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The Astronomical Journal, 127:3587–3608, 2004 June # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE SECOND EXTENSION OF THE INTERNATIONAL CELESTIAL REFERENCE FRAME: ICRF-EXT.1 A. L. Fey,1 C. Ma,2 E. F. Arias,3 P. Charlot,4 M. Feissel-Vernier,5 A.-M. Gontier,5 C. S. Jacobs,6 J. Li,7 and D. S. MacMillan2 Received 2004 January 8; accepted 2004 March 9

ABSTRACT We use very long baseline interferometry data obtained between mid-1995 and the end of 2002 May together with older data to extend and revise the International Celestial Reference Frame (ICRF). Revised positions of ICRF candidate and ‘‘other’’ sources, based on inclusion of the additional data, are presented. Positions, in the frame of the ICRF, for an additional 109 new sources are also presented. All but four of the new sources are located north of
=
30 . Positions of the ICRF defining sources remain unchanged. We present a summary of current astrometric and geodetic observing programs and discuss the evolution and future of the ICRF. Key words: astrometry — catalogs — quasars: general — radio continuum: galaxies — reference systems — techniques: interferometric

1. INTRODUCTION

obtained by a rotation of the positions into the system of the International Celestial Reference System (ICRS; Arias et al. 1995) and is consistent with the FK5 J2000.0 optical system (Fricke et al. 1988), within the limits of the latter system’s accuracy. The ICRF replaced the FK5 stellar catalog as the fundamental celestial reference frame as of 1998 January 1 and is the realization of the ICRS at radio wavelengths. The Hipparcos Catalogue (Perryman et al. 1997) is the realization of the ICRS at optical wavelengths (Kovalevsky et al. 1997). The ICRF has accomplished its primary goal of providing an accurate and stable frame conceptually independent of the motion of Earth in space. However, from its inception, it has been known that the ICRF has a less than desirable density of sources (Ma et al. 1998), with an average of only about one object per 8 square degrees on the sky (Charlot et al. 2000). In addition, the distribution of the ICRF sources on the sky is largely nonuniform. The deficit of sources is particularly pronounced in the southern hemisphere. Of the 212 ICRF defining sources, less than 30% are in the southern hemisphere. This nonuniform distribution between the northern and southern sky is due primarily to the fact that most VLBI arrays are physically located in the Northern Hemisphere. Consequently, astrometric and geodetic observations have historically concentrated on northern hemisphere sources. The nonuniform distribution of ICRF sources makes it difficult to assess and control any local deformations in the ICRF. Although systematic effects should be greatly reduced in frames derived from extragalactic radio sources, there is no a priori reason to assume that the radio frame will be completely free of systematic errors. Our studies have yielded evidence that mismodeling of both the troposphere and source structure contribute to systematic errors in the reported positions. We will not go into a full discussion here but merely make mention of the types of errors encountered and refer the reader to the literature for a more complete discussion. For example, modeling of tropospheric gradient effects in the ICRF analysis corrected a systematic position offset of nearly 0.4 mas for sources near the celestial equator (Ma et al. 1998). The effect is much larger than the formal errors of the position estimates and is caused by the greater tropospheric thickness near the equator (MacMillan & Ma 1997). Another source of frame deformation is the apparent motions of the sources due

At the 23rd General Assembly of the International Astronomical Union (IAU) held on 1997 August 20 in Kyoto, Japan, the International Celestial Reference Frame (ICRF; Ma et al. 1998) was adopted as the fundamental celestial reference frame. As a consequence, the definitions of the axes of the celestial reference system are no longer related to the equator or the ecliptic but have been superseded by the defining coordinates of the ICRF. The ICRF is currently defined by the radio positions of 212 extragalactic objects (Ma et al. 1998). These ‘‘defining’’ sources set the direction of the ICRF axes and were chosen based on their observing histories and the stability and accuracy of their position estimates. The IAU recommendation adopting the ICRF states that any further issue of the ICRF keep the same coordinate axes by means of a statistical no-net-rotation condition, while the coordinates of individual sources may change to better match the available observations. The ICRF radio positions were based on a general solution for all applicable dual-frequency 2.3 GHz and 8.4 GHz Mark III very long baseline interferometry (VLBI) data available through the middle of 1995, consisting of 1.6 million pairs of groupdelay and phase delay rate observations. The positional accuracy of the ICRF sources is at the sub-milliarcsecond level in both coordinates. In addition to the defining sources, positions for 294 less observed ‘‘candidate’’ sources along with 102 less suitable ‘‘other’’ sources were also given in Ma et al. (1998) to densify the frame. The final orientation of the frame axes was 1 US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420. 2 Space Geodesy Branch, Code 926, NASA Goddard Space Flight Center, Greenbelt, MD 20771. 3 Bureau International des Poids et Mesures, Pavillon de Breteuil, F-92312 Se`vres Cedex, France. 4 Observatoire Aquitain des Sciences de l’Univers, UMR 5804, CNRS, B.P. 89, F-33270 Floirac, France. 5 Syste`mes de Re´fe´rence Temps Espace, UMR 8630, Observatoire de Paris, 61 avenue de l’Observatoire, F-75014 Paris, France. 6 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099. 7 Shanghai Astronomical Observatory, 80 Nandan Road, 200030 Shanghai, China.

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to variable intrinsic structure. It is well known that most of the compact extragalactic sources that make up the ICRF have variable emission structure on scales larger than the accuracy of their position estimates (Fey, Clegg, & Fomalont 1996). This variable emission can, in some cases, induce temporal variations of the sources’ derived astrometric positions. Modeling such effects has been tested by Sovers et al. (2002), whose results confirm that intrinsic source structure significantly affects VLBI analysis. It should also be noted that the deficit and nonuniform distribution of ICRF sources precludes the use of the ICRF as a catalog of calibrators serving as fiducial points to determine the relative positions of nearby weaker objects (radio stars, pulsars, weak quasars) with the phase-referencing technique. The separation between the calibrator and target sources should be a few degrees at most in such observations. In recent years, various dedicated observing programs have been initiated to address concerns about the less than optimal distribution of ICRF sources and to monitor the sources for structural variations. As a consequence of these and various other observing programs, the amount of available data has almost doubled since the ICRF was defined, and many new sources have been added. In this paper, we use VLBI data obtained between mid-1995 and the end of 2002 May together with older data to extend and revise the ICRF. We report accurate positions for extragalactic radio sources observed subsequent to the definition of the ICRF, together with revised positions for ICRF candidate and ‘‘other’’ sources. A description of the VLBI data used in the analysis and a summary of the dedicated observing programs contributing data to the analysis are given in x 2. The analysis procedures used to estimate astrometric positions are discussed in x 3, and the positions are presented in x 4. The evolution of the ICRF in terms of observational and analysis improvements, observations at higher radio frequencies, and space-based optical astrometry are discussed in x 5. Our thoughts on the future of the ICRF are presented in x 6. Finally, a brief summary of our observational results is presented in x 7. 2. THE DATA VLBI observations for geodesy and astrometry using Mark III compatible systems (Clark et al. 1985) have been conducted since about mid-1979. These observations are made in a bandwidth synthesis mode (Rogers 1970) at standard frequencies of 2.3 GHz (S band) and 8.4 GHz (X band). Dualfrequency observations allow for an accurate calibration of the frequency-dependent propagation delay introduced by the ionosphere, while the multiplicity of channels facilitates the determination of a precise group delay. Observing sessions are typically of 24 hours’ duration, as this period of time is required to recover (separate) parameters for nutation and polar motion. In addition, 24 hr duration observations allow for coverage of the full range of catalog right ascensions. Many observing programs contributed data to the definition of the ICRF. A more detailed description of the data used in the ICRF can be found in Ma et al. (1998). Details of some of the newer observing programs designed for improvement of the ICRF are described below. 2.1. ICRF Observing Programs To address concerns about the nonuniform distribution of ICRF sources and to monitor sources for structural variations, various observing programs have been initiated. The goals of

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these dedicated observing programs include (1) obtaining additional data on astrometric sources for more accurate positions, (2) obtaining source position stability information from time series at useful time resolution, (3) densification of defining sources, especially in the southern hemisphere, and (4) source structure monitoring. As a consequence of these and various other observing programs, the amount of available data has almost doubled since the ICRF was defined, and many new sources have been added. Below we list some of the larger programs contributing data to the maintenance and improvement of the ICRF. The International VLBI Service for Geodesy and Astrometry (IVS) schedules regular celestial reference frame (CRF) observations for the specific purpose of maintenance of the ICRF. In recent years, these CRF sessions have concentrated primarily on observations of sources in the southern hemisphere. These sessions, using geodetic antennas, provide additional astrometric observations for estimating improved source positions. Very Long Baseline Array (VLBA) observations for maintenance of the celestial and terrestrial reference frames have been carried out since about 1994. Since 1997, these VLBA observations have been part of a joint program between the the US Naval Observatory (USNO), Goddard Space Flight Center (GSFC), and the National Radio Astronomy Observatory (NRAO). During each 24 hr VLBA session, about 80 ICRF sources are observed using the VLBA together with up to 10 additional geodetic antennas. An observing program proposed by Charlot et al. (2000) is currently under way to densify the ICRF in the northern hemisphere using the European VLBI Network and other geodetic antennas. The approach used in this program is designed to improve the overall sky distribution of the sources. A secondary objective of the program is to identify new sources of high astrometric value that could potentially be used as defining sources in a future realization of the ICRF. The USNO currently has a joint program with the Australia Telescope National Facility (ATNF) for astrometry and imaging of southern hemisphere ICRF sources (Fey et al. 2000). The goals of this joint USNO-ATNF program are to image all southern hemisphere ICRF sources at least twice for structure monitoring and to search for new astrometric sources for densification of the ICRF in the southern hemisphere. The IVS and other observing programs designed for specific purposes, such as for Earth orientation monitoring and for geodesy, also serve the dual purpose of maintenance of the ICRF in the northern hemisphere. Finally, the VLBA Calibrator Survey (VCS1; Beasley et al. 2002) is the largest high-resolution radio survey ever undertaken and triples the number of sources available to the radio astronomy community for VLBI applications. The VCS1 resulted in a catalog containing milliarcsecond-accurate positions of 1332 extragalactic radio sources distributed over the northern sky, and in a majority of cases, images of the sources are also available. Additional observations for the Second VLBA Calibrator Survey (VCS2; Fomalont et al. 2003) were undertaken to fill in regions of the sky that were not completely covered by the previous VCS1 calibrator survey. The VCS2 included calibrator sources near the Galactic plane,
45 <
<
30, and Very Large Array calibrators. VCS data are not currently used in the extension solution described in this paper. We mention it here only for completeness. VCS data may be useful for future ICRF extensions.

