Astrometric results of 1978-1985 Deep Space Network radio interferometry - The JPL 1987-1 extragalactic source catalog

07/1988; DOI: 10.1086/114761
Source: NTRS

ABSTRACT An astrometric radio reference frame has been determined from intercontinental dual-frequency radio interferometric measurements. These measurements were carried out on a regular basis during 1978-1985 between NASA's Deep Space Network stations in California, Spain, and Australia. Analysis of 6800 pairs of delay and delay-rate observations made during 51 sessions produced estimates of 1300 parameters. The most significant of these are geophysical quantities and positions of extragalactic sources. The source catalog resulting from this analysis includes 106 sources fairly uniformly distributed over the celestial sphere, north of -45 deg declination. Almost all of the resulting source positions have formal uncertainties between 0.5 and 3 milliarcseconds (mas), with rms values of 2 mas in both angular coordinates. Internal consistency checks, as well as comparisons with independently determined source catalogs of comparable quality, indicate that relative source coordinates determined by VLBI contain systematic errors at the level of 1 to 2 mas.

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    ABSTRACT: To compare the validity of current algorithms that map zenith tropospheric delay to arbitrary elevation angles, 10 difierent tropospheric mapping functions are used to analyze the current data base of Deep Space Network Mark III intercontinental very long baseline interferometric (VLBI) data. This analysis serves as a stringent test because of the high proportion of low-elevation observations necessitated by the extremely long baselines. Postflt delay and delay-rate residuals are examined, as well as the scatter of baseline lengths about the time{linear model that character- izes tectonic motion. Among the functions that utilize surface meteorological data as input parameters, the Lanyi 1984 mapping shows the best performance both for residuals and baselines, though the 1985 Davis function is statistically nearly iden- tical. The next best performance is shown by the recent function of Niell, which is based on an examination of global atmospheric characteristics as a function of sea- son and uses no weather data at the time of the measurements. The Niell function shows a slight improvement in residuals relative to Lanyi, but also an increase in baseline scatter that is signiflcant for the California{Spain baseline. Two variants of the Chao mapping function, as well as the Chao tables used with the inter- polation algorithm employed in the Orbit Determination Program software, show substandard behavior for both VLBI residuals and baseline scatter. The length of the California{Australia baseline (10,600 km) in the VLBI solution can vary by as much as 5 to 10 cm for the 10 mapping functions.
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    ABSTRACT: ABSTRACTA new method for combined calculations of proper motions (PMs) based on accurate measurements of POSS1 and POSS2 epoch plates is given. The positional accuracy of various surveys and catalogues is estimated, and statistical weights for each of them are established. To achieve the best positions, weighted averaging of direct measurements on DSS1/DSS2, and data from APM, MAPS, USNO-A2.0, USNO-B1.0 and GSC 2.3.2 catalogues were used. The rms accuracy of positions achieved for POSS1 is 119 mas in each coordinate and 168 mas total, and for POSS2 it is 69 mas in each coordinate and 98 mas total. Using these accurate positions and the large separation in years between POSS1 and POSS2, we calculated the best possible PMs: 3.9 mas yr−1 in each coordinate and 5.5 mas yr−1 total. We developed methods to control and exclude the accidental errors that appear in any survey. We compared and combined our PMs with those given in USNO-B1.0, SDSS DR7 and Tycho-2 catalogues and obtained even better results: 3.0 mas yr−1 in each coordinate and 4.2 mas yr−1 total PM. This approach was applied to the First Byurakan Survey blue stellar objects, containing significant numbers of white dwarfs and subdwarfs. In total, 640 objects were revealed with PM ≥10 mas yr −1, the detection limit for this method, and an electronic table of these objects is given. For more confident PMs, we adopt the limit 20 mas yr−1 (333 objects). Adopting a 50 km s−1 upper limit for tangential velocities, we calculated maximum distances and absolute magnitudes and estimated luminosity types for these objects, obtaining 185 probable white dwarfs (M > 8 mag), 69 possible white dwarfs (6 < M < 8 mag), and 42 candidate subdwarfs/white dwarfs (3 < M < 6 mag). Given that we rediscovered 141 genuine white dwarfs among the classified objects, the success rate for the discovery of white dwarfs is estimated as 83 per cent, and as 38 per cent for our probable and possible white dwarfs, respectively.
    Monthly Notices of the Royal Astronomical Society 07/2010; 407(1):681 - 690. · 5.52 Impact Factor
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    ABSTRACT: Celestial reference frames determined from measurements of extragalactic radio sources are used in interplanetary navigation, Earth orientation measurements, geodesy, and astrometry. The JPL 1997-3 celestial reference frame is derived from very long baseline interferometric (VLBI) measurements of 291 compact extragalactic radio sources. The observations were done principally at 8.4 GHz, supplemented by simultaneous 2.3-GHz observations needed to calibrate the charged-particle effects of the solar plasma and the Earth's ionosphere. The radio source positions that constitute this frame have median formal precisions of 123 and 188 microarcseconds (μas) in α cos δ and δ, respectively. Within the presently available 17.5-year span of observations, these sources are characterized by coordinate drifts that have median uncertainties of approximately 70 μas/yr. Only a few of these rates are statistically significant, and they probably are caused by variations in the internal structure of the sources. In agreement with the general relativistic theory of gravity, the parameterized post-Newtonian γPPN factor is determined to be γPPN = 1.001 ± 0.001. On the basis of internal consistency tests and comparisons to independent celestial frame determinations, we estimate that the formal uncertainties must be increased by a factor of 2 in order to more realistically describe the accuracy of the source positions. The dominant error comes from inaccurate modeling of the troposphere and, to lesser extents, from the lack of radio source structure models and imperfectly calibrated instrumentation. We briefly describe models of the observation covariances caused by these classes of errors and assess the size of remaining unmodeled errors. The absence of an all Southern-Hemisphere baseline makes the positions of southern sources especially sensitive to tropospheric mismodeling. As a result, zonal errors in the south may approach 1 milliarcsecond. The JPL frame provides an important independent verification of the International Astronomical Union's (IAU's) new International Celestial Reference Frame at the level of a few hundred μas. Finally, we review the work done to link the VLBI extragalactic radio frame to the planetary ephemeris frame and the Hipparcos optical frame, thereby creating a unified system that is much more valuable than the sum of the separate constituent frames.
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