i o... more
Kashima Space R e s e a r c h C e n t e r C o m m u n i c a t i o n s R e s e a r c h L a b o r a t o r y M i n i s t r y o f P o s t s and T e l e c o m m u n i c a t i o n s 693-1 H i r a i Kashima-machi ABSTRACT C o m m u n i c a t i o n s R e s e a r c h L a b o r a t o r y (CRL) has d e v e
After detailed field (stratigraphic and structural) and laboratory analyses of intact and deformed rocky outcrops, we studied the multi-scale properties of strike-slip faults nucleated and developed in Pleistocene carbonate grainstones of... more
After detailed field (stratigraphic and structural) and laboratory analyses of intact and deformed rocky outcrops, we studied the multi-scale properties of strike-slip faults nucleated and developed in Pleistocene carbonate grainstones of Favignana Island ( ...
- by Paul Tregoning and +1
- •
- Multidisciplinary, Satellite Geodesy, Wave propagation, Geophysical
One-baseline 1-h Very Long Baseline Interferometry (VLBI) Intensive sessions are carried out every day to determine Universal Time (UT1). Azimuthal asymmetry of tropospheric delays around the stations is usually ignored and not estimated... more
One-baseline 1-h Very Long Baseline Interferometry (VLBI) Intensive sessions are carried out every day to determine Universal Time (UT1). Azimuthal asymmetry of tropospheric delays around the stations is usually ignored and not estimated because of the small number of observations. In this study we use external information about the asymmetry for the Intensive sessions between Tsukuba (Japan) and Wettzell (Germany), which are carried out on Saturdays and Sundays (1) from direct ray-tracing for each observation at Tsukuba and (2) in the form of linear horizontal north and east gradients every 6 h at both stations. The change of the UT1 estimates is at the 10 µs level with maximum differences of up to 50 µs, which is clearly above the formal uncertainties of the UT1 estimates (between 5 and 20 µs). Spectral analysis reveals that delays from direct ray-tracing for the station Tsukuba add significant power at short periods (1-2 weeks) w.r.t. the state-of-the-art approach, and comparisons with length-of-day (LOD) estimates from Global Positioning System (GPS) indicate that these ray-traced delays slightly improve the UT1 estimates from Intensive sessions.
1] From Gibraltar to Anatolia, the active tectonics in the Mediterranean is studied by means of an integrated approach based on geophysical, geodetic, and seismological methodologies. The aim of this study is to gain a deep insight into... more
1] From Gibraltar to Anatolia, the active tectonics in the Mediterranean is studied by means of an integrated approach based on geophysical, geodetic, and seismological methodologies. The aim of this study is to gain a deep insight into the kinematics and dynamics of the crustal and lithospheric processes affecting the Mediterranean. Major tectonic processes, such as continental collision and subduction, characterize this region, which marks a broad transition zone between the African/Arabian and Eurasian plates. A thin-shell finite element approach allows us to simulate the deformation pattern in the Mediterranean, from 10°W to 40°E and from 30°to 50°N. The global plate motion model NUVEL-1A is used to account for the convergence, while the relative velocities of the overriding and subduction plates are obtained from another family of models. These models simulate the effects of the negatively buoyant density contrasts of the subducted lithosphere on the horizontal velocity at the surface. A systematic comparison between model results and the seismic strain rates obtained from the National Earthquake Information Center catalog, the geodetic velocity field and strain resulting from GPS, satellite laser ranging, and very long baseline interferometry analyses and the World Stress Map, indicate that Africa/Arabia versus Eurasia convergence and subduction in the Aegean Sea and Calabrian Arc are the major tectonic mechanisms controlling the deformation style in the Mediterranean. It is shown that in order to carry into coincidence the modeled and the seismic strain rate patterns and the geodetically retrieved strain rate tensors, a deep subduction in the Aegean Arc must be included in the modeling.