No. 6, 2004

ICRF EXTENSION 2 3. ANALYSIS 3.1. Software

The ICRF extension solutions were made at the USNO using the GSFC analysis system. The GSFC analysis system (Ryan, Ma, & Vandenberg 1980; Ma et al. 1986; Caprette, Ma, & Ryan 1990; Ryan, Ma, & Caprette 1993) consists of the astrometric and geodetic VLBI reduction software CALC and SOLVE. The data analysis methods using the GSFC system are covered in detail in Ma et al. (1986). A typical analysis combines data from many different observing sessions, allowing some parameters (e.g., source positions) to be estimated from a combination of many sessions. Application of the analysis methods to the ICRF are described in detail in Ma et al. (1998). To obtain a solution, the data from individual observing sessions are combined sequentially using arc-parameter elimination (Ma et al. 1990). All solutions give weighted leastsquares estimates for parameters. Time-invariant or ‘‘global’’ parameters, that is, parameters dependent on all data sets, are carried from step to step, resulting in a single estimate derived from the combined data of all sessions in the solution. These global parameters may include station positions, station velocities, source positions, source velocities (proper motions), nutation series coefficients, the precession constant, Love numbers for the solid-Earth tides, and the relativistic gamma factor. Local or ‘‘arc’’ parameters depend only on the data from an individual session and are estimated separately for each epoch of observation. Arc parameters include those for the station clocks and atmospheres, Earth’s orientation, and nutation offsets in obliquity and longitude. Station positions and source positions can also be arc parameters if the solution is to follow changes over time. 3.2. Analysis Configuration The configuration of the ICRF analysis (Ma et al. 1998) was developed as a balance between competing goals: the most data and the least systematic error, the best models and available options, the largest number of useful estimated parameters and computer speed, etc. To maintain consistency with the ICRF, the extension solution reported here was parameterized similarly to the ICRF solution. Rather than describe the complete analysis configuration for the extension solution, we will endeavor to only point out salient features of the analysis and to describe where the two analysis configurations differ. The weighting of the data followed the ICRF solution. For each session, an added noise value was computed for group delays and delay rates on a station-by-station basis, which caused the reduced 2 (the 2 per degree of freedom) to be close to unity when added to the variance of the observations derived from the correlation and fringe-finding process, as well as the calibration of the ionosphere. Other modifications of the observation errors such as elevation-dependent and source-dependent noise were not used. Similar to the ICRF solution, the primary geodetic parameters—the station positions—were estimated separately for each session in the extension solution. In this way, any nonlinear motion of the stations (e.g., unmodeled tectonic motion, long-term antenna motion, or earthquake displacements) does not affect the integrity of the invariant source positions. The relative source positions derived from a single 24 hr session are not distorted by forcing the station positions for that day to conform exactly to a linear model. Station motions within a

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day, from solid-Earth tides and ocean loading, were derived from unadjusted a priori models (McCarthy 1996). As mentioned previously, parameters were estimated using arc-parameter elimination (Ma et al. 1990), which is an incremental least-squares method that can accommodate large numbers of parameters if they are associated only with particular data intervals or ‘‘arcs.’’ As in the ICRF analysis, several classes of parameters were adjusted for the extension solution. For each observing session, the adjusted arc parameters included positions of sources with identified excessive apparent motion or random variation; celestial pole offsets in ecliptic longitude and obliquity to account for errors in the standard precession-nutation models; positions of the stations; the rate of UT1 relative to a good a priori time series; 20 minute piecewise linear continuous troposphere parameters; tropospheric gradients in the east-west and north-south directions, linear in time, estimated once per session; quadratic clock polynomials for the gross clock behavior; 60 minute piecewise linear continuous clock parameters; and necessary nuisance parameters such as clock jumps and baseline clock offsets (i.e., separate bias parameters for each VLBI baseline to accommodate small, constant, baseline-dependent instrumental and correlator errors). The same set of sources were treated as arc parameters in the extension solutions as in the ICRF solution with the exception of the following sources, which were downgraded to arc parameters because of excessive position variation: 0804+499, 1308+326, 1606+106, 2037+511 (3C 418), and 2145+067. The remaining parameters, including source positions, were adjusted as invariant, or ‘‘global,’’ quantities from the entire data set. Alignment of the positions onto the system of the ICRS was achieved through a no-net-rotation constraint imposed on the positions of 207 of the 212 ICRF defining sources using their published positions from Ma et al. (1998). The five sources listed above were excluded from this constraint for the stated reason. 3.2.1. ICRF-Ext.1: The First ICRF Extension

The data added to the ICRF in ICRF-Ext.1 spanned 1994 December through 1999 April and were obtained from both geodetic and astrometric observing programs. Approximately 0.6 million new observations from 461 sessions were added. There were 59 new sources. The positions and errors for the defining sources were unchanged from the ICRF. Revised positions and errors for the candidate and other sources were estimated and reflected the changes in the data set and the analysis. Positions for the 59 new sources were reported in the frame of the ICRF. The distribution on the sky of the new sources from ICRF-Ext.1 is shown in Figure 1. The ICRFExt.1 revised and new positions are available from the IERS (IERS 2000). Note that ICRF-Ext.1 has been superseded by the ICRF-Ext.2 positions listed in this paper. We describe ICRF-Ext.1 here only for historical purposes and to set the stage for presentation of ICRF-Ext.2. 3.2.2. ICRF-Ext.2: The Second ICRF Extension

The ICRF-Ext.2 solution differed from that of the ICRF and ICRF-Ext.1 solutions primarily in the use of the NMF mapping function (Niell 1996) for troposphere modeling. The reader is referred to Niell (1996) for a more thorough discussion of this mapping function and its benefits for VLBI analysis. The data added in ICRF-Ext.2 spanned 1999 May through 2002 May and were obtained from both geodetic and astrometric observing programs. Approximately 1.2 million new

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Fig. 1.—Distribution of the 59 new sources in ICRF-Ext.1 on an Aitoff equal-area projection of the celestial sphere. The dotted line represents the Galactic equator.

observations from approximately 400 sessions were added. There were 50 additional new sources. The positions and errors for the defining sources were unchanged from the ICRF. Revised positions and errors for the candidate and other sources were estimated and reflect the changes in the data set and the analysis. Positions for the 50 new sources are reported in the frame of the ICRF. The distribution on the sky of the new sources from ICRF-Ext.2 is shown in Figure 2. After completing a series of test solutions to refine various aspects of the analysis, a final solution was run in the fall of

2002, which included 3,386,128 group-delay and delay-rate observations spanning the time period from 1979 August through 2002 May. The post-fit weighted rms residuals were 24.75 ps (75.73 fs s
1) for delay (rate) with a combined reduced 2 of 0.964. There were 1241 global parameters, 1,700,787 arc parameters, and 5,772,662 degrees of freedom. ICRF-Ext.2 marks a milestone in that it utilizes the available VLBA data described in x 2.1. A total of over 652,000 delay observations (almost 20% of all available observations!) from 30 VLBA geodesy/astrometry sessions were included in

Fig. 2.—Distribution of the 50 new sources in ICRF-Ext.2 on an Aitoff equal-area projection of the celestial sphere. The dotted line represents the Galactic equator.

No. 6, 2004

ICRF EXTENSION 2

the ICRF-Ext.2 solution. Although preliminary analysis of the VLBA sessions suggested that these data had systematic errors of unknown origin, an analysis performed by Fey & Boboltz (2002) helped to show that the initial concerns were unfounded and that these data should be incorporated into the general astrometric and geodetic database. 4. POSITIONS OF CANDIDATE, OTHER, AND NEW SOURCES The primary result obtained from the final ICRF-Ext.2 least-squares solution is the set of invariant source positions and their formal uncertainties. For the ‘‘arc’’ sources, an additional step was taken to calculate the weighted mean positions and weighted rms scatter as a measure of error. To be consistent with the original ICRF (and ICRF-Ext.1), it was assumed that a realistic error estimate for the invariant source positions could be made by inflating the formal errors by a factor of 1.5 followed by a root sum square increase of 0.25 mas. For the most frequently observed sources, the 0.25 mas is the dominant error. Positions for 109 new sources in the ICRF (from both ICRF-Ext.1 and ICRF-Ext.2) are listed in Table 1. Revised positions for the 294 ICRF candidate and 102 ICRF ‘‘other’’ sources are listed in Tables 2 and 3, respectively. The column labeled C -
lists the correlation between right ascension and declination. The column labeled Nexp lists the number of 24 hr observing sessions, and column Nobs lists the number of observations. All positions listed in these tables are taken from the ICRF-Ext.2 solution. The position errors are the inflated values as described above. The positions and errors for the ICRF defining sources were unchanged from the those reported in Ma et al. (1998) and are not listed in this paper. 5. EVOLUTION OF THE ICRF The ICRF provides an accurate and stable celestial reference frame conceptually independent of the motion of Earth in space. However, it was recognized from the beginning that there were several intrinsic deficiencies, particularly the uneven distribution of defining sources and the weakness of the criteria and information for selecting these sources. With continued applicable VLBI observations and improvements in analysis, it is clear that a better realization of the ICRS is now possible and an even better realization is feasible in the foreseeable future. In the following sections, we address various issues related to this question. 5.1. Analysis Improvements 5.1.1. Modeling Capabilities

The largest contributors to inaccuracy and instability in VLBI astrometric results are the propagation media and source structure. While the charged-particle propagation media effects have been effectively calibrated using two observing frequencies, the modeling of the troposphere has improved in discrete steps associated with development of new troposphere mapping algorithms (Niell 1996; Niell & Tang 2002) and modeling of asymmetry and variability (MacMillan & Ma 1997). Current research is directed toward the use of global weather data in computing the mapping function through ray tracing or a proxy. Computational limits, as well as temporal and spatial resolution of the weather models, preclude direct ray tracing for every observation in place of estimating troposphere variations. A different approach is the use of troposphere structure functions to provide additional information.

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Weather models should also provide better a priori information for tropospheric gradients. In addition, close attention is being given in designing observing schedules to achieve optimal geometry and temporal resolution for estimating troposphere parameters. Source structure and changes in source structure put a floor on the stability of source positions, although a set of sources with acceptable consistency has been identified (see x 5.1.3). Modeling such effects has been tested on massive scales by Sovers et al. (2002). Results of their analysis show that the effects of intrinsic source structure on astrometric position estimation amounts to a significant fraction of the systematic error budget of the ICRF, thus confirming that source structure does affect VLBI analysis even though it is not currently the dominant error. For some sources with very extended or highly variable emission structures, the scatter of the ‘‘arc’’ source positions over time was also found to decrease substantially (Charlot 2002). Modeling source structure may be especially worthwhile in the specific task of refining positions for orienting a precise optical frame to the ICRF (Fey et al. 2001). The geometric distribution of terrestrial and celestial references in many of the early VLBI sessions was not good enough to ensure proper decorrelation of the source coordinate errors from errors in the terrestrial frame. Therefore, in the analysis strategy that was selected for the ICRF the station coordinates were set as arc parameters. The drawback of this strategy is that the link between the celestial frame and the terrestrial frame is lost, while the maintenance of this link is a major strength of VLBI with respect to the satellite geodesy techniques. As shown by Feissel-Vernier (2003), this is no longer the case, and it should be possible to include the station positions and velocities as global parameters in the analysis without contamination of the celestial reference frame. Consequently, the contribution of VLBI to the unification of reference frames would be strengthened. 5.1.2. Source Suitability Based on Intrinsic Structure

As stated previously, source structure and changes in source structure put a floor on the stability of estimated source positions using VLBI astrometric data. Fey & Charlot (1997, 2000) investigated the astrometric suitability of a large sample of ICRF sources (about two-thirds of the total number of ICRF sources) by calculating the average structural VLBI groupdelay effect for each source. This analysis was based on dualfrequency (S-band and X-band) VLBI images of 388 ICRF sources generated from dedicated VLBA observations. A source ‘‘structure index’’ was introduced as an indicator of the source quality. Values of the structure index range from 1 to 4, with increasing values indicating increasing average structural VLBI group-delay corrections. Structure index values of 1 and 2 point to excellent and good astrometric suitability, respectively, while values of 3 and 4 point to poor suitability (a given source may have differing structure index values at S band and X band depending on the source structure at each frequency band). Based on this indicator, correlations between the observed radio structure and the astrometric position accuracy and stability of the sources were found. These correlations indicate that the more extended sources have larger position uncertainties and are less positionally stable than the more compact sources. Overall, about 60% of the sources for which a structure index was calculated had a value of either 1 or 2 at X band (see Table 4), indicating compact or very compact structure

TABLE 1 ICRF Coordinates of 109 New Sources

Designation

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ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J003939.6+141157...... J004847.1+315725...... J005748.8+302108...... J012238.8+250231...... J013027.6+084246...... J013243.4
165448..... J020434.7+090349...... J020935.9+135200...... J022311.4+425931...... J030826.2+040639...... J031049.8+381453...... J032536.8+222400...... J032754.1+023341...... J033409.9+022609...... J033654.0
361606..... J041243.6+230505...... J041821.2+380135...... J042655.7+232739...... J042952.9+272437...... J043337.8+290555...... J044848.5
741731..... J053435.7
610607..... J055704.7+241355...... J060351.5+215937...... J060550.8+403008...... J061350.1+260436...... J063243.1+155957...... J065510.0+410010...... J065917.9+081330...... J073730.0+594103...... J075828.1+374711...... J080757.5+043234...... J082511.8+133232...... J083740.2+245423...... J083930.7+180247...... J084715.1+383110...... J085657.2+211143...... J094014.7+260329......

