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ABSTRACT: The source position time-series for many of the frequently observed radio sources in the NASA geodetic very long baseline
interferometry (VLBI) program show systematic linear and non-linear variations of as much as 0.5 mas (milli-arc-seconds) to
1.0 mas, due mainly to source structure changes. In standard terrestrial reference frame (TRF) geodetic solutions, it is a
common practice to only estimate a global source position for each source over the entire history of VLBI observing sessions.
If apparent source position variations are not modeled, they produce corresponding systematic variations in estimated Earth
orientation parameters (EOPs) at the level of 0.02–0.04 mas in nutation and 0.01–0.02 mas in polar motion. We examine the
stability of position time-series of the 107 radio sources in the current NASA geodetic source catalog since these sources
have relatively dense observing histories from which it is possible to detect systematic variations. We consider different
strategies for handling source instabilities where we (1) estimate the positions of unstable sources for each session they
are observed, or (2) estimate spline parameters or rate parameters for sources chosen to fit the specific variation seen in
the position-time series. We found that some strategies improve VLBI EOP accuracy by reducing the biases and weighted root
mean square differences between measurements from independent VLBI networks operating simultaneously. We discuss the problem
of identifying frequently observed unstable sources and how to identify new sources to replace these unstable sources in the
NASA VLBI geodetic source catalog.
Journal of Geodesy 05/2007; 81(6):443-453. · 2.41 Impact Factor
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ABSTRACT: A catalog containing milliarcsecond--accurate positions of 1332 extragalactic radio sources distributed over the northern sky is presented - the Very Long Baseline Array Calibrator Survey (VCS1). The positions have been derived from astrometric analysis of dual-frequency 2.3 and 8.4 GHz VLBA snapshot observations; in a majority of cases, images of the sources are also available. These radio sources are suitable for use in geodetic and astrometric experiments, and as phase-reference calibrators in high-sensitivity astronomical imaging. The VCS1 is the largest high-resolution radio survey ever undertaken, and triples the number of sources available to the radio astronomy community for VLBI applications. In addition to the astrometric role, this survey can be used in active galactic nuclei, Galactic, gravitational lens and cosmological studies. The VCS1 catalog will soon be available at www.nrao.edu/vlba/VCS1 . Comment: 25 pages, 5 figures. Accepted to ApJS
01/2002;
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ABSTRACT: A catalog containing milliarcsecond-accurate positions of 1332 extragalactic radio sources distributed over the northern sky is presented-the Very Long Baseline Array Calibrator Survey (VCS1). The positions have been derived from astrometric analysis of dual-frequency 2.3 and 8.4 GHz VLBA snapshot observations; in a majority of cases, images of the sources are also available. These radio sources are suitable for use in geodetic and astrometric experiments, and as phase-reference calibrators in high-sensitivity astronomical imaging. The VCS1 is the largest high-resolution radio survey ever undertaken and triples the number of sources available to the radio astronomy community for VLBI applications. In addition to the astrometric role, this survey can be used in active galactic nuclei, Galactic, gravitational lens, and cosmological studies.
Astrophysical Journal Supplement Series - ASTROPHYS J SUPPL SER. 01/2002; 141(1):13-21.
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01/1998;
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ABSTRACT: Gradients in the atmospheric refractive index can lead to errors in estimated vertical and horizontal station coordinates. These errors produce systematic errors in the terrestrial and celestial reference frames determined from our very long baseline interferometry (VLBI) measurements. Estimation of gradients for our global VLBI data set changes the terrestrial reference frame length scale by −0.7 ppb and produces station position adjustments that vary approximately monotonically with latitude. Estimating gradients reduces the radio source declinations by an amount that peaks at about 0.5 mas near the equator and decreases toward the poles. VLBI gradient estimates are consistent with gradients derived from a global three-dimensional model of assimilated meteorological data. Both indicate that mean atmospheric delay gradients point toward the equator in both the northern and southern hemispheres. The correlation coefficient between VLBI and meteorological model gradients for VLBI sessions for the VLBI antenna at Westford, Massachusetts was 0.56.
Geophysical Research Letters - GEOPHYS RES LETT. 01/1997; 24(4):453-456.
