The Definition and Stability of Local Inertial Reference Frames

Inertial reference frames spanning approximately 10°-30° square on the sky and capable of locating objects to few-hundred microarcsecond accuracies are useful for a broad class of astrometric measurements. Deep space tracking and general relativistic angular deflection experiments are examples of astrometric measurements which can profitably reference the positions and/or motions of objects to a field of radio sources in a local frame. A method for defining local inertial reference frames has been developed based on Very Long Baseline Interferometry (VLBI) measurements of extragalactic radio sources. By observing the radio emission from the object to be located in the frame, as well as that from about five radio sources which define the frame, dominant systematic astrometric errors can be minimized through parameter estimation. The entire reference frame measurement is of the order of 30 minutes including all the sources in a frame. The limiting error for single-epoch position determination in a local frame is the unknown structure of both target and reference objects. Structure can cause systematic milliarcsecond-level errors. The limiting error for epoch-to-epoch differential position measurements is tropospheric fluctuations, assuming that the radio source structures do not change from one epoch to the next. Preliminary results of an epoch-to-epoch measurement of relativistic gravitational deflection by Jupiter, in which the total deflection was about 600 microarcseconds, suggest that the local reference frame is stable at the 240-microarcsecond level over twelve days. Data have been taken at longer time intervals to determine the annual stability of the frames. At the time of preparation of these proceedings, those data have not yet been analyzed.

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The Possibility to Link Radio and Optical Reference Frames With Artificial Satellites

Symposium - International Astronomical Union ◽

10.1017/s0074180900086794 ◽

1990 ◽

Vol 141 ◽

pp. 194-194

Author(s):

Tong Fu

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Artificial Satellites ◽

Radio Sources ◽

Bl Lac Objects ◽

Extragalactic Radio Sources ◽

Bl Lac ◽

Optical Reference ◽

Radio Stars ◽

Fundamental Reference

Based on extragalactic radio sources, a new high precision extragalactic radio reference frame can be established from radio interferometric measurements. To link the optical fundamental reference frame presently represented by the FK4/5 to the extragalactic radio frame, the optical counterparts of extragalactic radio sources (quasars, BL Lac objects etc.) and radio stars are the most important classes of objects. Besides these two classes of objects, are there any other objects which can be used to link the optical and radio frames? A posible answer is that artificial satellites could be a candidate class of objects contributing to this subject.

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Some Considerations in the Use of Very-Long-Baseline-Interferometry to Establish Reference Coordinate Systems for Geodynamics

International Astronomical Union Colloquium ◽

10.1017/s0252921100081276 ◽

1980 ◽

Vol 56 ◽

pp. 205-216

Author(s):

Douglas S. Robertson

Keyword(s):

Reference Frames ◽

Geodetic Survey ◽

Limiting Factors ◽

Radio Sources ◽

National Geodetic Survey ◽

Terrestrial Reference Frames ◽

Long Baseline ◽

Lithospheric Plates ◽

Long Time ◽

The Stability

AbstractPresent knowledge of the number, distribution, proper motion and structures of extragalactic radio sources indicates that there should be no problem in defining a celestial reference frame with stabilities of a few milliseconds of arc over time spans of the order of a decade. One of the limiting factors appears to be the structure of the sources. By measuring and monitoring these structures, the stability could probably be improved by as much as one or two orders of magnitude. Even without this improvement, a network of properly distributed fixed observatories making regular interferometric observations of these radio sources could be used to define a terrestrial coordinate system that could be maintained at the few centimeter level over indefinitely long time periods. Such a stable terrestrial reference system would be useful for a host of modern geodetic and geodynamic applications, including, in particular, studies of the time varying deformations and relative motions of lithospheric plates. The National Geodetic Survey has already begun work on a three station base network of permanent observatories under project POLARIS as a first step toward implementing the new celestial and terrestrial reference frames. It is hoped that others will join in the effort and make the new reference frames a reality by the middle of this decade.

