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  • ASTRONOMY  (4)
  • INSTRUMENTATION AND PHOTOGRAPHY  (2)
  • 1
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    Submilliarcsecond astrometry via VLBI. I - Relative position of the radio sources 3C 345 and NRAO 512 (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The relative position and relative proper motion of the radio sources 3C 345 and NRAO 512 are estimated from four sets of VLBI observations spread out over the period from October 1971 to May 1974. The use of phase-connection techniques yields the separation, in 1950.0 coordinates, of the centers of brightness of the compact components of the two sources. An upper bound of 0.0005 arcsec/yr is placed on the relative proper motion (70% estimated confidence limits). Bounds that can be placed on the distances to the two sources are considered, prospects for improvement in the determination of relative position and proper motion of these sources are discussed, and other possible applications of the basic technique are described.
    Keywords: ASTRONOMY
    Type: Astronomical Journal; 84; Oct. 197
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  • 2
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    A very-long-baseline interferometer system for geodetic applications (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: A very-long-baseline interferometer system was designed and built for geodetic applications. Each interferometer terminal records a 360-kHz spectral band of noise from a compact extragalactic radio source. The center frequency of the spectral band can be selected to sample sequentially bands covering a much wider frequency range to obtain subnanosecond accuracy in group-delay measurements. A tunnel-diode pulse generator is used to calibrate the delays in the receiver. The necessary sets of algorithms and computer programs have been developed to analyze the data and have allowed the system to be employed to make accurate determinations of vector baselines, radio-source positions, polar motion, and universal time.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: Radio Science; 11; May 1976
    Format: text
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  • 3
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    Radio source positions from very-long-baseline interferometry observations (1976)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Accurate positions of compact radio sources have been determined from very-long-baseline interferometry (VLBI) observations based on the bandwidth-synthesis technique. The coordinates for 18 extragalactic sources were obtained from sets of observations spread over the period from April 1972 to January 1975; the scatter among the independent determinations of the source coordinates from the separate sets of observations is about 0.05 arcsec, except for the declinations of near-equatorial sources, where the scatter is about 0.15 arcsec. Comparison of these positions with those determined with the Cambridge 5-km radio interferometer shows the rms scatter about the mean difference to be about 0.04 arcsec in each coordinate (no sources of low declination were in common). A similar comparison of the present results with those obtained by the Jet Propulsion Laboratory from separate VLBI observations yields a slightly larger rms scatter, after exlusion of the declinations of the near-equatorial sources. A position is also obtained for the galactic object Beta Persei (Algol), which agrees well with the position given in the FK 4 catalogue.
    Keywords: ASTRONOMY
    Type: Astronomical Journal; 81; Aug. 197
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  • 4
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    Synchronization of clocks by very-long-baseline interferometry (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Two hydrogen-maser clocks, one at Haystack Observatory and one at the National Radio Astronomy Observatory, were synchronized by means of observations of several extragalactic radio sources on March 28, and again on September 23, 1977. Observations were made sequentially in eight 360-kHz bands distributed between about 8.4 and 8.5 GHz with spacings designed to enable the group-delay difference between the signals received at the two observatories from a given source to be estimated unambiguously, within an uncertainty of less than 1 ns set by receiver noise. The epoch and the rate differences between the observatories' clocks for each experiment were estimated by analysis of observations that spanned several hours. The application of corrections for the contributions to the delays of the antennas, feeds, receiver systems, and recorders yielded absolute determinations of the clock epoch differences. During each experiment, portable cesium clocks were flown from the U.S. Naval Observatory to the observatories and back. The traveling-clock data, analyzed in each case after the VLBI synchronization had been completed, confirmed the VLBI results to within 18 and 14 ns for the first and second experiments, respectively.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: IEEE Transactions on Instrumentation and Measurement; IM-28; Sept
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  • 5
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    A high declination search at 8 GHz for compact radio sources (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: With the Haystack-NRAO interferometer (baseline length of 20 million wavelengths at 3.8 cm) 37 sources were observed whose declinations were above 50 deg. Seven of these sources have compact cores with diameters smaller than 5 milliarcsec and with correlated flux densities greater than about 0.5 Jy; the remaining sources have no cores with flux densities above about 0.3 Jy, the sensitivity limit of the interferometer. Two of the sources with detected compact cores, 4C 67.05 and 3C 418, were also observed with longer-baseline interferometers; the diameter of the core of 4C 67.05 was estimated to be smaller than 1 milliarcsec and that of 3C 418 to be smaller than 0.4 milliarcsec. All diameter estimates were based on an assumed circular Gaussian distribution of radio brightness and refer to the contour with brightness density e to the -1/2 power times that of the center. Positions for the detected sources were also obtained from the interferometric data, the uncertainty in these coordinate estimates ranging from 0.04 to 0.6 arcsec. The compact core detected in 3C 390.3 was found to lie near the center of this extended (approximately 4 arcmin in diameter) double radio source and to be coincident to within 1 arcsec with an N galaxy previously identified with 3C 390.3.
    Keywords: ASTRONOMY
    Type: Astronomical Journal; 83; June 197
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  • 6
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    An unusually strong radio outburst in Algol - VLBI observations (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: For 8 hours during a strong radio flare on Jan. 15, 16, 1975, the close binary system Beta Persei (Algol) was observed with a three-station VLBI array operating at 7850 MHz. The size of the radio source was estimated to have been about 1.7 milliarcsec (0.05 AU), based on a model of a uniformly bright disk. The corresponding brightness temperature was nearly 10 billion K, indicating that the emission was probably nonthermal. There was no evidence for expansion of the source; the upper limit on the rate of any expansion was 100 km/s. The position of the Algol radio source with respect to an extragalactic reference frame was also determined from these observations with an uncertainty of about 0.1 arcsec in each coordinate.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal; 206; June 1
    Format: text
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