Millimetron is a Russian-led 12 m diameter submillimeter and far-infrared space observatory which is included in the Space Plan of the Russian Federation for launch around 2017. With its large collecting area and state-of-the-art... more
Millimetron is a Russian-led 12 m diameter submillimeter and far-infrared space observatory which is included in the Space Plan of the Russian Federation for launch around 2017. With its large collecting area and state-of-the-art receivers, it will enable unique science and allow at least one order of magnitude improvement with respect to the Herschel Space Observatory. Millimetron will be operated in two basic observing modes: as a single-dish observatory, and as an element of a ground-Exp Astron (space very long baseline interferometry (VLBI) system. As single-dish, angular resolutions on the order of 3 to 12 arc sec will be achieved and spectral resolutions of up to a million employing heterodyne techniques. As VLBI antenna, the chosen elliptical orbit will provide extremely large VLBI baselines (beyond 300,000 km) resulting in micro-arc second angular resolution.
In geodetic VLBI (Very Long Baseline Interferometry) the observations of extragalactic radio sources are performed at two frequencies (2.3 and 8.4 GHz) by a global network of radio telescopes. The most important observable for geodesy is... more
In geodetic VLBI (Very Long Baseline Interferometry) the observations of extragalactic radio sources are performed at two frequencies (2.3 and 8.4 GHz) by a global network of radio telescopes. The most important observable for geodesy is the time delay between the arrival times of a plane wavefront at two radio telescopes. From a sufficient number of these observables various parameters can be estimated with highest accuracy, e.g. the Earth orientation parameters or baseline lengths and differences of the station coordinates. The variable tropospheric refraction is a major error source for the estimation of these geodetic parameters by VLBI. The influence of the tropospheric refraction is taken into account by appropriate models which usually contain a hydrostatic and a wet part. By special mapping functions the tropospheric zenith delay is mapped to the elevation of the observation. The tropospheric zenith delays (total or wet) have also to be estimated within the VLBI least-square...
We describe the development of an FX style correlator for Very Long Baseline Interferometry (VLBI), implemented in software and intended to run in multi-processor computing environments, such as large clusters of commodity machines... more
We describe the development of an FX style correlator for Very Long Baseline Interferometry (VLBI), implemented in software and intended to run in multi-processor computing environments, such as large clusters of commodity machines (Beowulf clusters) or computers specifically designed for high performance computing, such as multi-processor shared-memory machines. We outline the scientific and practical benefits for VLBI correlation, these chiefly being due to the inherent flexibility of software and the fact that the highly parallel and scalable nature of the correlation task is well suited to a multi-processor computing environment. We suggest scientific applications where such an approach to VLBI correlation is most suited and will give the best returns. We report detailed results from the Distributed FX (DiFX) software correlator, running on the Swinburne supercomputer (a Beowulf cluster of ∼300 commodity processors), including measures of the performance of the system. For example, to correlate all Stokes products for a 10 antenna array, with an aggregate bandwidth of 64 MHz per station and using typical time and frequency resolution presently requires of order 100 desktop-class compute nodes. Due to the effect of Moore's Law on commodity computing performance, the total number and cost of compute nodes required to meet a given correlation task continues to decrease rapidly with time. We show detailed comparisons between DiFX and two existing hardware-based correlators: the Australian Long Baseline Array (LBA) S2 correlator, and the NRAO Very Long Baseline Array (VLBA) correlator. In both cases, excellent agreement was found between the correlators. Finally, we describe plans for the future operation of DiFX on the Swinburne supercomputer, for both astrophysical and geodetic science.
Very Long Baseline Interferometry (VLBI) allows to monitor universal time (UT1) by conducting regular international experiments. Such dedicated observation networks are equipped with different hardware components, which require different... more
Very Long Baseline Interferometry (VLBI) allows to monitor universal time (UT1) by conducting regular international experiments. Such dedicated observation networks are equipped with different hardware components, which require different processing strategies when the data are correlated. As the timing units at each stations are usually offset with respect to universal time (UTC) this effect should be considered during correlation processing. Thus, it is investigated how neglecting of these offsets theoretically impacts the estimation of UT1. Three different strategies for the proper handling of the timing offset will be discussed and their advantages/drawbacks will be pointed out. Moreover, it is studied how neglecting of these timing offsets affects UT1 time-series and how such a missing correction can be applied a-posteriori. Although the discussed effect is for most of the UT1 experiments smaller than the formal error of the estimates, it is important to consider station clock offsets properly in next-generation VLBI systems, which are expected to improve accuracy of results by about one order of magnitude.