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Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308019546 ◽

2007 ◽

Vol 3 (S248) ◽

pp. 348-351

Author(s):

Y. Y. Kovalev ◽

A. P. Lobanov ◽

A. B. Pushkarev ◽

J. A. Zensus

Keyword(s):

Reference Frames ◽

Reference Sample ◽

Physical Factors ◽

Radio Sources ◽

Positional Accuracy ◽

Extragalactic Radio Sources ◽

Long Baseline ◽

Spectral Bands ◽

Optical Reference ◽

Primary Reference

AbstractAccurate alignment of the radio and optical celestial reference frames requires detailed understanding of physical factors that may cause offsets between the positions of the same object measured in different spectral bands. Opacity in compact extragalactic jets (due to synchrotron self-absorption and external free-free absorption) is one of the key physical phenomena producing such an offset, and this effect is well-known in radio astronomy (“core shift”). We have measured the core shifts in a sample of 29 bright compact extragalactic radio sources observed by Very Long Baseline Interferometry (VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and estimate that the average shift between radio and optical positions of distant quasars could be of the order of 0.1--0.2 mas. This shift exceeds the expected positional accuracy of Gaia and SIM. We suggest two possible approaches to carefully investigate and correct for this effect in order to align accurately the radio and optical positions. Both approaches involve determining a Primary Reference Sample of objects to be used for tying the radio and optical reference frames together.

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Post-Newtonian Reference Frames for Advanced Theory of the Lunar Motion and for a New Generation of Lunar Laser Ranging

Acta Physica Slovaca Reviews and Tutorials ◽

10.2478/v10155-010-0004-0 ◽

2010 ◽

Vol 60 (4) ◽

Cited By ~ 2

Author(s):

Yi Xie ◽

Sergei Kopeikin

Keyword(s):

Solar System ◽

Reference Frame ◽

Reference Frames ◽

Laser Ranging ◽

Lunar Laser Ranging ◽

Local Frame ◽

The Earth ◽

Lunar Laser ◽

Lunar Motion ◽

New Generation

Post-Newtonian Reference Frames for Advanced Theory of the Lunar Motion and for a New Generation of Lunar Laser RangingWe overview a set of post-Newtonian reference frames for a comprehensive study of the orbital dynamics and rotational motion of Moon and Earth by means of lunar laser ranging (LLR). We employ a scalar-tensor theory of gravity depending on two post-Newtonian parameters, β and γ, and utilize the relativistic resolutions on reference frames adopted by the International Astronomical Union (IAU) in 2000. We assume that the solar system is isolated and space-time is asymptotically flat at infinity. The primary reference frame covers the entire space-time, has its origin at the solar-system barycenter (SSB) and spatial axes stretching up to infinity. The SSB frame is not rotating with respect to a set of distant quasars that are forming the International Celestial Reference Frame (ICRF). The secondary reference frame has its origin at the Earth-Moon barycenter (EMB). The EMB frame is locally-inertial and is not rotating dynamically in the sense that equation of motion of a test particle moving with respect to the EMB frame, does not contain the Coriolis and centripetal forces. Two other local frames - geocentric (GRF) and selenocentric (SRF) - have their origins at the center of mass of Earth and Moon respectively and do not rotate dynamically. Each local frame is subject to the geodetic precession both with respect to other local frames and with respect to the ICRF because of their relative motion with respect to each other. Theoretical advantage of the dynamically non-rotating local frames is in a more simple mathematical description. Each local frame can be aligned with the axes of ICRF after applying the matrix of the relativistic precession. The set of one global and three local frames is introduced in order to fully decouple the relative motion of Moon with respect to Earth from the orbital motion of the Earth-Moon barycenter as well as to connect the coordinate description of the lunar motion, an observer on Earth, and a retro-reflector on Moon to directly measurable quantities such as the proper time and the round-trip laser-light distance. We solve the gravity field equations and find out the metric tensor and the scalar field in all frames which description includes the post-Newtonian multipole moments of the gravitational field of Earth and Moon. We also derive the post-Newtonian coordinate transformations between the frames and analyze the residual gauge freedom.

