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G. E. Lanyi,
D. A. Boboltz,
P. Charlot,
A. L. Fey,
E. B. Fomalont,
B. J. Geldzahler,
D. Gordon,
C. S. Jacobs,
C. Ma,
C. J. Naudet,
J. D. Romney, O. J. Sovers,
and L. D. Zhang
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ABSTRACT: We present astrometric results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24 hr observing sessions made over a five-year period. These observations were motivated by the need to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies to enable improved deep space navigation after 2016 and to improve state-of-the-art astrometry. Source coordinates for 268 sources were estimated at 24 GHz and for 131 sources at 43 GHz. The median formal uncertainties of right ascension and declination at 24 GHz are 0.08 and 0.15 mas, respectively. Median formal uncertainties at 43 GHz are 0.20 and 0.35 mas, respectively. Weighted root-mean-square differences between the 24 and 43 GHz positions and astrometric positions based on simultaneous 2.3 and 8.4 GHz Very Long Baseline Interferometry observations, such as the ICRF, are less than about 0.3 mas in both coordinates. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted systematic errors limit the overall accuracy of the catalogs.
The Astronomical Journal 03/2010; 139(5):1695. · 4.03 Impact Factor
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P. Charlot,
D. A. Boboltz,
A. L. Fey,
E. B. Fomalont,
B. J. Geldzahler,
D. Gordon,
C. S. Jacobs,
G. E. Lanyi,
C. Ma,
C. J. Naudet,
J. D. Romney, O. J. Sovers,
and L. D. Zhang
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ABSTRACT: We have measured the submilliarcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high-frequency celestial reference frame (CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of ~5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.
The Astronomical Journal 03/2010; 139(5):1713. · 4.03 Impact Factor
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ABSTRACT: A program of survey observations using the Very Long Baseline Array
(VLBA) at K-band (24 GHz) and Q-band (43 GHz) was begun in 2002 as a
first step toward the goal of extending the ICRF to frequencies above
8.4 GHz. Each session covers the full range of right ascension and
declinations down to ≈ -30 deg. Analysis of the first three sessions
has produced a full sky catalog of 108 sources with median K-band formal
position uncertainties of 210 and 280 μas in α cos(δ) and
δ, respectively. For Q-band the corresponding results are 292 and
404 μas. Group delay residuals were excellent at ≈ 20 ps
weighted RMS. An external comparison of the K-band frame to the
S/X-band ICRF shows systematic errors at about 2 times larger than the
formal precision. These systematic differences include a zonal trend vs.
declination. We suspect that these errors are due in part to the
North-South geometric weakness of the array.
11/2005; 340:523.
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ABSTRACT: The current realization of the International Celestial Reference Frame (ICRF) comprises a total of 717 extragalactic radio sources distributed over the entire sky. An observing program has been developed to densify the ICRF in the northern sky using the European VLBI network (EVN) and other radio telescopes in Spitsbergen, Canada and USA. Altogether, 150 new sources selected from the Jodrell Bank-VLA Astrometric Survey were observed during three such EVN+ experiments conducted in 2000, 2002 and 2003. The sources were selected on the basis of their sky location in order to fill the "empty" regions of the frame. A secondary criterion was based on source compactness to limit structural effects in the astrometric measurements. All 150 new sources have been successfully detected and the precision of the estimated coordinates in right ascension and declination is better than 1 milliarcsecond (mas) for most of them. A comparison with the astrometric positions from the Very Long baseline Array Calibrator Survey for 129 common sources indicates agreement within 2 mas for 80% of the sources.
01/2005;
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ABSTRACT: Astrometric observations of distant active galactic nuclei (AGN) have
been used to construct quasi-inertial global reference frames, most
notably the International Celestial Reference Frame (ICRF) which now
forms the basis for all astrometry including deep space navigation. The
ICRF frame was defined using X- (8.4 GHz) and S-band (2.3 GHz)
observations over the past 20+ years. There are several motivations for
extending this work to higher radio frequencies, namely, to construct a
more stable frame based on more compact sources, to provide calibrators
for phase referencing, and to support spacecraft navigation at higher
frequencies. As a first step toward these goals, in 2002 we began a
series of survey observations using the Very Long Baseline Array (VLBA)
of ten radio telescopes at K-band (24 GHz) and Q-band (43 GHz). Each
session covers the full 24 hours of right ascension and covers
declinations down to the VLBA's southern limit (approx. -30 deg).