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ABSTRACT: We determine the improvement in baseline length precision and accuracy using new atmospheric delay mapping functions and MTT by analyzing the NASA Crustal Dynamics Project research and development (R&D) experiments and the International Radio Interferometric Surveying (IRIS) A experiments. These mapping functions reduce baseline length scatter by about 20% below that using the CfA2.2 dry and Chao wet mapping functions. With the newer mapping functions, average station vertical scatter inferred from observed length precision (given by length repeatabilites) is 11.4 mm for the 1987-1990 monthly R&D series of experiments and 5.6 mm for the 3-week-long extended research and development experiment (ERDE) series. The inferred monthly R&D station vertical scatter is reduced by 2 mm or by 7 mm is a root-sum-square (rss) sense. Length repeatabilities are optimum when observations below a 7-8 deg elevation cutoff are removed from the geodetic solution. Analyses of IRIS-A data from 1984 through 1991 and the monthly R&D experiments both yielded a nonatmospheric unmodeled station vertical error or about 8 mm. In addition, analysis of the IRIS-A exeriments revealed systematic effects in the evolution of some baseline length measurements. The length rate of change has an apparent acceleration, and the length evolution has a quasi-annual signature. We show that the origin of these effects is unlikely to be related to atmospheric modeling errors. Rates of change of the transatlantic Westford-Wettzell and Richmond-Wettzell baseline lengths calculated from 1988 through 1991 agree with the NUVEL-1 plate motion model (Argus and Gordon, 1991) to within 1 mm/yr. Short-term (less than 90 days) variations of IRIS-A baseline length measurements contribute more than 90% of the observed scatter about a best fit line, and this short-term scatter has large variations on an annual time scale.
02/1994;
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ABSTRACT: Despite improvement of space geodetic techniques, the troposphere continues to be a source of random and systematic error. We have investigated the possibility of correcting tropospheric delay models in space geodetic analysis with data from a meteorological data assimilation model. Four radiosonde sites in different climatic regions were selected for study. The precision and accuracy at a 5° elevation angle of commonly used tropospheric mapping functions and of data assimilation model delays were evaluated by comparisons with raytracings of radiosonde profiles. We determined the model delays by raytracing site profiles calculated by interpolation of the gridded (2°×2.5°) model profiles. Monthly variations in the atmospheric profile at a typical continental site result in hydrostatic mapping function delay variations at a 5° observation elevation of 50–100 mm, which results in about 12–25 mm of variation in the estimated site vertical coordinate. Rms differences between radiosonde delays and the common mapping functions delays are 9–35 mm. Rms differences between radiosonde delays and data assimilation model delays are 5–6 mm. Along with mapping function errors, unmodeled atmospheric delay gradients produce errors in geodetic analysis. The gradient delay at an elevation of 5° has an rms variation of about 45 mm over a year. For the 1990–1993 VLBI sessions using the antenna at Westford, Massachusetts, we found that the gradient vector computed from the data assimilation model is correlated with corresponding VLBI gradient vector estimates with correlation coefficients of 0.60 and 0.49 for the north and east components respectively. When the model gradient includes only the hydrostatic part of the refractivity, this correlation is reduced to 0.53 and 0.48.
Physics and Chemistry of The Earth.
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ABSTRACT: This paper presents an extension of the Very Long Baseline Array Calibrator Survey, called VCS2, containing 276 sources. This survey fills in regions of the sky that were not completely covered by the previous VCS1 calibrator survey. The VCS2 survey includes calibrator sources near the Galactic plane, À30 < < À45 , and VLA calibrators. The positions have been derived from astrometric analysis of the group delays measured at 2.3 and 8.4 GHz using the Goddard Space Flight Center CALC/SOLVE package. From the VLBA snapshot observations, images of the calibrators are available, and each source is given a quality code for anticipated use. The VCS2 catalog is available from the NRAO Web site.
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ABSTRACT: Intensive work in recent years has demonstrated the value of high precision and high temporal resolution in collecting Earth rotation data, paving the way for continuous measurement that will open exciting new research areas in geophysics as the new millennium unfolds. Very long baseline interferometry (VLBI) will continue to be a principal technique. However, it will have greatly improved precision and temporal resolution, making possible more in-depth studies of geophysical phenomena affecting Earth rotation. Originally developed as a radio astronomical technique for high-resolution mapping of distant radio sources, VLBI, when ``turned around,'' has become a valuable tool in geodesy. Because of its past success, a new VLBI project, Continuous Observation of the Rotation of the Earth (CORE), was launched last year. It is being implemented in phases by the NASA Goddard Space Flight Center in collaboration with the U.S. Naval Observatory (USNO) [see Ma et al., 1997].
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