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Astrometry and Geodesy from VLBI

Symposium - International Astronomical Union ◽

10.1017/s0074180900173140 ◽

1993 ◽

Vol 156 ◽

pp. 159-171

Author(s):

C. Ma ◽

J. L. Russell

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Radio Sources ◽

Dual Frequency ◽

Terrestrial Reference Frames ◽

Vlbi Observations ◽

Using Data ◽

Celestial Reference Frame ◽

Right Ascension ◽

Better Than

Dual frequency Mark III VLBI observations acquired since 1979 by several geodetic and astrometric observing programs have been used to establish precise celestial and terrestrial reference frames. The program to establish a uniformly distributed celestial reference frame of ∼400 compact radio sources with optical counterparts was begun in 1987. Some 700 sources have been considered as part of this effort and a preliminary list of ∼400 has been observed. At present, 308 sources have formal 1σ errors less than 1 mas in right ascension and 308 have similar precision in declination. The astrometric results include some data acquired for geodetic purposes. The geodetic results using data to September, 1992 include the positions of 105 sites with formal 1σ horizontal errors generally less than 1 cm at 1992.6 and the velocities of 64 sites with formal 1σ horizontal errors generally better than 2 mm/yr.

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Models for Source Structure Corrections

International Astronomical Union Colloquium ◽

10.1017/s0252921100000075 ◽

2000 ◽

Vol 180 ◽

pp. 29-39

Author(s):

P. Charlot

Keyword(s):

Reference Frame ◽

Very Long Baseline Interferometry ◽

Radio Sources ◽

Time Varying ◽

Long Baseline ◽

Vlbi Observations ◽

Spatially Extended ◽

Astrometric Data ◽

Celestial Reference Frame ◽

Source Structure

AbstractAt the milliarcsecond scale, most of the extragalactic radio sources exhibit spatially-extended intrinsic structures which are variable in both time and frequency. Such radio structures set limits on the accuracy of source positions determined with the Very Long Baseline Interferometry (VLBI) technique unless their effects in the astrometric data can be accounted for. We review the modeling scheme for calculating source structure corrections and discuss the magnitude and impact of these effects for the sources that are part of the International Celestial Reference Frame (ICRF). Results obtained by applying source structure corrections to actual VLBI observations on the time-varying source 4C39.25 (0923 + 392) are also presented.

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Dense optical reference frames: UCAC and URAT

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308019431 ◽

2007 ◽

Vol 3 (S248) ◽

pp. 310-315

Author(s):

N. Zacharias

Keyword(s):

Reference Frame ◽

Focal Plane ◽

Reference Frames ◽

Field Of View ◽

Radio Sources ◽

Proper Motions ◽

Naval Observatory ◽

Optical Reference ◽

Celestial Reference Frame ◽

The U.S

AbstractA series of ground-based, dedicated astrometric, observational programs have been performed or are in preparation which provide a dense and accurate optical reference frame. Integral to all these programs are new observations to link the Hipparcos Celestial Reference Frame (HCRF) to the International Celestial Reference Frame (ICRF), based on compact, extragalactic radio sources.The U.S. Naval Observatory CCD Astrograph Catalog (UCAC) 3rd release is in preparation. A pixel re-reduction is in progress to improve astrometric and photometric accuracy as well as completeness of this all-sky reference catalog to 16th magnitude. Optical counterparts of ICRF radio sources have been observed with 0.9-meter telescopes contemporaneously. Scanning of over 5000 early-epoch astrograph plates on StarScan has been completed. These data will improve the proper motions of stars in the 10 to 14 mag range for the UCAC3 release.A 111 million-pixel CCD was successfully fabricated in 2006 and test observations at the USNO astrograph are underway. Four of such detectors will be used for the USNO Robotic Astrometric Telescope (URAT) focal plane assembly. Phase I of URAT will use the astrograph to reach 18th magnitude, while the new 0.85-meter telescope with a 4.5 deg diameter field of view will reach 21st magnitude. The URAT primary mirror has been fabricated.