Preliminary analysis of the first session produced a full sky catalog of
65 sources with formal position uncertainties of about 0.5 mas. Group
delay residuals were an excellent 15-20 psec WRMS. We will present
evidence from an external comparison to the S/X-band ICRF that shows
zonal errors at several times the level of the formal precision. We
expect these errors to be reduced as futher sessions are added to the
analysis thereby strengthening the observation geometry. --- The
research described in this paper was in part performed at the Jet
Propulsion Laboratory, California Institute of Technology, under a
contract with the National Aeronautics and Space Administration, Goddard
Space Flight Center, U.S. Naval Observatory, National Radio Astronomical
Observatory, and Bordeaux Observatory.
11/2002; 34:1227.
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[show abstract]
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ABSTRACT: The goal of Part II was to create the definitive catalogue for the ICRF
(International Celestial Reference Frame) using the best data and
methods available at the time the work was done.
IERS Technical Note. 05/1997; 23:1.
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ABSTRACT: 24 years of Lunar Laser Ranging (LLR) observations and 16 years of Very Long Baseline Interferometry (VLBI) observations are combined in a global analysis to yield improved estimates of the Earth's precession and nutation. The correction to the International Astronomical Union (IAU) (1976) precession constant inferred from this joint VLBI/LLR analysis is -3.00 +/- 0.20 milliarcsec/yr (mas/yr). A significant obliquity rate correction of -0.20 +/- 0.08 mas/yr is also found. In all, 32 forced nutation coefficients are estimated. These coefficients confirm that the IAU (1980) nutation theory is in error by several mas. The estimated nutation coeficients are found to vary by as much as several tenths of mas, depending on the a priori nutation model used to analyze the VLBI and LLR data. Forced circular nutations derived from this analysis agree with the ZMOA-1990-2 nutation theory at the 0.2 mas level for the 18.6 yr terms, and at the 0.05 mas level for the other terms (periods less than or = 1 yr). A retrograde free core nutation with an amplitude of 0.20 mas is also detected. Its phase is found to be very sensitive to the precise value of the free core nutation period used in the solution. Separate analyses of four independent subsets of the LVBI data indicate no significant variations of the free core nutation since 1988. The pre-1988 estimates of the free core nutation are consistent with the post-1988 estimates but are not accurate enough to rule out possible variations of the free core nutation at these earlier epochs.
02/1995;
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ABSTRACT: This report is a revision of the document Observation Model and Parameter Partials for the JPL VLBI Parameter Estimation Software 'MODEST'---1991, dated August 1, 1991. It supersedes that document and its four previous versions (1983, 1985, 1986, and 1987). A number of aspects of the very long baseline interferometry (VLBI) model were improved from 1991 to 1994. Treatment of tidal effects is extended to model the effects of ocean tides on universal time and polar motion (UTPM), including a default model for nearly diurnal and semidiurnal ocean tidal UTPM variations, and partial derivatives for all (solid and ocean) tidal UTPM amplitudes. The time-honored 'K(sub 1) correction' for solid earth tides has been extended to include analogous frequency-dependent response of five tidal components. Partials of ocean loading amplitudes are now supplied. The Zhu-Mathews-Oceans-Anisotropy (ZMOA) 1990-2 and Kinoshita-Souchay models of nutation are now two of the modeling choices to replace the increasingly inadequate 1980 International Astronomical Union (IAU) nutation series. A rudimentary model of antenna thermal expansion is provided. Two more troposphere mapping functions have been added to the repertoire. Finally, corrections among VLBI observations via the model of Treuhaft and lanyi improve modeling of the dynamic troposphere. A number of minor misprints in Rev. 4 have been corrected.
09/1994;
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ABSTRACT: Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources provide the basis for defining an accurate non-rotating reference frame in terms of angular positions of the sources. Measurements of the distance from the Earth to the Moon and to the inner planets provide the basis for defining an inertial planetary ephemeris reference frame. The relative orientation, or frame tie, between these two reference frames is of interest for combining Earth orientation measurements, for comparing Earth orientation results with theories referred to the mean equator and equinox, and for determining the positions of the planets with respect to the extragalactic reference frame. This work presents an indirect determination of the extragalactic-planetary frame tie from a combined reduction of VLBI and Lunar Laser Ranging (LLR) observations. For this determination, data acquired by LLR tracking stations since 1969 have been analyzed and combined with 14 years of VLBI data acquired by NASA's Deep Space Network since 1978. The frame tie derived from this joint analysis, with an accuracy of 0.003 sec, is the most accurate determination obtained so far. This result, combined with a determination of the mean ecliptic (defined in the rotating sense), shows that the mean equinox of epoch J2000 is offset from the x-axis of the extragalactic frame adopted by the International Earth Rotation Service for astrometric and geodetic applications by 0.078 sec +/- 0.010 sec along the y-direction and y 0.019 sec +/- 0.001 sec. along the z-direction.