Gravitational physics of VLBI experiment conducted on September 8, 2002 and dedicated to measure the speed of gravity is treated in the first post-Newtonian approximation. Explicit speed-of-gravity parameterization is introduced to the... more
Gravitational physics of VLBI experiment conducted on September 8, 2002 and dedicated to measure the speed of gravity is treated in the first post-Newtonian approximation. Explicit speed-of-gravity parameterization is introduced to the Einstein equations to single out the retardation effect associated with the finite speed of gravity in the relativistic time delay of light propagating in the gravitational field of moving Jupiter. Velocity-dependent 1.5 post-Newtonian correction to the Shapiro time delay is derived and compared with our previous result obtained by making use of the post-Minkowskian approximation. We prove that the 1.5 post-Newtonian correction to the Shapiro delay depends on the speed of propagation of gravity cg that is a directly measurable parameter in the VLBI experiment. Subject headings: gravitation – relativity – techniques: interferometric – (galaxies:) quasars: individual (QSO J0842+1835) 1.
A novel method to determine the •xis intersection •n(1 offset of the Hartebeesthoek Radio Astronomy Observatory (HartRA()) radio telescope using GPS is described. This method •llows the very long b•seline interferometry (VLBI), satellite... more
A novel method to determine the •xis intersection •n(1 offset of the Hartebeesthoek Radio Astronomy Observatory (HartRA()) radio telescope using GPS is described. This method •llows the very long b•seline interferometry (VLBI), satellite laser ranging (SLR) and Global Positioning System ((_]PS) reference frames to be tied to • single point.
1] In the framework of the activities of the Combination Research Centers (CRC) of the International Earth Rotation and Reference Systems Service (IERS), the French Groupe de Recherche en Géodésie Spatiale (GRGS) studies the benefit of... more
1] In the framework of the activities of the Combination Research Centers (CRC) of the International Earth Rotation and Reference Systems Service (IERS), the French Groupe de Recherche en Géodésie Spatiale (GRGS) studies the benefit of combining space-geodetic techniques (Doppler orbitography and radiopositioning integrated by satellite, GPS, satellite laser ranging, and very long baseline interferometry) at the observational level. This combination aims to produce a global and consistent solution for Earth orientation parameters (EOPs), polar motion x p and y p , and universal time UT1 with a 1-day or a 6-hour sampling, as well as weekly station positions. In this paper we present a methodology for multitechnique combination at the observational level. We process the measurements of the four techniques over a 1-year period (the year 2002) in order to illustrate and validate our method. All techniques are processed with the same computational framework, thus with the same models and a priori values for parameters. By using the same software and conventions, we avoid inconsistencies in individual computations. We process each technique individually and inside the combination. The comparison between these solutions is a way of analyzing the power of our method even if the actual status of our software does not reproduce the state-of-the-art analyses of each technique. However, we produce an analysis of the quality of our individual computations so that readers can get an informed appreciation of the current capabilities of our software. Finally, we present the capability of such combinations in terms of accuracy and precision, we underline the main issues of our method and propose solutions to solve them in the future. Citation: Coulot, D., P. Berio, R. Biancale, S. Loyer, L. Soudarin, and A.-M. Gontier (2007), Toward a direct combination of spacegeodetic techniques at the measurement level: Methodology and main issues,
Citation: Collilieux, X., Z. Altamimi, D. Coulot, J. Ray, and P. Sillard , Comparison of very long baseline interferometry, GPS, and satellite laser ranging height residuals from ITRF2005 using spectral and correlation methods,
The usage of Very Long Baseline Interferometry (VLBI) fringe-phase information in geodetic VLBI is a new field of research, which can be used for the detection of short-period (i.e., several minutes) variations (scintillations) of the... more
The usage of Very Long Baseline Interferometry (VLBI) fringe-phase information in geodetic VLBI is a new field of research, which can be used for the detection of short-period (i.e., several minutes) variations (scintillations) of the ionosphere. This paper presents a method for the extraction of such disturbances and discusses how dispersive influences can be separated from intra-scan delay variations. A proper functional and stochastic model for the separation of the different effects is presented and the algorithms are applied to real measurements. In an example, it is shown that a traveling ionospheric disturbance in Antarctica can be detected very precisely. A possible physical origin and the propagation properties of the disturbance are presented and the results are compared with GPS measurements. The benefit of this method for other applications is also discussed.