 (J2000.0)

Source 0037+139 0046+316 0055+300 0119+247 0127+084 0130
171 0201+088 0206+136 0220+427 0305+039 0307+380 0322+222 0325+023 0331+022 0335
364 0409+229 0415+379 0423+233 0426+273 0430+289 0450
743 0534
611 0554+242 0600+219 0602+405 0610+260 0629+160 0651+410 0656+082 0733+597 0755+379 0805+046 0822+137 0834+250 0836+182 0844+387 0854+213 0937+262

00 00 00 01 01 01 02 02 02 03 03 03 03 03 03 04 04 04 04 04 04 05 05 06 06 06 06 06 06 07 07 08 08 08 08 08 08 09

39 48 57 22 30 32 04 09 23 08 10 25 27 34 36 12 18 26 29 33 48 34 57 03 05 13 32 55 59 37 58 07 25 37 39 47 56 40

39.619530 47.141484 48.883347 38.815979 27.634458 43.487475 34.759299 35.998314 11.411245 26.223802 49.879955 36.814355 54.194843 09.947731 54.023515 43.666865 21.277204 55.734774 52.960761 37.829859 48.557653 35.772641 04.713569 51.557093 50.855352 50.139196 43.135585 10.024734 17.996024 30.089071 28.108162 57.538569 11.890947 40.245686 30.721362 15.168672 57.244764 14.722742

Epoch of Observation (MJD)


(J2000.0)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

14 11 57.55653 31 57 25.08509 30 21 08.81218 25 02 31.79287 08 42 46.17173
16 54 48.52226 09 03 49.26336 13 52 00.75128 42 59 31.38402 04 06 39.30092 38 14 53.83716 22 24 00.36551 02 33 41.98203 02 26 09.64824
36 16 06.22449 23 05 05.45251 38 01 35.80068 23 27 39.63363 27 24 37.87632 29 05 55.47704
74 17 31.24924
61 06 07.08232 24 13 55.29851 21 59 37.69755 40 30 08.10358 26 04 36.71982 15 59 57.62155 41 00 10.15981 08 13 30.95348 59 41 03.18097 37 47 11.80727 04 32 34.53110 13 32 32.53870 24 54 23.12172 18 02 47.14323 38 31 09.99236 21 11 43.65853 26 03 29.94655

0.000054 0.000019 0.000018 0.000027 0.000032 0.000019 0.000066 0.000031 0.000105 0.000018 0.000122 0.000017 0.000110 0.000170 0.000035 0.000018 0.000099 0.000030 0.000018 0.000017 0.002885 0.002376 0.000018 0.000083 0.000020 0.000020 0.000026 0.000020 0.000019 0.001440 0.000053 0.000020 0.000019 0.000018 0.000025 0.002786 0.000018 0.000035

0.00062 0.00032 0.00027 0.00046 0.00099 0.00034 0.00107 0.00071 0.00144 0.00031 0.00096 0.00026 0.00176 0.00290 0.00054 0.00028 0.00134 0.00062 0.00028 0.00025 0.00572 0.01378 0.00028 0.00352 0.00028 0.00032 0.00053 0.00032 0.00040 0.00957 0.00135 0.00039 0.00039 0.00028 0.00052 0.01778 0.00028 0.00065


0.582
0.073
0.327
0.044 0.079
0.029
0.189 0.163
0.198
0.158
0.397
0.126 0.470 0.255 0.127
0.250 0.000
0.155
0.041
0.105 0.891 0.827 0.298
0.255 0.156
0.246 0.298
0.253
0.390
0.970
0.395
0.227
0.026
0.433
0.674
0.966
0.203
0.209

52,399.8 51,013.0 51,133.0 51,967.7 51,367.8 51,088.0 50,343.8 52,408.5 51,449.7 51,006.8 49,939.8 51,502.1 51,579.8 50,731.8 51,333.3 50,938.3 51,538.7 51,057.8 51,028.9 51,221.8 50,182.6 50,182.6 50,579.8 50,168.7 51,373.0 51,285.0 52,399.3 51,410.9 52,044.9 50,685.8 51,255.8 51,395.7 51,883.6 51,883.6 51,981.8 51,449.7 51,883.6 51,408.8

52,397.8 50,303.8 50,303.8 51,967.7 51,184.8 50,800.8 50,065.8 52,404.8 51,449.7 50,303.8 49,939.8 51,324.8 51,492.8 50,554.8 50,919.8 50,519.8 51,464.7 50,561.8 50,526.8 50,043.8 50,182.6 50,182.6 50,065.8 50,168.7 50,255.8 50,065.8 52,397.8 51,296.8 51,520.8 50,303.8 51,246.6 50,960.8 51,883.6 51,883.6 51,841.6 51,449.7 51,883.6 51,408.8

52,401.7 51,660.8 51,814.8 51,967.7 51,927.8 52,254.7 51,386.3 52,408.7 51,449.7 51,772.8 49,939.8 51,967.7 51,927.8 50,800.8 52,254.7 51,774.7 51,681.8 51,927.8 51,709.7 52,403.7 50,182.6 51,074.2 51,723.7 50,168.7 51,774.7 51,732.8 52,401.7 51,927.8 52,423.7 51,449.7 51,449.7 51,870.7 51,883.6 51,883.6 52,039.8 51,449.7 51,883.6 51,408.8

2 10 17 1 5 7 3 2 1 20 1 20 3 3 13 12 5 9 21 35 1 2 25 1 4 19 2 5 8 2 2 11 1 1 2 1 1 1

25 125 423 20 17 131 19 19 10 224 8 755 5 14 40 239 136 29 244 1303 5 2 284 4 189 177 46 140 88 3 22 69 64 242 65 2 183 13

TABLE 1—Continued

Designation

3593

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J094538.1+353455...... J095232.0+351252...... J095622.6+575355...... J095738.1+552257...... J102556.2+125349...... J102838.7
084438..... J103334.0+071126...... J104552.7+062436...... J110741.7+723236...... J113626.4+700927...... J114505.0+193622...... J114722.1+350107...... J114850.3+592456...... J121923.2+054929...... J122006.8+291650...... J122011.8+020342...... J122503.7+125313...... J122836.9
030439..... J124129.5+602041...... J124209.8+372005...... J131059.4+323334...... J131931.6
123925..... J132952.8+315411...... J133245.2+472222...... J135446.5
104102..... J135706.0
174401..... J141604.1+344436...... J142549.0+142456...... J142637.0+362509...... J142921.8+540611...... J143023.7+420436...... J143239.8+361807...... J143853.6+371035...... J143923.6+325354...... J144635.3+172107...... J145353.6+264833...... J151053.5
054307..... J152441.6+152121...... J160332.0+171155......

 (J2000.0)

Source 0942+358 0949+354 0952+581 0954+556 1023+131 1026
084 1030+074 1043+066 1104+728 1133+704 1142+198 1144+352 1146+596 1216+061 1217+295 1217+023 1222+131 1226
028 1239+606 1239+376 1308+328 1316
123 1327+321 1330+476 1352
104 1354
174 1413+349 1423+146 1424+366 1427+543 1428+422 1430+365 1436+373 1437+331 1444+175 1451+270 1508
055 1522+155 1601+173

09 09 09 09 10 10 10 10 11 11 11 11 11 12 12 12 12 12 12 12 13 13 13 13 13 13 14 14 14 14 14 14 14 14 14 14 15 15 16

45 52 56 57 25 28 33 45 07 36 45 47 48 19 20 20 25 28 41 42 10 19 29 32 54 57 16 25 26 29 30 32 38 39 46 53 10 24 03

38.120712 32.026181 22.634451 38.184490 56.285332 38.796358 34.024287 52.733258 41.722577 26.408415 05.009035 22.130561 50.358250 23.216211 06.825633 11.884594 03.743330 36.917301 29.590644 09.812382 59.402729 31.669639 52.864905 45.246423 46.518671 06.074190 04.186226 49.018023 37.087499 21.878781 23.741635 39.829604 53.610985 23.654496 35.346294 53.600649 53.591420 41.611456 32.083343


(J2000.0) 35 35 57 55 12
08 07 06 72 70 19 35 59 05 29 02 12
03 60 37 32
12 31 47
10
17 34 14 36 54 42 36 37 32 17 26
05 15 17

34 12 53 22 53 44 11 24 32 09 36 01 24 49 16 03 53 04 20 20 33 39 54 22 41 44 44 24 25 06 04 18 10 53 21 48 43 21 11

55.08848 52.40311 55.90445 57.76914 49.02220 38.53375 26.14780 36.45288 36.00480 27.30697 22.74139 07.52264 56.38203 29.69482 50.71239 42.22486 13.13927 39.31120 41.32233 05.69233 34.44961 25.07765 11.05455 22.66776 02.65672 01.90429 36.42811 56.90181 09.57374 11.12295 36.49110 07.93269 35.41670 54.82378 07.58094 33.40985 07.41721 21.05026 55.31110

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000020 0.000019 0.000081 0.000162 0.000024 0.000117 0.000017 0.000030 0.000042 0.000045 0.000020 0.000024 0.000040 0.016435 0.000801 0.000036 0.000019 0.000034 0.000084 0.000022 0.000017 0.000126 0.000017 0.000018 0.000018 0.000023 0.000035 0.000021 0.000026 0.000024 0.000020 0.000030 0.000023 0.000062 0.000041 0.000019 0.000018 0.000027 0.000265

0.00030 0.00028 0.00066 0.00148 0.00036 0.00363 0.00027 0.00091 0.00031 0.00032 0.00043 0.00268 0.00037 0.15935 0.00605 0.00145 0.00043 0.00097 0.00063 0.00037 0.00026 0.00539 0.00026 0.00026 0.00032 0.00047 0.00070 0.00052 0.00031 0.00030 0.00030 0.00058 0.00035 0.00121 0.00064 0.00037 0.00030 0.00084 0.00966


0.044
0.234
0.035 0.252
0.282
0.318
0.227
0.556
0.315 0.162 0.060 0.000
0.419
0.999
0.325
0.321
0.098 0.214
0.376 0.098
0.099
0.674
0.121
0.170
0.284
0.058 0.102 0.013
0.028
0.292 0.056
0.287 0.038
0.403 0.009
0.453
0.059
0.296
0.836

50,854.6 50,998.8 50,974.8 50,987.8 52,425.8 51,169.7 51,585.8 50,699.3 51,042.1 50,826.5 50,804.0 50,788.4 51,020.3 50,303.8 50,258.8 50,555.8 50,994.1 50,633.6 51,303.9 51,512.7 50,966.2 51,449.7 51,883.6 50,978.0 51,122.8 51,114.3 51,685.0 52,415.3 51,562.1 50,974.0 50,969.1 51,322.5 51,687.6 51,687.6 51,967.7 51,883.6 51,247.3 52,421.9 50,685.8

50,854.6 50,800.8 50,974.8 48,482.8 52,425.8 51,086.8 50,855.8 50,168.7 50,799.8 50,303.8 50,386.8 50,303.8 50,303.8 50,246.8 50,113.8 50,441.6 50,303.8 50,526.8 51,284.6 50,168.7 49,706.7 51,449.7 51,883.6 50,750.8 48,353.6 50,456.8 51,646.8 52,411.8 50,868.7 50,736.8 50,764.8 50,303.8 51,687.6 51,687.6 51,967.7 51,883.6 50,610.8 52,418.8 50,303.8

50,854.6 51,674.8 50,974.8 50,989.8 52,425.8 51,169.7 52,376.8 51,751.8 51,927.8 51,774.7 51,576.8 51,184.7 51,927.8 50,303.8 50,303.8 50,680.8 51,716.8 50,654.8 51,380.7 52,039.8 51,814.8 51,449.7 51,883.6 51,309.6 51,774.7 51,927.8 51,687.6 52,415.7 51,687.6 51,604.8 51,716.8 51,774.7 51,687.6 51,687.6 51,967.7 51,883.6 51,611.7 52,423.7 51,449.7

1 5 1 3 1 2 145 7 7 9 13 3 7 2 3 3 17 6 3 10 35 1 1 3 18 17 2 2 4 6 13 10 1 1 1 1 14 2 2

100 122 10 29 46 15 598 28 59 129 129 16 109 1 3 9 157 79 39 107 611 2 407 632 251 46 64 35 162 145 185 84 65 35 10 118 269 25 3

TABLE 1—Continued

Designation

3594

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J163011.2+213134...... J163231.9+823216...... J163745.1+471733...... J164125.2+225704...... J173548.0+361645...... J174456.6+554217...... J175559.7+182021...... J181657.0+530744...... J182536.5
171849..... J183220.8
103511..... J183243.4+135744...... J184511.1+400751...... J185027.5+282513...... J185700.4
532500..... J190414.3+153638...... J191254.2+051800...... J192218.6+084157...... J192439.4+154043...... J195511.5+135816...... J195734.4+342754...... J195928.3+404402...... J200210.4+472528...... J210841.0+143027...... J212313.3+100754...... J214518.7+111527...... J214755.2+083011...... J214935.2+075625...... J220326.8+172548...... J223222.5
165901..... J223638.5+732252...... J225307.3+194234...... J231147.4+454356......

