U.S. Naval Observatory Astrometry Department

IVS Special Analysis Center
for Source Structure
Operation

The Analysis Center for Source Structure is supported and operated by the United States Naval Observatory (USNO). The main mission of the Analysis Center is to analyze VLBI data to extract intrinsic source structure information for use in maintaining the International Celestial Reference Frame (ICRF). Source structure information is provided in the form of synthesis images and source models suitable for evaluating sources for astrometric and/or geodetic use and for long-term monitoring of sources. Research into the effects of source structure on astrometric position determination is also carried out with emphasis on improving the long-term stability of the ICRF. The web server for the Analysis Center is provided by the USNO.

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Products

Our primary product is the Radio Reference Frame Image Database which holds information on the radio frequency intrinsic structure of most ICRF sources with declination greater than -30 degrees.

Both the raw data and processed images (with their associated source models) can be obtained from this site.

Staff

Dr. Alan L. Fey, Dr. David A. Boboltz,
Dr. Roopesh Ojha, Dr. Ralph A. Gaume,
& Dr. Kerry A. Kingham
<Home | Current Activities | Publications | Future Activities>
VLBA High Frequency Imaging

Very Long Baseline Array observations to extend the ICRF to 24 and 43 GHz began in May 2002. One of the goals of these observations is to study source structure variation at 24 and 43 GHz in order to improve the astrometric accuracy at these frequencies. These observation are part of a joint program between NASA, the U.S. Naval Observatory, NRAO and Bordeaux Observatory. The following experiments have been processed and imaged.

VLBA RDV Imaging

Very Long Baseline Array (VLBA) observations for maintenance of the celestial and terrestrial reference frames have been carried out since 1994. Since 1997 these VLBA RDV observations have been part of a joint program between the U.S. Naval Observatory, Goddard Space Flight Center (GSFC) and the National Radio Astronomy Observatory (NRAO). Since the inception of the USNO Analysis Center, the following VLBA RDV experiments have been processed and imaged.

* These results were contributed by Whitier College who calibrated, edited and imaged the data.
ICRF Maintenance in the Southern Hemisphere

The USNO and the Australia Telescope National Facility (ATNF) are collaborating in a continuing VLBI research program in Southern Hemisphere source imaging and astrometry using USNO, ATNF and ATNF-accessible facilities. These observations are aimed specifically toward improvement of the ICRF in the Southern Hemisphere. Plans include strengthening the ICRF in the Southern Hemisphere by a) increasing the reference source density with additional 2.3/8.4 GHz bandwidth-synthesis astrometric VLBI observations, and b) VLBI imaging at 8.4 GHz of ICRF sources south of -20 degrees declination. VLBI images for a total of 111 Southern Hemisphere ICRF sources have been made at a frequency of 8.4 GHz using the Australian Long Baseline Array (Ojha, et al. 2005, AJ, 130, 2529; Ojha, et al. 2004, AJ, 127, 3609).

These data were used to quantify the magnitude of the expected effect of intrinsic source structure on astrometric bandwidth synthesis VLBI observations by calculating a ``structure index'' for the sources; the structure index yields an estimate of their astrometric quality. Approximately 35% of sources in the sample have a structure index indicative of compact or very compact structures. The remaining two-thirds of the sources are less compact and should probably be avoided in astrometric and geodetic VLBI experiments requiring the highest accuracy unless intrinsic source structure can be accounted for in the astrometric/geodetic analysis.

Last modified: Tue Dec 29 09:51:58 EST 2009
Send comments to afey@usno.navy.mil.