A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon. Realizing this goal would open a new window on the study of General Relativity... more
A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon. Realizing this goal would open a new window on the study of General Relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of an event
We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United... more
We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United States. Northwest motion of the central Sierra Nevada block relative to stable North America, a measure of integrated Basin and Range deformation, is 12.1+1.2 mm/yr oriented N38øW+5 ø (one standard error), in agreement with previous geological estimates within uncertainties. This velocity reflects both east-west extension concentrated in the eastern Basin and Range and north-northwest directed right lateral shear concentrated in the western Basin and Range. Ely, Nevada is moving west at 4.9+1.3 mm/yr relative to stable North America, consistent with dip-slip motion on the north striking Wasatch fault and other north striking normal faults. Comparison with ground-based geodetic data suggests that most of this motion is accommodated within N50 km of the Wasatch fault zone. Paleoseismic data for the Wasatch fault zone and slip rates based on seismic energy release in the region both suggest much lower slip rates. The discrepancy may be explained by some combination of additional deformation away from the Wasatch fault itself, aseismic slip, or a seismic rate that is anomalously low with respect to longer time averages. Deformation in the western Basin and Range province is also largely confined to a relatively narrow boundary zone and in our study area is partitioned into the eastern California shear zone, accommodating 10.7+1.6 mm/yr of north-northwest directed right-lateral shear, and a small component (-1 mm/yr) of west-southwest-east-northeast
Las observaciones interferométricas de la emisión maser de diversas moléculas (p.ej., H 2 O, OH, CH 3 OH) están constituyendo una herramienta muy eficaz para estudiar con gran resolución angular el gas molecular cercano a las... more
Las observaciones interferométricas de la emisión maser de diversas moléculas (p.ej., H 2 O, OH, CH 3 OH) están constituyendo una herramienta muy eficaz para estudiar con gran resolución angular el gas molecular cercano a las protoestrellas. Gracias a ellas se ha observado que, en algunos casos, los máseres parecen trazar el disco que se forma alrededor del objeto estelar joven, mientras que en otros casos parecen trazar el propio flujo de material eyectado. En este artículo resumimos algunos de esos estudios observacionales, incluyendo los realizados muy recientemente mediante la poderosa técnica de interferometría de base muy larga (VLBI). Esas observaciones VLBI permiten estudiar los máseres con resolución angular de una pocas décimas de milisegundo de arco (equivalente a una resolución lineal de unas pocas décimas de unidad astronómica para objetos que se encuentran a una distancia de 500 pc), permitiendo con ello medir sus movimientos propios.
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by... more
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by the propagation delay of radio waves in the atmosphere, while the remote sensing technique is sensitive to the atmospheric emission close to the center of the 22 GHz water vapor emission line. We present a comparison of estimated equivalent zenith wet delay and linear horizontal delay gradients from an independent analysis of simultaneous VLBI, GPS, and WVR observations. Using different constraints for the variability of the delay and the horizontal gradient in the analysis of the VLBI and the GPS data did not have a large influence on the agreement with the WVR estimates. We found that the weighted rms differences between wet delay estimates from the geodetic techniques and the WVR estimates generally increased for an increased variability in the atmosphere.
Loading of the Earth's crust due to variations of global atmospheric pressure can displace the positions of geodetic sites by more than 1 cm, both vertically and horizontally on annual to sub-diurnal time scales. Such atmosphere pressure... more
Loading of the Earth's crust due to variations of global atmospheric pressure can displace the positions of geodetic sites by more than 1 cm, both vertically and horizontally on annual to sub-diurnal time scales. Such atmosphere pressure loading (APL) effects have been observed in high-precision space geodetic data, i.e. Global Navigation Satellite Systems (GNSS), Very Long Baseline Interferometry (VLBI), and Satellite Laser Ranging (SLR). These observational data are primarily used for geodynamic studies and it is important to remove the displacement signals due to APL which - if uncorrected - can propagate into other parameters and effects. In order to accurately model APL corrections, we established the project GGOS Atmosphere at TU Vienna. The overall goal of the project is not only to determine APL corrections but also to develop consistent and homogeneous models for atmospheric angular momentum, troposphere delays of radio signals, and gravity field coefficients for the atmosphere. Within project GGOS Atmosphere, we set up an accurate and rigorous procedure for the calculation of APL effects including the unambiguous definition and accurate calculation of a reference pressure. We use operational analysis as well as re-analysis data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF) with a horizontal resolution of 1 degree. The full APL displacements are divided into tidal and non-tidal components and 6-hourly radial and horizontal corrections are provided for all VLBI sites as well as for the nodes of a global 1 degree grid. The displacements are determined in the Center of Mass (CM) and in the Center of the solid Earth (CE) frames. We validate the correction values by VLBI observations and by comparison with results from other providers. Users can freely access the correction values at http://ggosatm.hg.tuwien.ac.at/.