 (J2000.0)

Source 1628+216 1637+826 1636+473 1639+230 1734+363 1744+557 1753+183 1815+531 1822
173 1829
106 1830+139 1843+400 1848+283 1852
534 1901+155 1910+052 1919+086 1922+155 1952+138 1955+343 1957+406 2000+472 2106+143 2120+099 2142+110 2145+082 2147+077 2201+171 2229
172 2235+731 2250+190 2309+454

16 16 16 16 17 17 17 18 18 18 18 18 18 18 19 19 19 19 19 19 19 20 21 21 21 21 21 22 22 22 22 23

30 32 37 41 35 44 55 16 25 32 32 45 50 57 04 12 22 24 55 57 59 02 08 23 45 47 49 03 32 36 53 11

11.230781 31.969926 45.130558 25.227567 48.086650 56.607067 59.781951 57.070843 36.532437 20.834331 43.471096 11.131484 27.589831 00.452763 14.361634 54.257638 18.633622 39.455876 11.571404 34.446464 28.356628 10.418247 41.032143 13.358592 18.775089 55.219399 35.263789 26.893665 22.564043 38.597039 07.369174 47.408964

Epoch of Observation (MJD)


(J2000.0)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

21 31 34.30950 82 32 16.39994 47 17 33.83139 22 57 04.03285 36 16 45.61156 55 42 17.16119 18 20 17.67132 53 07 44.50075
17 18 49.85210
10 35 11.16923 13 57 44.40025 40 07 51.57716 28 25 13.15532
53 25 00.35077 15 36 38.45292 05 18 00.42182 08 41 57.37176 15 40 43.94196 13 58 16.23957 34 27 54.55785 40 44 02.09695 47 25 28.77367 14 30 27.01253 10 07 54.94427 11 15 27.31247 08 30 11.89868 07 56 25.34762 17 25 48.24726
16 59 01.89180 73 22 52.66232 19 42 34.62869 45 43 56.01632

0.000291 0.000037 0.000021 0.000018 0.000022 0.000020 0.001023 0.000131 0.000206 0.004967 0.000028 0.000145 0.000031 0.001049 0.000454 0.000101 0.000128 0.000019 0.000041 0.009708 0.000717 0.000028 0.000017 0.000044 0.000019 0.000040 0.000080 0.000031 0.000480 0.000022 0.000017 0.000032

0.00537 0.00026 0.00031 0.00030 0.00030 0.00027 0.00878 0.00122 0.00436 0.03541 0.00052 0.00158 0.00050 0.02320 0.00153 0.00097 0.00442 0.00032 0.00135 0.09571 0.00470 0.00035 0.00029 0.00146 0.00037 0.00158 0.00204 0.00061 0.00508 0.00026 0.00028 0.00041

0.831
0.195
0.024
0.079 0.194 0.162 0.430
0.382
0.690
0.819
0.236 0.196
0.177 0.451 0.817
0.076
0.613
0.049
0.172 0.959
0.761
0.181
0.153
0.448
0.174
0.635
0.074
0.070 0.116 0.215
0.149
0.139

50,854.6 51,159.9 51,355.9 51,621.6 51,186.4 51,365.6 51,067.7 51,704.5 51,732.8 51,732.8 52,400.9 50,053.5 51,967.7 50,989.8 52,408.7 50,919.8 50,700.6 50,710.4 52,425.8 49,690.0 50,974.8 51,802.4 50,800.8 50,700.6 50,654.8 50,654.8 50,700.6 51,967.7 50,953.8 51,734.5 50,984.3 51,967.7

50,854.6 50,820.8 51,268.8 50,168.7 49,939.8 50,303.8 50,303.8 51,704.5 51,732.8 46,875.8 52,397.8 50,022.8 51,967.7 50,989.8 52,404.8 50,919.8 50,700.6 50,654.8 52,425.8 49,690.0 50,974.8 51,704.5 50,800.8 50,700.6 50,654.8 50,654.8 50,700.6 51,967.7 50,946.8 51,072.8 50,771.8 51,967.7

50,854.6 52,376.8 51,499.8 52,403.7 51,386.3 52,039.8 51,449.7 51,704.5 51,732.8 51,732.8 52,401.7 50,065.8 51,967.7 50,989.8 52,408.7 50,919.8 50,700.6 52,423.7 52,425.8 49,690.0 50,974.8 51,967.7 50,800.8 50,700.6 50,654.8 50,654.8 50,700.6 51,967.7 50,960.8 52,390.8 51,870.7 51,967.7

1 60 3 10 4 17 3 1 1 6 2 3 1 1 2 1 1 3 1 1 1 2 1 1 1 1 1 1 2 101 15 1

9 742 46 192 206 385 3 11 16 2 42 7 14 3 7 8 9 350 8 1 2 43 293 31 124 26 17 19 4 521 242 25

Note.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Includes all sessions in which a source was observed.

TABLE 2 Revised Coordinates of the 294 ICRF Candidate Sources

Designation

3595

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J000435.6
473619..... J000613.8
062335..... J001052.5
415310..... J001101.2
261233..... J001611.0
001512..... J002442.9
420203..... J003824.8+413706...... J005846.5
565911..... J005905.5+000651...... J011137.3+390628...... J011327.0+494824...... J011343.1+022217...... J011517.0
012704..... J011935.0+321050...... J012031.6
270124..... J012141.5+114950...... J012156.8+042224...... J014922.3+055553...... J015002.6
072548..... J015310.1
331025..... J015456.2+474326...... J015537.0
404842..... J020213.6
762003..... J020346.6+113445...... J020450.4+151411...... J020457.6
170119..... J022428.4+065923...... J023145.8+132254...... J023752.4+284808...... J023945.4
023440..... J023951.2+041621...... J024457.6+622806...... J025246.1
710435..... J025329.1
544151..... J025334.8+180542...... J030230.5+121856...... J030642.6+624302...... J031155.2
765150..... J031951.2+190131...... J033553.9
543025..... J033717.1+013722...... J033930.9
014635.....

Source 0002
478 0003
066 0008
421 0008
264 0013
005 0022
423 0035+413 0056
572 0056
001 0108+388 0110+495 0111+021 0112
017 0116+319 0118
272 0119+115 0119+041 0146+056 0147
076 0150
334 0151+474 0153
410 0202
765 0201+113 0202+149 0202
172 0221+067 0229+131 0234+285 0237
027 0237+040 0241+622 0252
712 0252
549 0250+178 0259+121 0302+625 0312
770 0317+188 0334
546 0334+014 0336
019

 (J2000.0) 00 04 00 06 00 10 00 11 00 16 00 24 00 38 00 58 00 59 01 11 01 13 01 13 01 15 01 19 01 20 01 21 01 21 01 49 01 50 01 53 01 54 01 55 02 02 02 03 02 04 02 04 02 24 02 31 02 37 02 39 02 39 02 44 02 52 02 53 02 53 03 02 03 06 03 11 03 19 03 35 03 37 03 39

35.655514 13.892889 52.519721 01.246737 11.088552 42.989932 24.843593 46.581208 05.514893 37.316805 27.006821 43.144949 17.099945 35.000586 31.663361 41.595041 56.861699 22.370886 02.697281 10.121711 56.289893 37.059393 13.693974 46.657060 50.413899 57.674350 28.428191 45.894050 52.405678 45.472274 51.263047 57.696849 46.155778 29.180436 34.882262 30.546766 42.659550 55.250341 51.256736 53.924887 17.108944 30.937788


(J2000.0)
47
06
41
26
00
42 41
56 00 39 49 02
01 32
27 11 04 05
07
33 47
40
76 11 15
17 06 13 28
02 04 62
71
54 18 12 62
76 19
54 01
01

36 19.60366 23 35.33532 53 10.79115 12 33.37684 15 12.44531 02 03.95069 37 06.00037 59 11.47049 06 51.62168 06 28.10388 48 24.04321 22 17.31635 27 04.57706 10 50.05444 01 24.65235 49 50.41311 22 24.73438 55 53.56866 25 48.48884 10 25.86125 43 26.53942 48 42.35670 20 03.05664 34 45.40955 14 11.04358 01 19.84044 59 23.34153 22 54.71627 48 08.99006 34 40.91470 16 21.41189 28 06.51470 04 35.27416 41 51.43638 05 42.52412 18 56.75222 43 02.02406 51 50.84832 01 31.29082 30 25.11474 37 22.73643 46 35.80404

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000040 0.000017 0.000548 0.000022 0.000017 0.000157 0.000018 0.000055 0.000021 0.000018 0.000027 0.000017 0.000017 0.001362 0.000029 0.000017 0.000017 0.000017 0.000071 0.000046 0.000020 0.000193 0.002348 0.000017 0.000025 0.000018 0.000017 0.000017 0.000017 0.000017 0.000018 0.000168 0.003702 0.000040 0.002261 0.000021 0.000024 0.000207 0.000018 0.000050 0.003005 0.000017

0.00058 0.00025 0.00614 0.00034 0.00027 0.00143 0.00026 0.00074 0.00040 0.00027 0.00037 0.00026 0.00028 0.00960 0.00055 0.00025 0.00025 0.00026 0.00368 0.00082 0.00029 0.00140 0.00560 0.00025 0.00049 0.00031 0.00027 0.00025 0.00025 0.00029 0.00030 0.00097 0.02811 0.00054 0.02882 0.00040 0.00030 0.00062 0.00029 0.00089 0.01531 0.00025

0.005
0.044
0.633
0.138
0.194
0.449
0.159 0.390
0.118 0.097
0.393
0.345
0.422 0.369
0.411
0.024
0.025
0.212
0.332
0.205 0.169
0.046 0.272
0.150 0.000
0.102
0.424
0.034
0.049
0.248
0.403 0.141 0.328 0.098
0.998
0.221
0.176 0.500
0.380 0.042 0.524
0.051

50,413.6 51,304.6 49,051.0 50,343.7 50,379.1 49,123.6 50,851.3 49,645.4 49,731.3 50,830.5 50,639.4 50,662.5 50,128.3 49,339.1 51,121.8 51,421.5 49,583.0 50,415.4 49,992.3 49,316.2 51,094.1 49,272.6 48,780.2 50,889.6 50,043.8 50,696.3 50,361.1 49,153.6 49,029.0 50,963.6 49,993.3 49,933.2 48,162.4 49,671.5 48,977.5 50,207.3 50,136.2 49,330.5 50,283.1 49,855.5 49,350.2 50,761.1

49,330.5 47,176.5 48,162.4 47,686.1 47,394.1 48,162.4 49,422.9 47,626.5 47,005.8 49,099.7 49,422.9 47,023.7 47,278.8 47,019.9 47,512.0 47,394.1 46,977.9 47,288.7 49,519.8 47,511.1 49,750.8 48,766.9 48,110.9 47,512.0 45,997.7 46,840.8 47,394.1 44,773.8 44,447.0 49,253.8 47,941.3 47,165.8 48,162.4 47,626.5 48,977.5 47,941.3 48,614.0 48,110.9 48,942.5 47,626.5 49,177.8 44,448.8