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Comparison of the Optical and Radio Reference Frames

Highlights of Astronomy ◽

10.1017/s1539299600006304 ◽

1986 ◽

Vol 7 ◽

pp. 103-108 ◽

Cited By ~ 1

Author(s):

K. J. Johnston ◽

Chr. de Vegt

Keyword(s):

Radio Emission ◽

Reference Frame ◽

Radio Telescope ◽

Reference Frames ◽

Radio Sources ◽

Large Array ◽

Very Large Array ◽

Extragalactic Radio Sources ◽

Optical Reference ◽

Lack Of Knowledge

The Very Large Array (VLA) has made possible the measurement of the precise positions of the radio emission associated with stars. This allows the direct comparison of the optical reference frame (FK4) with the radio reference frame which is defined by the quasi-absolute positions of extragalactic radio sources. This comparison is limited by the small number of bright stars that display detectable radio emission and the lack of knowledge of the precise coincidence of the radio emission with the optical photocenter of the star. Since the VLA is the most sensitive astrometrically capable radio telescope, positions of the largest number of stars north of declination -20 degrees can be measured. The accuracy of the positions on the extragalactic reference frame should approach a milliarcsecond.

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Link between Hipparcos and VLBI Celestial Reference Frames

International Astronomical Union Colloquium ◽

10.1017/s0252921100063764 ◽

1991 ◽

Vol 127 ◽

pp. 202-205

Author(s):

E.F. Arias ◽

J.-F. Lestrade

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Inertial Frame ◽

Radio Sources ◽

Proper Motions ◽

Extragalactic Radio Sources

AbstractThe celestial frame of HIPPARCOS astrometric satellite will be materialized by the positions and proper motions of about 120000 stars relative to arbitrary origins. As the HIPPARCOS reference frame is not naturally related to fixed directions in space, it has to be linked to an inertial frame of similar quality. The technique of VLBI determines the coordinates of extragalactic radio sources precise at the milliarcsecond level in an equatorial frame. The precision expected for HIPPARCOS normal mission is 0.002ʺ for the positions, yearly proper motions and parallaxes.

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Reference Frame Studies at JPL/Caltech

Highlights of Astronomy ◽

10.1017/s1539299600006274 ◽

1986 ◽

Vol 7 ◽

pp. 93-96

Author(s):

J. O. Dickey ◽

P. B. Esposito ◽

J.-F. Lestrade ◽

R. P. Linfield ◽

W. G. Melbourne ◽

...

Keyword(s):

Reference Frame ◽

Artificial Satellites ◽

Radio Sources ◽

Coordinate Systems ◽

New Techniques ◽

Long Baseline ◽

Celestial Bodies ◽

Spacecraft Tracking ◽

Orbiting Spacecraft ◽

Radar Ranging

In recent years, a revolution in astronomical position measurements has been taking place with the advent of modern space techniques. These new techniques, which supplement the traditional astrometric measurements, include laser ranging to the moon and artificial satellites, very-long-baseline interferometry (VLBI) of galactic and extra-galactic radio sources and spacecraft, radio tracking of satellites, and radar-ranging and spacecraft tracking during planetary encounters. Impressive accuracies have been achieved and further improvements are forthcoming. Each technique can be expected to establish its own reference frame which is derived from observations of a particular class of objects. The celestial and terrestrial coordinate systems are related through adopted constants and definitions. Contemporary astronomy has led to the development of three principal celestial coordinate systems: the optical frame (FK4/FK5) based on positions of galactic stars; the planetary/lunar ephemeris frame based on the major celestial bodies of the solar system; and the radio frame constructed from observations of extragalactic radio sources (quasars). Each frame is rotated with respect to others; furthermore, the optical frame offset is time variable. It is important that all frames be interconnected and unified. The optical frame is being connected to the radio frame by VLBI observations of radio emitting stars. The radio frame is being tied to the ephemeris frame in several ways – one is via differential VLBI measurements between quasars and planet-orbiting spacecraft.

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