We estimate the velocity field in central and southern Calitbrnia using Global Positioning System (GPS) observations from 1986 to 1902 and very long baseline interferometry (VLB!) observations from 1984 to 1991. Our core network includes... more
We estimate the velocity field in central and southern Calitbrnia using Global Positioning System (GPS) observations from 1986 to 1902 and very long baseline interferometry (VLB!) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 05% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north-south, and a semimajor axis of approximately 3 mm/yr oriented east-west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position ("repeatability"), (2) independent velocity estimates from separate analyses of the GPS and VLBi data, and (3) rates of change in baseline length estimated t¾om the joint GPS+VLB! solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model l¾om the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 _.+ I, 5 _.+ I, 5 _.+ 1, and 2 _.+ I mm/yr, respectively. North of Ihe Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnctic declinations. Our estimates indicate that the "San Andreas discrepancy" can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of I-2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the ofl•horc islands and the mainland.
We show the influence of horizontal displacements due to ocean tide loading on the determination of polar motion and UT1 (PMU) on the daily and subdaily timescale. So called 'virtual PMU variations' due... more
We show the influence of horizontal displacements due to ocean tide loading on the determination of polar motion and UT1 (PMU) on the daily and subdaily timescale. So called 'virtual PMU variations' due to modelling errors of ocean tide loading are predicted for ...
We present new high spatial resolution Karl G. Jansky Very Large Array (VLA) HI absorption and Very Long Baseline Array (VLBA) continuum observations of the Active Galactic Nucleus (AGN)-driven molecular outflow candidate NGC 1266.... more
We present new high spatial resolution Karl G. Jansky Very Large Array (VLA) HI absorption and Very Long Baseline Array (VLBA) continuum observations of the Active Galactic Nucleus (AGN)-driven molecular outflow candidate NGC 1266. Although other well-known systems with molecular outflows may be driven by star formation in a central molecular disk, the molecular mass outflow rate reported in Alatalo et al. (2011) in NGC 1266 of 13 M_ year^-1 exceeds star formation rate estimates from a variety of tracers. This suggests that an additional energy source, such as an AGN, may play a significant role in powering the outflow. Our high spatial resolution HI absorption data reveal compact absorption against the radio continuum core co-located with the putative AGN, and the presence of a blueshifted spectral component re-affirms that gas is indeed flowing out of the system. Our VLBA observations at 1.65 GHz reveal one continuum source within the densest portion of the molecular gas, with a d...
From a conceptual point of view water vapour radiometers (WVRs) are ideal instruments for a direct determination of the water vapour content in the atmosphere above a space geodetic observing site. For various reasons the application of... more
From a conceptual point of view water vapour radiometers (WVRs) are ideal instruments for a direct determination of the water vapour content in the atmosphere above a space geodetic observing site. For various reasons the application of WVRs for a direct correction of geodetic Very Long Baseline Interferometry (VLBI) observations for the wet component of atmospheric refraction has not been brought to a stage where it could be employed routinely. The installation of a new type of WVR at the 100 m Effelsberg radio telescope in Germany opens up new possibilities for a direct correction of the VLBI observations using line-ofsight WVR observations. The WVR has been in operation during two geodetic VLBI sessions of a purely European network. In addition, some WVR data at Onsala have been revisited to embark on a project which aims at applying wet delay corrections from WVRs to VLBI sessions of the European geodetic VLBI network for the Effelsberg, Madrid, Onsala and Wettzell sites. In this paper we present the analysis and results of a few examples of Effelsberg and Onsala observing sessions.