08/1994;
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ABSTRACT: Ocean tidal effects on universal time and polar motion (UTPM) are investigated at four nearly diurnal (K(sub 1), P(sub 1), O(sub 1), and Q(sub 1)) and four nearly semidiurnal (K(sub 2), S(sub 2), M(sub 2), and N(sub 2)) frequencies by analyzing very long baseline interferometry (VLBI) data extending from 1978 to 1992. We discuss limitations of comparisons between experiment and theory for the retograde nearly diurnal polar motion components due to their degeneracy with prograde components of the nutation model. Estimating amplitudes of contributions to the modeled VLBI observables at these eight frequencies produces a statistically highly significant improvement of 7 mm to the residuals of a fit to the observed delays. Use of such an improved UTPM model also reduces the 14-30 mm scatter of baseline lengths about a time-linear model of tectonic motion by 3-14 mm, also withhigh significance levels. A total of 28 UTPM ocean tidal amplitudes can be unambiguously estimated from the data, with resulting UTI and PM magnitudes as large as 21 micro secs and 270 microarc seconds and formal uncertainties of the order of 0.3 micro secs and 5 microarc secs for UTI and PM, respectively. Empirically determined UTPM amplitudes and phases are com1pared to values calculated theoretically by Gross from Seiler's global ocean tide model. The discrepancy between theory and experiment is larger by a factor of 3 for UTI amplitudes (9 micro secs) than for prograde PM amplitudes (42 microarc secs). The 14-year VLBI data span strongly attenuates the influence of mismodeled effects on estimated UTPM amplitudes and phases that are not coherent with the eight frequencies of interest. Magnitudes of coherent and quasi-coherent systematic errors are quantified by means of internal consistency tests. We conclude that coherent systematic effects are many times larger than the formal uncertainties and can be as large as 4 micro secs for UTI and 60 microarc secs for polar motion. On the basis of such ealistic error estimates, 22 of the 31 fitted UTPM ocean tidal amplitudes differ from zero by more than 2 sigma.
12/1993;
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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.
07/1988;
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ABSTRACT: Intercontinental radio interferometric measurements between NASA's Deep Space Network stations yield a catalog of positions of 106 extragalactic radio sources, uniformly distributed between -45 and +85 deg declination. Almost all of the source positions have formal uncertainties between 0.5 and 3 milliarcseconds. Estimates of three independent sets of 32 source coordinates at average epochs 1980.0, 1983.2, and 1984.5 assess the level of stability of the reference frame over a five-year period. Comparisons of the three sets of source coordinates show a number of cases of greater than 2 sigma differences, particularly between the 1984-85 and 1981-83 periods. No sources, however, show 2 sigma shifts among both pairs of epochs. Upper limits of the order of 1 mas/yr can be placed on the time rates of change of the 32 source coordinates.
02/1988;
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ABSTRACT: This report is a revision of the document of the same title (1986), dated August 1, which it supersedes. Model changes during 1986 and 1987 included corrections for antenna feed rotation, refraction in modelling antenna axis offsets, and an option to employ improved values of the semiannual and annual nutation amplitudes. Partial derivatives of the observables with respect to an additional parameter (surface temperature) are now available. New versions of two figures representing the geometric delay are incorporated. The expressions for the partial derivatives with respect to the nutation parameters have been corrected to include contributions from the dependence of UTI on nutation. The authors hope to publish revisions of this document in the future, as modeling improvements warrant.
01/1988;
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ABSTRACT: Very Long Baseline Interferometry (VLBI) results are presented for the two baseline sectors between the Goldstone DSN antenna complex and the overseas sites at Canberra, Australia and Madrid, Spain. Results from solutions using data taken between 1978 September and 1983 May show an apparent California-Spain baseline length increase of 21 cm during this time span, while the California-Australia length has remained constant. Statistical investigations of the integrity of the data are discussed along with dominant systematic error sources and their effect on baseline length determination. Results and interpretation of the time behavior of the angle between DSN baselines are also described.
12/1984;
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ABSTRACT: Progress has been made toward the realization of the potential of radio interferometry for measuring crustal motions and global rotations of the earth with accuracies at the centimeter level. In this connection, a series of experiments, primarily with NASA's Deep Space Network (DSN) antennas, has been conducted to develop two generations of very long baseline interferometric (VLBI) systems. A description is presented of the employed techniques, an analysis of the experiments, and the results of geophysical significance. Attention is given to the interferometry technique, the geometric delay model, propagation media calibrations, and the observing strategy.