52,388.7 52,425.8 50,049.5 52,254.7 51,618.8 52,067.8 51,967.7 52,388.7 51,309.6 52,403.7 51,967.7 52,390.8 52,340.7 50,413.7 52,254.7 52,418.8 52,425.8 51,660.8 52,067.8 51,927.8 51,967.7 52,067.8 51,211.6 52,425.8 52,425.8 51,732.8 51,751.8 52,425.8 52,403.7 51,883.6 51,939.6 51,927.8 49,330.5 52,388.7 49,662.8 51,927.8 51,774.7 52,388.7 51,814.8 51,927.8 49,694.8 52,423.7

9 560 5 15 74 8 5 9 17 7 7 216 65 12 11 483 1546 57 12 9 6 10 4 437 550 14 58 2057 1150 15 28 25 3 12 9 20 25 7 39 10 3 801

42 12250 9 97 619 11 491 29 234 410 78 2135 475 9 100 16999 33563 988 14 34 283 10 8 6830 20914 358 501 50587 47870 368 300 44 2 32 1 181 356 25 372 23 3 22680

TABLE 2—Continued

Designation

3596

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J034423.1+155943................ J035721.9
481215............... J040221.2
314725............... J040659.0
382628............... J040748.4
121136............... J040820.3
654509............... J040905.7
123848............... J042356.0+415002................ J042747.5+045708................ J043221.1
510925............... J044017.1
433308............... J044331.6+344106................ J044923.3+633209................ J045314.6
280737............... J045550.7
461558............... J050112.8
015914............... J050215.4+060907................ J050401.7
604952............... J050927.4+101144................ J051349.1
215916............... J051637.7
723707............... J052234.4
610757............... J052531.4
455754............... J053056.4+133155................ J053238.9+073243................ J053932.0
155030............... J055530.8+394849................ J060309.1+174216................ J060752.6+672055................ J060759.6
083449............... J060940.9
154240............... J061357.6+130645................ J061635.9
345616............... J061732.3
363414............... J062331.7
441302............... J062419.0+385648................ J063920.9
334600............... J064524.0+212151................ J064814.0
304419............... J064848.4
473427............... J065358.2+370540................ J070001.5+170921................ J070134.5
463436...............

 (J2000.0)

Source 0341+158 0355
483 0400
319 0405
385 0405
123 0407
658 0406
127 0420+417 0425+048 0431
512 0438
436 0440+345 0444+634 0451
282 0454
463 0458
020 0459+060 0503
608 0506+101 0511
220 0517
726 0522
611 0524
460 0528+134 0529+075 0537
158 0552+398 0600+177 0602+673 0605
085 0607
157 0611+131 0614
349 0615
365 0622
441 0620+389 0637
337 0642+214 0646
306 0647
475 0650+371 0657+172 0700
465

03 03 04 04 04 04 04 04 04 04 04 04 04 04 04 05 05 05 05 05 05 05 05 05 05 05 05 06 06 06 06 06 06 06 06 06 06 06 06 06 06 07 07

44 57 02 06 07 08 09 23 27 32 40 43 49 53 55 01 02 04 09 13 16 22 25 30 32 39 55 03 07 07 09 13 16 17 23 24 39 45 48 48 53 00 01

23.172170 21.917864 21.266005 59.035329 48.430983 20.378844 05.769722 56.009789 47.570538 21.178272 17.179986 31.635198 23.310571 14.646784 50.772484 12.809883 15.445931 01.701144 27.457063 49.114327 37.719082 34.425506 31.400154 56.416742 38.998495 32.010136 30.805610 09.130270 52.671604 59.699237 40.949529 57.692755 35.981373 32.323915 31.786247 19.021311 20.904636 24.099514 14.096457 48.451521 58.282827 01.525536 34.547049


(J2000.0) 15 59
48 12
31 47
38 26
12 11
65 45
12 38 41 50 04 57
51 09
43 33 34 41 63 32
28 07
46 15
01 59 06 09
60 49 10 11
21 59
72 37
61 07
45 57 13 31 07 32
15 50 39 48 17 42 67 20
08 34
15 42 13 06
34 56
36 34
44 13 38 56
33 46 21 21
30 44
47 34 37 05 17 09
46 34

43.36950 15.16027 25.94551 28.04227 36.65953 09.08062 48.14395 02.71299 08.32565 25.18663 08.60392 06.66410 09.43396 37.32744 58.68203 14.25623 07.49403 52.53888 44.60013 16.09202 07.46590 57.13356 54.68506 55.14955 43.34553 30.32129 49.16501 16.81065 55.40985 49.97831 40.67260 45.40102 16.56450 14.80330 02.54243 48.73599 00.11339 51.20155 19.65953 27.18574 40.60646 21.70154 36.62134

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000018 0.000078 0.000020 0.000018 0.000018 0.000850 0.000018 0.000018 0.000025 0.000121 0.000022 0.000017 0.000018 0.000019 0.000052 0.000017 0.000018 0.000153 0.000018 0.000023 0.000199 0.000062 0.000045 0.000017 0.000064 0.000021 0.000017 0.000018 0.000018 0.000017 0.000017 0.000018 0.005235 0.004367 0.000624 0.000020 0.000045 0.000019 0.000019 0.004920 0.000018 0.000017 0.000456

0.00030 0.00200 0.00036 0.00029 0.00031 0.00541 0.00032 0.00027 0.00039 0.00179 0.00034 0.00026 0.00025 0.00031 0.00255 0.00025 0.00030 0.00085 0.00029 0.00040 0.00071 0.00062 0.00049 0.00025 0.00088 0.00046 0.00025 0.00028 0.00025 0.00027 0.00026 0.00031 0.03786 0.03911 0.00205 0.00031 0.00048 0.00030 0.00033 0.01977 0.00029 0.00025 0.00382

0.047 0.738
0.330 0.088 0.067
0.204
0.462
0.151
0.765 0.603
0.114
0.062
0.073
0.309
0.260
0.090
0.073 0.542
0.356
0.387 0.307 0.368
0.005
0.071 0.169 0.012
0.049
0.518
0.050
0.221
0.155
0.182
0.943
0.966
0.255
0.109 0.000
0.480
0.254
0.729
0.133
0.136
0.342

50,687.1 49,126.0 50,937.7 50,939.4 50,884.4 49,383.7 50,476.5 50,232.0 50,095.3 49,571.6 49,557.5 50,627.3 50,846.0 50,908.1 49,015.7 50,230.8 50,895.5 49,374.8 50,601.0 50,055.5 49,590.9 48,610.6 50,568.0 49,344.6 49,079.3 50,486.5 49,545.9 50,449.1 51,631.9 50,741.4 51,173.2 50,558.8 48,766.9 48,766.9 48,419.8 50,085.2 50,332.4 50,097.4 50,068.3 48,766.9 49,879.1 50,614.4 48,162.4

47,394.1 48,162.4 48,766.9 48,162.4 48,766.9 48,162.4 47,777.3 47,568.6 47,394.1 47,626.5 47,941.3 47,605.1 49,422.9 47,176.5 49,015.7 44,773.8 47,394.1 48,110.9 47,394.1 47,176.5 48,757.4 47,626.5 49,750.8 44,448.8 44,773.8 47,777.3 44,090.5 47,394.1 49,750.8 44,773.8 45,466.3 47,394.1 48,766.9 48,766.9 48,162.4 49,391.7 47,511.1 47,605.1 47,640.2 48,766.9 48,348.6 47,517.4 48,162.4

51,732.8 51,074.2 52,388.7 52,254.7 51,927.8 50,259.6 51,883.6 51,981.6 51,687.6 52,067.8 51,927.8 51,967.7 51,569.8 52,254.7 49,407.6 52,425.8 52,340.7 52,388.7 51,841.6 51,617.6 51,211.6 50,324.8 52,254.7 52,425.8 50,460.3 52,353.7 52,425.8 51,927.8 52,423.7 52,403.7 52,423.7 51,841.6 50,765.8 49,909.6 50,413.7 51,120.7 51,927.8 50,460.3 52,254.7 52,067.8 50,849.8 52,017.7 49,909.6

29 7 11 121 9 4 27 21 22 8 27 39 7 19 2 1351 14 5 35 19 5 4 10 2637 19 21 3107 36 17 40 34 16 5 3 6 9 9 23 16 3 40 163 3

278 13 214 511 260 14 218 470 147 10 119 1407 1102 181 6 31123 225 23 314 96 15 17 67 85493 75 142 205219 385 2660 660 765 228 2 3 6 179 41 251 307 2 268 2834 3

TABLE 2—Continued

Designation

3597

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J072905.4
363945............... J073816.9
332212............... J073918.0+013704................ J074202.7+490015................ J074331.6
672625............... J075052.0+123104................ J080815.5
075109............... J081108.8
492943............... J081126.7+014652................ J081815.9+422245................ J082526.8
501038............... J082538.6+615728................ J082550.3+030924................ J082601.5
223027............... J083052.0+241059................ J083322.3
444138............... J083520.6
451035............... J084124.3+705342................ J084127.0
754027............... J085448.8+200630................ J090216.8
141530............... J090910.0+012135................ J091437.9+024559................ J092246.4
395935............... J092314.4+384939................ J092751.8
203451............... J093032.5
853359............... J095524.7+690113................ J100159.9
443800............... J100614.0
501813............... J101353.4+244916................ J101725.8+611627................ J102232.7
103744............... J102444.8+191220................ J103507.0+562846................ J104142.9
474006............... J104455.9+065538................ J104827.6+714335................ J105104.7
313814............... J105653.6+701145................ J110331.5
325116 ............... J110352.2
535700............... J110427.3+381231................

Source 0727
365 0736
332 0736+017 0738+491 0743
673 0748+126 0805
077 0809
493 0808+019 0814+425 0823
500 0821+621 0823+033 0823
223 0827+243 0831
445 0833
450 0836+710 0842
754 0851+202 0859
140 0906+015 0912+029 0920
397 0920+390 0925
203 0936
853 0951+692 0959
443 1004
500 1011+250 1014+615 1020
103 1022+194 1031+567 1039
474 1042+071 1044+719 1048
313 1053+704 1101
325 1101
536 1101+384

 (J2000.0) 07 29 07 38 07 39 07 42 07 43 07 50 08 08 08 11 08 11 08 18 08 25 08 25 08 25 08 26 08 30 08 33 08 35 08 41 08 41 08 54 09 02 09 09 09 14 09 22 09 23 09 27 09 30 09 55 10 01 10 06 10 13 10 17 10 22 10 24 10 35 10 41 10 44 10 48 10 51 10 56 11 03 11 03 11 04

05.423794 16.949119 18.033892 02.748944 31.611577 52.045726 15.536039 08.803182 26.707316 15.999605 26.869000 38.612198 50.338352 01.572919 52.086185 22.315765 20.655245 24.365262 27.034834 48.874925 16.830910 10.091591 37.913422 46.418258 14.452928 51.824335 32.570404 24.774759 59.908265 14.008948 53.428766 25.887576 32.805317 44.809590 07.040267 42.936401 55.911246 27.619915 04.777525 53.617491 31.526436 52.221668 27.313940


(J2000.0)
36
33 01 49
67 12
07
49 01 42
50 61 03
22 24
44
45 70
75 20
14 01 02
39 38
20
85 69
44
50 24 61
10 19 56
47 06 71
31 70
32
53 38

39 22 37 00 26 31 51 29 46 22 10 57 09 30 10 41 10 53 40 06 15 21 45 59 49 34 33 01 38 18 49 16 37 12 28 40 55 43 38 11 51 57 12

45.28489 12.77760 04.61794 15.60894 25.54638 04.82826 09.88648 43.51013 52.22016 45.41497 38.48765 28.57939 24.52011 27.20401 59.82046 38.71499 35.15446 42.17301 27.87080 30.64089 30.87562 35.61776 59.24657 35.06796 39.91020 51.23290 59.69337 13.70260 00.60452 13.46899 16.44087 27.49669 44.36739 20.41561 46.79733 06.49535 38.26287 35.93832 14.30758 45.91575 16.69199 00.69641 31.79914