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by... more
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by the propagation delay of radio waves in the atmosphere, while the remote sensing technique is sensitive to the atmospheric emission close to the center of the 22 GHz water vapor emission line. We present a comparison of estimated equivalent zenith wet delay and linear horizontal delay gradients from an independent analysis of simultaneous VLBI, GPS, and WVR observations. Using different constraints for the variability of the delay and the horizontal gradient in the analysis of the VLBI and the GPS data did not have a large influence on the agreement with the WVR estimates. We found that the weighted rms differences between wet delay estimates from the geodetic techniques and the WVR estimates generally increased for an increased variability in the atmosphere.
The solid Earth tide model as recommended by the IERS standards [McCarthy, 1992] and used in standard Very Long Baseline Interferometry [VLBI] data analysis only approximates to the first order the frequency dependence of the tidal... more
The solid Earth tide model as recommended by the IERS standards [McCarthy, 1992] and used in standard Very Long Baseline Interferometry [VLBI] data analysis only approximates to the first order the frequency dependence of the tidal deformation due to the Free Core Nutation [FCN] resonance. It is shown that frequency dependent second degree Love and Shida numbers have to be taken into account if the tidal displacements are to be modeled to better than +/-5 mm. We also determined second degree Love and Shida numbers for different tidal bands (long-period, diurnal, semidiurnal) and third degree Love and Shida numbers from VLBI data. The results for the different parts of the tidal spectrum are slightly different and a clear resonance can be seen around the FCN. The FCN period was determined to 426+/-20 sidereal days in the celestial reference frame.
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by... more
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by the propagation delay of radio waves in the atmosphere, while the remote sensing technique is sensitive to the atmospheric emission close to the center of the 22 GHz water vapor emission line. We present a comparison of estimated equivalent zenith wet delay and linear horizontal delay gradients from an independent analysis of simultaneous VLBI, GPS, and WVR observations. Using different constraints for the variability of the delay and the horizontal gradient in the analysis of the VLBI and the GPS data did not have a large influence on the agreement with the WVR estimates. We found that the weighted rms differences between wet delay estimates from the geodetic techniques and the WVR estimates generally increased for an increased variability in the a...
Precollege science education is an issue with profound import for the future of our country. An effective program requires the active participation of the scientific commu nity. Thirty-one scientists and engineers from 14 states... more
Precollege science education is an issue with profound import for the future of our country. An effective program requires the active participation of the scientific commu nity. Thirty-one scientists and engineers from 14 states representing a number of disci plines and institutions (government laborato ries, industry, and universities) met on the Caltech campus March 7-13 for a working conference on this issue. The workshop was sponsored by the National Science Re sources Center (NSRC) and hosted by Jer ome Pine and Jim Bower of Caltech.
The VSOP terminal is a new data-acquisition system for the Very-Long-Baseline Interferometry (VLBI). This terminal was primarily designed for ground telescopes in the VLBI Space Observatory Programme (VSOP). New technologies; higher-order... more
The VSOP terminal is a new data-acquisition system for the Very-Long-Baseline Interferometry (VLBI). This terminal was primarily designed for ground telescopes in the VLBI Space Observatory Programme (VSOP). New technologies; higher-order sampling and digital filtering techniques, were introduced in the development. A cassette cart was also introduced, which supports 24-hour unattended operations at the maximum data rate of 256 Mbps. The higher-order sampling and digital filtering techniques achieve flat and constant phase response over bandwidth of 32 MHz without using expensive wide base-band converters. The digital filtering technique also enables a variety of observing modes defined on the VSOP terminal, even with a fixed sampling frequency in an A/D converter. The new terminals are installed at Nobeyama, Kashima, Usuda, Mizusawa, and Kagoshima radio observatories in Japan, and are being used in VSOP and other domestic VLBI observations. In this paper the key features of the VSOP terminal focusing on these advanced technologies are presented, and the results of performance tests are shown.