10/1984;
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ABSTRACT: Approximately 2400 observations of extragalactic radio sources were made between August 1971 and February 1980 during 48 separate sessions. These consisted of 259 delay rate observations at 2.3 GHz (S-band), 796 delay and delay rate observations at either S-band of 8.3 GHz (X-band) and 1325 delay and delay rate observations recorded simultaneously at both S- and X-band. A single multiparameter fit has been applied to the observed values of delay and delay rate to extract astrometric and geophysical parameters from this decade-long sequence. The fit produced estimates of 784 parameters, including station locations, radio source positions, polar motion, Universal Time, the precession constant, and solid earth tides. The a priori model included gravitational bending, the 1980 IAU nutation series, the 1976 IAU expressions for Greenwich mean sidereal time and precession, BIH estimates of Universal Time and polar motion, and monthly mean values for zenith troposphere delay. The rms residuals were 0.52 nsec for delay and 0.30 psec/sec for delay rate. Intercontinental baseline lengths were determined with formal uncertainties of 5 to 10 cm. Universal Time and polar motion were measured at 49 epochs, with formal uncertainties (for the more recent data) of 0.5 msec for UT1 and 6 and 2 mas, respectively, for the X and Y components of polar motion. Previously announced in STAR as N83-28038
08/1984;
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[show abstract]
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ABSTRACT: Approximately 2400 observations of extragalactic radio sources were made between August 1971 and February 1980 during 48 separate sessions. These consisted of 259 delay rate observations at 2.3 GHz (S-band), 796 delay and delay rate observations at either S-band of 8.3 GHz (X-band) and 1325 delay and delay rate observations recorded simultaneously at both S- and X-band. A single multiparameter fit has been applied to the observed values of delay and delay rate to extract astrometric and geophysical parameters from this decade-long sequence. The fit produced estimates of 784 parameters, including station locations, radio source positions, polar motion, Universal Time, the precession constant, and solid earth tides. The a priori model included gravitational bending, the 1980 IAU nutation series, the 1976 IAU expressions for Greenwich mean sidereal time and precession, BIH estimates of Universal Time and polar motion, and monthly mean values for zenith troposphere delay. The rms residuals were 0.52 nsec for delay and 0.30 psec/sec for delay rate. Intercontinental baseline lengths were determined with formal uncertainties of 5 to 10 cm. Universal Time and polar motion were measured at 49 epochs, with formal uncertainties (for the more recent data) of 0.5 msec for UT1 and 6 and 2 mas, respectively, for the X and Y components of polar motion.
06/1983;
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ABSTRACT: In the development of a celestial radio reference frame, there are now over 100 sources whose relative positions are known with an average uncertainty less than 5 milliarcseconds. These sources are fairly uniformly distributed over the celestial sphere north of -40 deg declination. Their positions are expressed in the new IAU system. This presentation describes the analysis involved in obtaining these results, as well as future plans for linking this system to the JPL planetary ephemerides.
02/1983;
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ABSTRACT: A series of experiments was conducted during the last decade to explore
the capability of very long baseline interferometry (VLBI) to measure
the crustal and rotational motions of the Earth with accuracies at the
centimeter level. The observing stations are those of NASA's Deep Space
Network in California, Spain and Australia. A multiparameter fit to the
observed values of delay and delay rate yields radio source positions,
polar motion, universal time, the precession constant, baseline vectors,
and solid Earth tides. Source positions are obtained with formal errors
of the order of 0''.01. UT1-UTC and polar motion are determined at 49
epochs, with formal error estimates for the more recent data of 0.5 msec
for UT1-UTC and 2 to 6 mas for polar motion. Intercontinental baseline
lengths are determined with formal errors of 5 to 10 cm. The Love
numbers and Earth tide phase lag agree with the commonly accepted
values.
10/1982; -1:1-7.
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J. L. Fanselow, O. J. Sovers,
J. B. Thomas,
F. R. Bletzacker,
T. J. Kearns,
E. J. Cohen,
Purcell,
G. H,
D. H. Rogstad,
L. J. Skjerve,
L. E. Young
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ABSTRACT: The Jet Propulsion Laboratory has been developing a radio-astrometric catalogue for use in the application of radio interferometry to interplanetary navigation and geodesy. The catalogue consists of approximately 100 compact extragalactic radio sources whose relative positions have formal uncertainties of the order of 0.01 arcsec. The sources cover nearly all of the celestial sphere above -40 deg declination. By using the optical counterparts of many of these radio sources, this radio reference frame has been tied to the FK4 optical system with a global accuracy of approximately 0.1 arcsec. This paper describes the status of this work.
02/1981;