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.017806 0.000143 0.000017 0.000022 0.000152 0.000017 0.000017 0.000775 0.000017 0.000017 0.000207 0.000020 0.000017 0.000065 0.000017 0.000622 0.001736 0.000018 0.002915 0.000017 0.000018 0.000018 0.000018 0.000017 0.000017 0.000021 0.003119 0.000094 0.000490 0.001219 0.000017 0.000018 0.006224 0.000017 0.000728 0.011730 0.000019 0.000042 0.000040 0.000018 0.000049 0.000020 0.000017

0.09417 0.00123 0.00026 0.00029 0.00055 0.00026 0.00029 0.00530 0.00027 0.00025 0.00134 0.00026 0.00025 0.00132 0.00025 0.00376 0.01137 0.00025 0.00232 0.00025 0.00030 0.00030 0.00030 0.00028 0.00026 0.00036 0.00338 0.00064 0.00524 0.01258 0.00026 0.00025 0.02141 0.00026 0.00449 0.11438 0.00035 0.00032 0.00044 0.00025 0.00121 0.00029 0.00025


0.990
0.424
0.312
0.099
0.194
0.091
0.350 0.200
0.128
0.038
0.480 0.195
0.062
0.359
0.139
0.257
0.410 0.018
0.509
0.058
0.313
0.154
0.174
0.291
0.067
0.356 0.064 0.117
0.940
0.981
0.046 0.062 0.836
0.015 0.000
0.999
0.290 0.000
0.045
0.022 0.109 0.167
0.088

48,766.9 49,497.2 50,412.8 50,715.6 48,600.5 50,215.1 50,481.5 49,188.0 50,565.7 49,148.8 48,968.6 51,406.1 50,445.3 48,500.1 50,945.3 49,027.6 48,132.8 51,250.3 48,205.3 49,151.4 50,466.2 49,706.9 50,629.5 51,122.5 50,550.8 50,271.6 48,887.3 49,238.2 48,757.8 50,379.3 51,351.8 50,812.8 49,650.8 51,034.6 48,461.1 49,535.0 50,635.8 50,878.5 49,078.6 51,160.6 51,115.8 49,632.6 51,234.5

48,766.9 48,766.9 44,773.8 49,750.8 48,110.9 44,773.8 47,176.5 47,626.5 46,977.9 45,138.8 48,162.4 49,422.9 45,466.3 46,875.8 47,023.7 48,043.8 48,043.8 46,977.9 48,110.9 44,203.7 46,875.8 47,005.8 47,407.6 47,640.2 49,736.9 47,777.3 48,162.4 49,141.8 47,626.5 49,535.0 48,353.6 49,422.9 49,650.8 47,407.6 47,023.6 49,535.0 47,407.6 47,605.0 47,640.2 49,125.7 47,511.1 47,626.5 49,519.8

52,254.7 51,288.7 51,883.6 51,386.3 50,324.8 52,415.7 51,981.6 52,067.8 52,423.7 51,667.8 52,388.7 51,687.6 52,403.7 52,067.8 52,291.8 50,413.7 51,116.2 52,039.8 49,330.5 52,423.7 51,617.6 52,403.7 51,309.6 52,403.7 51,751.8 52,254.7 49,650.8 49,267.8 50,765.8 52,254.7 52,340.7 51,731.8 52,067.8 51,772.8 49,690.0 49,535.0 51,841.6 52,423.7 52,388.7 52,340.7 52,254.7 52,388.7 52,425.8

3 6 49 5 5 25 34 7 38 147 10 12 841 12 64 7 4 11 4 2853 24 28 11 198 44 40 5 4 8 3 23 19 3 32 15 1 13 546 9 30 10 37 236

1 11 702 241 17 686 311 7 576 2206 24 678 27873 22 1393 15 4 1764 8 102867 326 384 213 1341 627 153 15 96 9 3 690 944 2 1673 93 1 171 58653 45 1302 24 308 7290

TABLE 2—Continued

Designation

3598

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J110712.6
682050............... J111826.9
463415 ............... J112027.8+142054................ J112704.3
185717............... J112813.3+592514................ J113007.0
144927............... J113130.5
050019............... J113143.2
581853............... J114701.3
381211 ............... J114751.5
072441............... J115043.8
002354............... J115912.7
094052............... J115931.8+291443................ J120935.2
401613............... J121546.7
173145............... J121806.2
460029............... J122131.6+281358................ J122452.4+033050................ J122454.3
831310............... J123049.4+122328................ J123715.2
504623............... J123943.0
102328............... J123959.4
113722............... J124251.3+375100................ J124604.2
073046............... J124646.8
254749............... J125359.5
405930............... J125614.2+565225................ J125759.0
315516............... J130533.0
103319............... J130933.9+115424................ J131607.9
333859............... J131736.4+342515................ J132304.2
445233............... J132527.6
430108............... J132616.5+315409................ J133108.2+303032................ J133237.5
664650............... J133752.4
650924............... J134022.9+375443................ J134733.3+121724................ J135256.5
441240............... J135406.8
020603...............

 (J2000.0)

Source 1105
680 1116
462 1117+146 1124
186 1125+596 1127
145 1128
047 1129
580 1144
379 1145
071 1148
001 1156
094 1156+295 1206
399 1213
172 1215
457 1219+285 1222+037 1221
829 1228+126 1234
504 1237
101 1237
113 1240+381 1243
072 1244
255 1251
407 1254+571 1255
316 1302
102 1307+121 1313
333 1315+346 1320
446 1322
427 1323+321 1328+307 1329
665 1334
649 1338+381 1345+125 1349
439 1351
018

11 11 11 11 11 11 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 13 13 13 13 13 13 13 13 13 13 13 13 13 13

07 18 20 27 28 30 31 31 47 47 50 59 59 09 15 18 21 24 24 30 37 39 39 42 46 46 53 56 57 05 09 16 17 23 25 26 31 32 37 40 47 52 54

12.695013 26.957570 27.807260 04.392446 13.340670 07.052577 30.516743 43.287619 01.370701 51.554030 43.870755 12.711732 31.833909 35.243542 46.751767 06.252055 31.690521 52.421940 54.384057 49.423380 15.239447 43.061500 59.431845 51.369065 04.232105 46.802037 59.533576 14.233870 59.060806 33.015026 33.932435 07.985934 36.494181 04.246159 27.615074 16.512109 08.288109 37.517746 52.444713 22.951801 33.361621 56.534906 06.895317

Epoch of Observation (MJD)


(J2000.0)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs


68 20 50.72766
46 34 15.00180 14 20 54.99142
18 57 17.44159 59 25 14.79859
14 49 27.38811
05 00 19.65729
58 18 53.44358
38 12 11.02338
07 24 41.14105
00 23 54.20513
09 40 52.04879 29 14 43.82694
40 16 13.10039
17 31 45.40296
46 00 29.00945 28 13 58.50024 03 30 50.29291
83 13 10.10672 12 23 28.04394
50 46 23.17437
10 23 28.69251
11 37 22.99545 37 51 00.02540
07 30 46.57454
25 47 49.28879
40 59 30.68921 56 52 25.23665
31 55 16.85161
10 33 19.42797 11 54 24.55316
33 38 59.17259 34 25 15.93259
44 52 33.85332
43 01 08.80533 31 54 09.51597 30 30 32.96001
66 46 50.45201
65 09 24.89879 37 54 43.83361 12 17 24.23994
44 12 40.38740
02 06 03.19038

0.000177 0.000050 0.000603 0.000017 0.000021 0.000018 0.000022 0.000832 0.000017 0.000017 0.000018 0.000025 0.000017 0.000727 0.000017 0.000951 0.000018 0.000054 0.004688 0.000017 0.001240 0.000017 0.000055 0.000018 0.000017 0.000017 0.000169 0.000054 0.000017 0.000017 0.000018 0.000017 0.000019 0.000482 0.000182 0.000422 0.000033 0.017558 0.001095 0.000024 0.000068 0.000031 0.000017

0.00095 0.00060 0.00853 0.00025 0.00027 0.00030 0.00046 0.00866 0.00025 0.00025 0.00032 0.00046 0.00025 0.00212 0.00028 0.00729 0.00027 0.00120 0.00966 0.00025 0.00497 0.00028 0.00231 0.00026 0.00028 0.00027 0.00481 0.00060 0.00026 0.00028 0.00032 0.00026 0.00030 0.00288 0.00525 0.00566 0.00048 0.20675 0.00429 0.00046 0.00212 0.00033 0.00025

0.747 0.333 0.000
0.032 0.124
0.220
0.053 0.773 0.025
0.132
0.214
0.458
0.047 0.415
0.066
0.836
0.231 0.000
0.486
0.063
0.666
0.194
0.825
0.218
0.226
0.294 0.518 0.274 0.051
0.180 0.085
0.101
0.248 0.020 0.000 0.000 0.349 0.612
0.271
0.280 0.000
0.517 0.029

49,927.1 50,339.1 49,316.2 51,278.4 51,062.4 49,633.1 50,225.0 49,535.0 50,879.3 50,988.1 49,862.0 50,327.7 50,197.5 47,830.8 50,943.5 48,746.5 49,620.6 49,075.5 48,043.8 51,276.1 49,679.7 51,279.9 50,250.2 51,286.7 50,563.1 49,635.1 49,814.0 50,499.4 50,887.9 50,591.9 50,616.8 51,044.3 50,319.9 49,191.2 50,158.7 49,394.3 50,707.2 48,766.9 48,969.7 50,930.2 49,387.0 49,697.9 51,296.3

48,388.4 48,110.9 49,098.5 46,875.8 49,422.9 45,259.2 49,099.7 49,535.0 47,654.0 47,176.5 47,023.7 47,777.3 46,977.9 47,511.1 46,840.8 48,162.4 44,447.0 46,502.7 47,626.5 46,502.8 48,766.9 49,398.5 49,883.8 49,429.9 47,176.5 46,875.8 49,629.6 49,690.0 47,640.2 47,176.5 48,378.8 47,415.7 47,946.4 48,766.9 48,110.9 48,223.6 48,357.8 48,766.9 47,626.5 48,942.5 47,659.7 48,110.9 48,573.8

52,388.7 52,353.7 49,533.8 52,423.7 51,393.6 51,927.8 51,120.7 49,535.0 52,425.8 52,403.7 51,939.6 51,687.6 52,425.8 51,074.2 52,254.7 49,960.8 52,340.7 51,499.7 51,386.3 52,425.8 52,067.8 51,883.6 51,002.8 51,927.8 51,927.8 51,883.6 51,107.8 51,386.3 52,402.7 51,709.7 52,401.7 52,403.7 52,039.8 51,386.3 51,927.8 51,386.3 51,841.6 48,766.9 49,895.6 52,039.8 52,408.7 51,927.8 52,425.8

6 11 2 482 4 33 8 1 519 139 24 25 632 5 18 3 39 65 4 439 7 24 14 6 43 103 8 5 242 58 13 343 29 8 7 7 11 1 6 20 11 25 526

36 36 4 8386 362 376 172 2 3927 5291 326 138 18546 10 341 6 552 603 4 14266 10 482 33 439 506 697 9 50 1346 496 271 3003 315 19 44 91 136 1 10 160 187 78 5129

TABLE 2—Continued

Designation

3599

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J135546.6
632642............... J135711.2
152728............... J135755.3+764321................ J135900.1
415252............... J140445.8
013021............... J140501.1+041535................ J140856.4
075226............... J141154.8+213423................ J141946.6+382148................ J141959.2+270625................ J142230.3+322310................ J142700.3+234800................ J143257.6
180135............... J143439.7+195200................ J143535.4+301224................ J143809.4
220454............... J144553.3
162901............... J145427.4
374733............... J145432.9
401232............... J150506.4+032630................ J150704.7
165230............... J151002.9+570243................ J151344.8
101200............... J151741.8
242219............... J152237.6
273010............... J154929.4+023701................ J155059.1
825806............... J155751.4
000150............... J155821.9
140959............... J155930.9+030448................ J160140.4+431647................ J160140.5+431646................ J160431.0
444131............... J161341.0+341247................ J161637.5+045932................ J162606.0
295126............... J165039.5
294346............... J165352.2+394536................ J165802.7+473749................ J165809.0+074127................ J165833.4+051516................ J170053.1
261051............... J170717.7+453610................