Very long baseline interferometry (VLBI) observations collected primarily under projects POLARIS (Polar-Motion Analysis by Radio Interferometric Surveying) and IRIS (International Radio Interferometric Surveying) since 1980 have been used... more
Very long baseline interferometry (VLBI) observations collected primarily under projects POLARIS (Polar-Motion Analysis by Radio Interferometric Surveying) and IRIS (International Radio Interferometric Surveying) since 1980 have been used to derive polar motion and UT1 time series. The typical 24-hour observing sessions using the multistation North America-Europe network yield estimates of the X and Y components of the pole with uncertainties of 1-2 milliseconds of arc (mas) and UT1 with uncertainties of 0.05-0.1 ms of time. (Preliminary experimental results indicate that UT1 can be tracked by a single VLBI interferometer to the 0.1-ms level in observing periods of about 1 hour.) The same observations have been used to estimate the relative rates of motion of the North American and European tectonic plates and investigate possible regional deformations of the North American plate, with uncertainties of a few millimeters per year; check the accuracy of the adopted nutation series to the millisecond of arc level' estimate earth tide Love numbers for each observatory site with uncertainties of about 1% in h and 25% in 1' and determine the coordinates of 20 radio sources to a few milliseconds of arc. Temporal variations in the apparent positions of the radio sources caused by relativistic deflections of the radio signals in the solar gravitational field have been used to estimate the relativistic deflection parameter 7, yielding a value of 1.000 _+ 0.003. ploit the improved capabilities of the combined POLARIS/ Wettzell network, the responsible geodetic agencies in the FRG and NGS jointly initiated project IRIS (International Radio Interferometric Surveying) in 1982 [Carter and Robertson, 1984]. The International Association of Geodesy (IAG) subsequently approved the formation of a permanent IRIS subcommission; organizations in several additional nations have agreed to participate or expressed interest in participating in IRIS activities. The Onsala Space Observatory, Sweden, is regularly participating in one IRIS observing session per month, and plans for the participation of the Kashima Observatory, Japan, and the Shanghai Observatory, People's Republic of China, are being formulated. Other stations which have provided valuable observing time for these measurements include the Effelsberg observatory in Bonn, FRG, the Chilbolton antenna in England, the Haystack observatory in Massachusetts, the Maryland Point antenna in Maryland, the National Radio Astronomy Observatory (NRAO) 140-foot (42.7-m) antenna in West Virginia, and the This paper is not subject to U.S.
Nine years of very long baseline interferometry (VLBI) observations have been analyzed to determine the magnitudes of the tidal variations in UT1 for periods between 5 and 35 days. Corrections for variations in atmospheric angular... more
Nine years of very long baseline interferometry (VLBI) observations have been analyzed to determine the magnitudes of the tidal variations in UT1 for periods between 5 and 35 days. Corrections for variations in atmospheric angular momentum (AAM) significantly reduce the scatter of the measured amplitudes across both time and frequency. The AAM corrections are found to reduce the scatter in the observed tidal amplitudes by as much as 60% for periods as short as 5.6 days; in contrast, earlier studies have shown a loss of coherence between AAM and length-of-day (LOD) for periods shorter than about 10 days.
The VLBI (Very-Long-Baseline-Interferometry) technique has enormous potential applications in radio astronomy and geodesy. A method to improve the VLBI signal processing is described. The method was based on an adaptive phase noise model... more
The VLBI (Very-Long-Baseline-Interferometry) technique has enormous potential applications in radio astronomy and geodesy. A method to improve the VLBI signal processing is described. The method was based on an adaptive phase noise model and the use of recursive Kalman filter in the time domain. Removing the phase noise from the fringe phase data partially eliminates the bias due to the phase noise. A computer simulation of a recursive prefiltering method was made. In the simulation the signals, assumed to be first order Markov processes were filtered by the use of recursive Kalman filters, then correlated. Through this process the signal to noise ratio of the interferometer was improved significantly.
The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the... more
The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300 MHz to a few GHz.
High accuracy differenced phase delay can be obtained by observing multiple point frequencies of two spacecraft using the same beam Very Long Baseline Interferometry (VLBI) technology. Its contribution in lunar spacecraft precision orbit... more
High accuracy differenced phase delay can be obtained by observing multiple point frequencies of two spacecraft using the same beam Very Long Baseline Interferometry (VLBI) technology. Its contribution in lunar spacecraft precision orbit determination has been performed during the Japanese lunar exploration mission SELENE. In consideration that there will be an orbiter and a return capsule flying around the moon during the Chinese lunar exploration future mission Chang'E-3, the contributions of the same beam VLBI in spacecraft precision orbit determination and lunar gravity field solution have been investigated. Our results show that the accuracy of precision orbit determination can be improved more than one order of magnitude after including the same beam VLBI measurements. There are significant improvements in accuracy of low and medium degree coefficients of lunar gravity field model obtained from combination of two way range and Doppler and the same beam VLBI measurements than the one that only uses two way range and Doppler data, and the accuracy of precision orbit determination can reach meter level.