Source

 (J2000.0)

1352
632 1354
152 1357+769 1355
416 1402
012 1402+044 1406
076 1409+218 1417+385 1417+273 1420+326 1424+240 1430
178 1432+200 1433+304 1435
218 1443
162 1451
375 1451
400 1502+036 1504
166 1508+572 1511
100 1514
241 1519
273 1546+027 1540
828 1555+001 1555
140 1557+032 1600+432 1600+431 1600
445 1611+343 1614+051 1622
297 1647
296 1652+398 1656+477 1655+077 1656+053 1657
261 1705+456

13 55 46.611679 13 57 11.244974 13 57 55.371526 13 59 00.183920 14 04 45.895462 14 05 01.119813 14 08 56.481191 14 11 54.862196 14 19 46.613756 14 19 59.297071 14 22 30.378955 14 27 00.391788 14 32 57.690638 14 34 39.793348 14 35 35.402172 14 38 09.469407 14 45 53.376292 14 54 27.409754 14 54 32.912354 15 05 06.477149 15 07 04.786961 15 10 02.922356 15 13 44.893425 15 17 41.813128 15 22 37.675984 15 49 29.436846 15 50 59.142390 15 57 51.433968 15 58 21.948119 15 59 30.972608 16 01 40.443911 16 01 40.515418 16 04 31.020692 16 13 41.064243 16 16 37.556813 16 26 06.020839 16 50 39.544132 16 53 52.216683 16 58 02.779590 16 58 09.011464 16 58 33.447348 17 00 53.154066 17 07 17.753437


(J2000.0)
63
15 76
41
01 04
07 21 38 27 32 23
18 19 30
22
16
37
40 03
16 57
10
24
27 02
82
00
14 03 43 43
44 34 04
29
29 39 47 07 05
26 45

26 27 43 52 30 15 52 34 21 06 23 48 01 52 12 04 29 47 12 26 52 02 12 22 30 37 58 01 09 04 16 16 41 12 59 51 43 45 37 41 15 10 36

42.57601 28.78657 21.05108 52.63850 21.94734 35.81902 26.66623 23.43734 48.47508 25.55269 10.44038 00.03756 35.24885 00.73608 24.51916 54.74812 01.61885 33.14475 32.51438 30.81280 30.26725 43.37596 00.26446 19.47597 10.78541 01.16363 06.84489 50.41365 59.05193 48.25704 47.75691 46.47611 31.97421 47.90911 32.73664 26.97115 46.95480 36.60897 49.23098 27.54075 16.44446 51.72537 10.55284

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000449 0.000017 0.000018 0.001317 0.000019 0.000017 0.000017 0.000024 0.000017 0.000019 0.000020 0.000021 0.000027 0.000018 0.000040 0.000020 0.000021 0.000017 0.000019 0.000018 0.000017 0.000018 0.000018 0.000017 0.000017 0.000017 0.002891 0.000017 0.000033 0.000018 0.000114 0.000157 0.000185 0.000017 0.000017 0.000018 0.000029 0.000017 0.000018 0.000017 0.000018 0.000017 0.000020

0.00286 0.00028 0.00025 0.00980 0.00035 0.00028 0.00027 0.00061 0.00026 0.00030 0.00032 0.00039 0.00044 0.00028 0.00060 0.00033 0.00034 0.00027 0.00031 0.00030 0.00027 0.00026 0.00030 0.00026 0.00026 0.00027 0.00635 0.00026 0.00066 0.00030 0.00184 0.00453 0.00159 0.00025 0.00027 0.00028 0.00039 0.00025 0.00026 0.00028 0.00028 0.00027 0.00029

0.249
0.314
0.026
0.449 0.052
0.191
0.272 0.124
0.043
0.183
0.247
0.233
0.732
0.044 0.152
0.487
0.637 0.069 0.110
0.218
0.313 0.003
0.288
0.136
0.085
0.209
0.019
0.238
0.270
0.113 0.203 0.264 0.060 0.029
0.263
0.059
0.651
0.047
0.119
0.187 0.071
0.300
0.229

49,927.2 49,631.8 51,036.8 49,613.3 51,240.7 50,212.6 50,884.2 50,491.5 51,405.5 50,800.6 51,298.1 50,862.2 50,024.9 51,137.9 50,272.3 50,338.7 50,361.7 51,162.1 50,878.7 50,835.2 50,453.5 50,401.7 50,434.9 51,227.0 50,582.4 50,621.0 48,779.1 50,733.7 50,589.4 50,360.6 50,184.6 49,883.8 51,348.1 49,902.7 50,394.4 49,940.7 50,006.4 50,892.8 51,004.3 50,549.0 48,908.8 51,221.1 50,657.2

49,535.0 46,875.8 47,011.4 48,110.9 48,643.8 48,888.7 47,176.5 48,863.2 49,750.8 48,863.2 48,863.2 48,863.2 48,160.3 48,863.2 48,863.2 47,176.5 47,941.3 47,511.1 47,640.2 48,853.8 46,840.8 49,541.8 46,875.8 46,840.8 46,875.8 47,005.8 47,626.5 44,773.8 48,353.6 49,541.8 49,883.8 49,883.8 49,535.0 44,773.8 47,605.1 46,840.8 48,346.0 45,997.8 48,853.8 47,407.6 44,773.8 46,875.8 48,434.7

52,048.6 51,541.8 52,425.8 52,067.8 52,353.7 51,660.8 51,981.6 51,617.6 52,423.7 51,687.6 51,732.8 51,939.6 50,459.3 52,403.7 51,939.6 51,386.3 51,927.8 52,403.7 52,403.7 51,687.6 52,353.7 51,731.8 51,883.6 52,403.7 52,017.7 51,981.6 49,330.5 52,376.8 52,067.8 51,674.8 50,560.6 50,560.6 52,254.7 52,425.8 52,017.7 51,681.7 52,388.7 52,425.8 51,732.8 51,617.6 51,687.6 52,403.7 51,981.6

4 132 930 6 15 47 44 5 14 5 5 5 17 8 5 31 21 198 39 9 70 49 29 261 240 43 6 299 20 28 3 3 2 1096 146 57 19 228 6 19 48 77 11

7 615 81242 8 242 658 457 103 754 212 217 160 88 441 138 154 166 1560 363 343 678 1264 310 5228 1578 707 7 1566 76 350 9 3 3 49472 1054 295 51 3236 434 396 845 993 323

TABLE 2—Continued

Designation

3600

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J173315.1
372232............... J173420.5+385751................ J173549.0+504911................ J173735.7
563403............... J173927.3+495503................ J173957.1+473758................ J174036.9+521143................ J174425.4
514443............... J174726.6+465850................ J175132.8+093900................ J175151.2
252400............... J180024.7+384830................ J180045.6+782804................ J180650.6+694928................ J180821.8+454220................ J180957.8
455241............... J181935.0
634548............... J182057.8
252812............... J182402.8+104423................ J183537.2
714958............... J183728.7
710843............... J191109.6
200655............... J192332.1
210433............... J192559.6+210626................ J192840.8+084848................ J193006.1
605609............... J193124.9+224331................ J193435.0+104340................ J193510.4+203154................ J193716.2
395801............... J193925.0
634245............... J193926.6
152543............... J193957.2
100241............... J194121.7
621121............... J194606.2+230004................ J195330.8+353759................ J195740.5+333827................ J200925.3
484953............... J202510.8+334300................ J204008.7
250746............... J205741.6
373402............... J210159.1
421916............... J210217.0+470216................

Source 1729
373 1732+389 1734+508 1733
565 1738+499 1738+476 1739+522 1740
517 1746+470 1749+096 1748
253 1758+388 1803+784 1807+698 1806+456 1806
458 1814
637 1817
254 1821+107 1829
718 1831
711 1908
201 1920
211 1923+210 1926+087 1925
610 1929+226 1932+106 1932+204 1933
400 1934
638 1936
155 1937
101 1936
623 1943+228 1951+355 1955+335 2005
489 2023+335 2037
253 2054
377 2058
425 2100+468

 (J2000.0) 17 33 17 34 17 35 17 37 17 39 17 39 17 40 17 44 17 47 17 51 17 51 18 00 18 00 18 06 18 08 18 09 18 19 18 20 18 24 18 35 18 37 19 11 19 23 19 25 19 28 19 30 19 31 19 34 19 35 19 37 19 39 19 39 19 39 19 41 19 46 19 53 19 57 20 09 20 25 20 40 20 57 21 01 21 02

15.192622 20.578538 49.005174 35.770329 27.390502 57.129077 36.977848 25.450768 26.647317 32.818570 51.263059 24.765366 45.683914 50.680651 21.885879 57.871844 35.002407 57.848703 02.855250 37.205070 28.714947 09.652890 32.189816 59.605362 40.855489 06.160061 24.916782 35.025543 10.472913 16.217370 25.026708 26.657743 57.256565 21.768656 06.251406 30.875706 40.549919 25.390704 10.842101 08.772891 41.603451 59.114192 17.056141


(J2000.0)
37 38 50
56 49 47 52
51 46 09
25 38 78 69 45
45
63
25 10
71
71
20
21 21 08
60 22 10 20
39
63
15
10
62 23 35 33
48 33
25
37
42 47

22 57 49 34 55 37 11 44 58 39 24 48 28 49 42 52 45 28 44 49 08 06 04 06 48 56 43 43 31 58 42 25 02 11 00 37 38 49 43 07 34 19 02

32.39667 51.44323 11.56587 03.15597 03.36838 58.36160 43.40749 43.79296 50.92641 00.72855 00.06067 30.69753 04.01849 28.10859 20.86631 41.01414 48.19259 12.58454 23.77425 58.21851 43.55455 55.10892 33.33306 26.16212 48.41332 09.18449 31.25906 40.36559 54.15418 01.55307 45.62549 43.05828 41.52086 21.05683 04.41419 59.35986 27.94333 53.72146 00.21437 46.66290 02.99044 16.16174 16.25393

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000587 0.000017 0.000028 0.000387 0.000019 0.000018 0.000017 0.000519 0.000021 0.000017 0.000457 0.000017 0.000018 0.000017 0.000019 0.000158 0.000135 0.000027 0.000018 0.001044 0.000032 0.000017 0.000017 0.000017 0.000018 0.000056 0.000019 0.000050 0.000018 0.000023 0.000344 0.000017 0.000018 0.000352 0.000022 0.000024 0.000034 0.000046 0.000020 0.000031 0.000031 0.000209 0.000241

0.00376 0.00026 0.00040 0.00196 0.00027 0.00026 0.00025 0.00337 0.00029 0.00025 0.00488 0.00026 0.00025 0.00025 0.00029 0.00093 0.00379 0.00053 0.00029 0.00448 0.00030 0.00025 0.00027 0.00026 0.00036 0.00064 0.00032 0.00171 0.00029 0.00032 0.00174 0.00027 0.00033 0.00445 0.00050 0.00043 0.00044 0.00050 0.00028 0.00040 0.00056 0.00329 0.00351


0.298
0.063 0.319 0.761
0.145 0.081 0.045 0.684 0.090 0.073 0.170
0.034 0.025 0.053
0.061
0.630
0.431
0.051
0.178 0.411
0.006
0.052
0.321
0.046
0.291
0.102
0.036
0.289
0.088
0.519 0.010
0.228
0.114 0.541
0.386 0.456
0.504 0.241
0.235
0.137
0.152 0.568 0.055

49,650.8 50,108.9 49,518.6 49,898.5 50,612.1 50,427.3 49,829.0 49,041.3 50,419.4 50,426.2 48,917.2 50,734.6 49,191.4 51,264.7 50,894.7 51,584.2 49,220.9 50,433.4 49,946.8 49,393.6 49,077.0 51,296.0 50,478.1 49,798.9 51,450.0 49,469.6 50,084.7 50,084.6 51,155.2 50,107.1 49,338.5 50,771.4 50,496.1 48,810.1 51,096.0 50,813.8 50,669.0 49,653.7 50,726.7 50,576.7 50,617.4 49,017.6 49,504.3