We have studied the archetypal Gigahertz Peaked Spectrum radio galaxy, PKS 1934−638, using the Australian Long Baseline Array, augmented with two new telescopes that greatly improve the angular resolution of the array. These VLBI... more
We have studied the archetypal Gigahertz Peaked Spectrum radio galaxy, PKS 1934−638, using the Australian Long Baseline Array, augmented with two new telescopes that greatly improve the angular resolution of the array. These VLBI observations represent the first scientific results from a new antenna in NZ and the first antenna of the Australian SKA Pathfinder (ASKAP). A compact double radio source, PKS 1934−638, has been monitored over a period of 40 years, and the observation described here provides the latest datum, eight years after the previous observation, to aid in the study of the long-term evolution of the source structure. We take advantage of these new long baselines to probe PKS 1934−638 at the relatively low frequency of 1.4 GHz, in order to examine the effects of optical depth on the structure of the radio source. Optical depth effects, resulting in the observation of frequency dependent structure, may have previously been interpreted in terms of an expansion of the source as a function of time. Expansion and frequency dependent effects are important to disentangle in order to estimate the age of PKS 1934−638. We show that frequency dependent structure effects are likely to be important in PKS 1934−638 and present a simple two-dimensional synchrotron source model in which opacity effects due
Compact and strong radio sources were repeatedly observed in regular geodetic very long baseline interferometry experiments under the Key Stone Project. The flux densities of these sources in the S and X bands were estimated from a set of... more
Compact and strong radio sources were repeatedly observed in regular geodetic very long baseline interferometry experiments under the Key Stone Project. The flux densities of these sources in the S and X bands were estimated from a set of correlated amplitudes obtained through the correlation processing of the observed data. From -•5 years of the observed data, irregular variations in the flux densities were detected for several radio sources using the source 2134+004 as the calibrator. The results showed the monitoring of flux density variations by means of very long baseline interferometry is feasible. Since the correlation processing of the observed data are performed real-time using a high-speed digital communication technique, this method gives a capability to monitor flux density variations of quasars and BL-Lac types of extragalactic radio sources.
The complacency with which Man inhabits continent-ocean margins or plate boundaries is based on the assumption that sea level rise and periodic seismicity are acceptable risks. The forecast of future sea level or future seismicity is... more
The complacency with which Man inhabits continent-ocean margins or plate boundaries is based on the assumption that sea level rise and periodic seismicity are acceptable risks. The forecast of future sea level or future seismicity is essential for informed assessment of these risks. The historical data from which forecasts may be derived are in each case of limited antiquity and uneven spatial distribu-tion. Developments in space geodesy and crustal deformation metrology in the past two decades offer the means to acquire a homogeneous global data set for monitoring relative vertical and horizontal motions of the Earth's surface to within several millimeters. These new tools are reviewed in terms of their application to forecasting trends in sea level rise and forecasting damaging earthquakes.
We present the results of the pilot observations of the Deep Extragalactic VLBI-Optical Survey (DEVOS). Our ultimate aim is to collect information on compact structures in a large sample of extragalactic radio sources (∼ 10 4 objects) up... more
We present the results of the pilot observations of the Deep Extragalactic VLBI-Optical Survey (DEVOS). Our ultimate aim is to collect information on compact structures in a large sample of extragalactic radio sources (∼ 10 4 objects) up to two orders of magnitude fainter than those studied in typical imaging Very Long Baseline Interferometry (VLBI) surveys up until now. This would lead to an unprecedented data base for various astrophysical, astrometric and cosmological studies. The first global VLBI observations of the DEVOS programme were successfully conducted in May 2002. We selected sources without any spectral criterion from the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-centimeters (FIRST) catalogue, that are also detected with the Multi-Element Radio Linked Interferometer Network (MERLIN). The DEVOS pilot sample sources are in the area of the sky that is covered by the Sloan Digital Sky Survey (SDSS). We describe the sample selection and present high resolution 5-GHz radio images of the sources. Based on the results of this pilot study, we estimate the outcome of and the resources needed for a full-scale DEVOS project.