49,629.6 46,977.9 49,429.9 48,388.4 49,422.9 47,288.7 47,165.8 48,388.4 49,422.9 44,447.0 48,093.0 49,429.9 45,138.8 45,997.8 49,422.9 49,629.6 48,162.4 47,407.6 45,466.3 48,766.9 47,626.5 46,840.8 47,407.6 45,138.8 49,541.8 47,626.5 48,614.0 49,541.8 48,804.9 47,640.2 48,766.9 47,176.5 48,110.9 48,162.4 48,741.7 48,614.0 47,946.4 47,626.5 45,775.8 47,176.5 48,162.4 48,162.4 48,949.1

52,254.7 51,351.6 51,074.2 52,067.8 51,386.3 51,774.7 52,403.7 50,413.7 51,927.8 52,425.8 52,067.8 51,967.7 52,423.7 52,425.8 51,870.7 52,254.7 49,895.6 52,353.7 52,291.8 52,048.6 52,388.7 52,425.8 51,723.7 52,423.7 52,340.7 52,388.7 51,927.8 52,353.7 51,774.7 52,039.8 51,211.6 51,695.8 51,688.8 52,048.6 51,732.8 52,403.7 51,841.6 52,402.7 51,981.6 52,254.7 51,575.6 52,067.8 49,690.0

5 48 3 7 7 18 1410 4 8 1632 13 24 1979 278 15 4 4 16 25 6 22 450 86 121 9 9 14 7 12 30 4 57 17 6 12 17 16 26 15 13 9 6 13

2 700 94 16 330 827 95905 6 248 51764 11 720 107017 6483 268 13 11 62 562 10 213 7714 650 1308 306 38 325 30 435 181 13 645 357 6 145 184 101 63 254 70 62 11 12

TABLE 2—Continued

Designation

3601

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF a

J211810.5
301911 ............... J212912.1
153841............... J213410.3
015317............... J214710.1+092946................ J214712.7
753613............... J215137.8+055212................ J215705.9
694123............... J215852.0
301332............... J221302.4
252930............... J221438.5
383545............... J221620.0+351814................ J222547.2
045701............... J222940.0
083254............... J223040.2
394252............... J223622.4+282857................ J223634.0
143322............... J224838.6
323552............... J225504.2
084404............... J225536.7+420252................ J225717.5+024317................ J230223.8
371806............... J230305.8
303011............... J230343.5
680737............... J231409.3
445549............... J232044.8+051349................ J232331.9
031705............... J232747.9
144755............... J233040.8+110018................ J233355.2
234340............... J233612.1
523621............... J233757.3
023057............... J234636.8+093045................ J235430.1
151311 ............... J235600.6
682003............... J235753.2
531113 ............... J235810.8
102008............... J235933.1+385042................

 (J2000.0)

Source 2115
305 2126
158 2131
021 2144+092 2142
758 2149+056 2152
699 2155
304 2210
257 2211
388 2214+350 2223
052 2227
088 2227
399 2234+282 2233
148 2245
328 2252
090 2253+417 2254+024 2259
375 2300
307 2300
683 2311
452 2318+049 2320
035 2325
150 2328+107 2331
240 2333
528 2335
027 2344+092 2351
154 2353
686 2355
534 2355
106 2356+385

21 18 21 29 21 34 21 47 21 47 21 51 21 57 21 58 22 13 22 14 22 16 22 25 22 29 22 30 22 36 22 36 22 48 22 55 22 55 22 57 23 02 23 03 23 03 23 14 23 20 23 23 23 27 23 30 23 33 23 36 23 37 23 46 23 54 23 56 23 57 23 58 23 59

Includes all sessions in which a source was observed.

10.597694 12.175899 10.309598 10.162968 12.730286 37.875492 05.980608 52.065107 02.497980 38.569665 20.009900 47.259291 40.084346 40.278585 22.470847 34.087151 38.685734 04.239789 36.707837 17.563103 23.890643 05.821145 43.566298 09.382803 44.856597 31.953753 47.964275 40.852252 55.237801 12.144506 57.339086 36.838542 30.195184 00.681458 53.266098 10.882405 33.180793

Epoch of Observation (MJD)


(J2000.0)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs


30 19 11.60641
15 38 41.04092
01 53 17.23863 09 29 46.67242
75 36 13.22500 05 52 12.95452
69 41 23.68549
30 13 32.11822
25 29 30.07966
38 35 45.00984 35 18 14.18060
04 57 01.39082
08 32 54.43543
39 42 52.06718 28 28 57.41325
14 33 22.18954
32 35 52.18784
08 44 04.02135 42 02 52.53279 02 43 17.51172
37 18 06.84230
30 30 11.47324
68 07 37.46225
44 55 49.23776 05 13 49.95266
03 17 05.02381
14 47 55.75140 11 00 18.70977
23 43 40.65798
52 36 21.95043
02 30 57.62925 09 30 45.51473
15 13 11.21313
68 20 03.47204
53 11 13.68938
10 20 08.61124 38 50 42.31841

0.000250 0.000017 0.000017 0.000018 0.000172 0.000017 0.000189 0.000021 0.000020 0.000177 0.000018 0.000017 0.000017 0.000048 0.000040 0.000019 0.000022 0.000018 0.000019 0.000018 0.002351 0.000554 0.011686 0.000089 0.000017 0.000017 0.000019 0.000018 0.000023 0.000130 0.000017 0.000017 0.000018 0.000073 0.000029 0.000017 0.000017

0.00263 0.00026 0.00027 0.00029 0.00057 0.00028 0.00129 0.00042 0.00044 0.00140 0.00029 0.00025 0.00027 0.00104 0.00057 0.00035 0.00032 0.00031 0.00028 0.00030 0.01459 0.00245 0.14877 0.00183 0.00026 0.00028 0.00035 0.00029 0.00035 0.00125 0.00029 0.00027 0.00029 0.00046 0.00040 0.00027 0.00025


0.743
0.062
0.307
0.157 0.297
0.362 0.810
0.148
0.221 0.187
0.147
0.039
0.325 0.590 0.000
0.450
0.564
0.256
0.105
0.396
0.780 0.194
0.994 0.092
0.236
0.437
0.121
0.321
0.690 0.546
0.206
0.010
0.449
0.220 0.255
0.270
0.046

48,904.1 50,904.1 50,282.7 49,186.8 48,289.6 50,187.3 49,095.1 51,568.4 50,810.7 50,837.9 50,894.4 51,543.3 50,228.9 48,896.9 49,558.5 50,480.5 50,362.1 50,641.6 49,557.7 50,074.9 49,180.8 48,591.5 49,650.8 48,971.1 51,080.1 50,215.9 50,810.5 49,826.2 49,533.5 49,306.2 50,703.0 50,573.6 50,442.6 49,467.6 48,861.6 49,615.9 51,336.5

48,162.4 47,176.5 44,448.8 45,997.8 47,626.5 45,466.3 48,110.9 48,766.9 46,875.8 48,766.9 49,750.8 44,448.8 45,466.3 48,162.4 45,725.7 47,176.5 47,394.1 47,394.1 47,005.8 47,394.1 48,162.4 48,110.9 49,650.8 48,162.4 47,019.9 47,394.1 47,176.5 46,977.9 46,875.8 48,110.9 47,941.3 45,725.8 47,394.1 48,162.4 47,626.5 47,394.1 49,519.8

49,398.5 52,425.8 51,939.6 51,883.6 50,324.8 52,411.8 52,048.6 52,291.8 52,254.7 52,067.8 51,774.7 52,425.8 51,814.8 49,895.6 52,423.7 51,617.6 51,927.8 52,039.8 51,927.8 51,774.7 50,049.5 49,706.7 49,650.8 50,210.5 52,423.7 51,870.7 52,353.7 51,883.6 51,576.8 50,049.5 51,841.6 52,291.8 51,282.8 52,388.7 52,388.7 51,716.8 52,425.8

4 243 68 32 5 51 5 11 13 10 7 313 77 4 1773 29 35 30 54 30 4 4 1 6 160 91 10 32 33 8 53 39 39 7 21 169 240

5 1290 568 569 14 498 15 158 189 26 292 6855 524 16 58307 194 212 309 538 350 3 6 1 11 2084 614 288 349 101 19 317 551 383 39 90 990 2541

TABLE 3 Revised Coordinates of the 102 ICRF Other Sources

Designation

3602

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J001945.7+732730................ J002232.4+060804................ J002914.2+345632................ J005041.3
092905............... J010245.7+582411................ J010645.1
403419............... J010838.7+013500................ J011612.5
113615 ............... J013741.2+330935................ J021046.2
510101............... J021730.8+734932................ J024008.1
230915............... J024104.7
081520............... J030335.2+471616................ J031948.1+413042................ J032153.1+122113................ J032957.6+275615................ J033413.6
400825............... J033630.1+321829................ J034035.6
211931............... J035929.7+505750................ J040353.7
360501............... J040534.0
130813............... J040820.3+303230................ J042315.8
012033............... J043311.0+052115................ J043701.4
184448............... J045703.1
232452............... J050321.1+020304................ J053007.9
250329............... J053850.3
440508............... J053942.3+143345................ J071338.1+434917................ J071424.8+353439................ J073019.1
114112 ............... J073807.3+174218................

 (J2000.0)

Source 0016+731 0019+058 0026+346 0048
097 0059+581 0104
408 0106+013 0113
118 0134+329 0208
512 0212+735 0237
233 0238
084 0300+470 0316+413 0319+121 0326+277 0332
403 0333+321 0338
214 0355+508 0402
362 0403
132 0405+304 0420
014 0430+052 0434
188 0454
234 0500+019 0528
250 0537
441 0536+145 0710+439 0711+356 0727
115 0735+178

00 00 00 00 01 01 01 01 01 02 02 02 02 03 03 03 03 03 03 03 03 04 04 04 04 04 04 04 05 05 05 05 07 07 07 07

19 22 29 50 02 06 08 16 37 10 17 40 41 03 19 21 29 34 36 40 59 03 05 08 23 33 37 57 03 30 38 39 13 14 30 38

45.786421 32.441215 14.242476 41.317386 45.762383 45.107958 38.771100 12.522024 41.299465 46.200407 30.813373 08.174459 04.798505 35.242225 48.160102 53.103500 57.669424 13.654486 30.107605 35.607850 29.747262 53.749894 34.003387 20.377578 15.800725 11.095519 01.482738 03.179228 21.197176 07.962790 50.361551 42.365985 38.164125 24.817473 19.112469 07.393744


(J2000.0) 73 06 34
09 58
40 01
11 33
51 73
23
08 47 41 12 27
40 32
21 50
36
13 30
01 05
18
23 02
25
44 14 43 35
11 17

27 08 56 29 24 34 35 36 09 01 49 09 15 16 30 21 56 08 18 19 57 05 08 32 20 21 44 24 03 03 05 33 49 34 41 42

30.01750 04.26883 32.24667 05.21020 11.13664 19.96031 00.31731 15.43430 35.13343 01.89203 32.62179 15.73021 20.75172 16.27550 42.10314 13.95466 15.49925 25.39793 29.34227 31.17212 50.16150 01.91327 13.69103 30.49045 33.06529 15.61916 48.61351 52.02007 04.67647 29.89953 08.93906 45.56173 17.20702 39.79609 12.60043 18.99825

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000070 0.000041 0.000320 0.000032 0.000028 0.000095 0.000057 0.000047 0.000355 0.000110 0.000104 0.000233 0.000062 0.000041 0.000186 0.000039 0.000080 0.000093 0.000036 0.000051 0.000124 0.000070 0.000023 0.000080 0.000046 0.000053 0.000052 0.000050 0.000038 0.000037 0.000088 0.000025 0.000073 0.000080 0.000035 0.000035

0.00037 0.00115 0.00262 0.00071 0.00032 0.00183 0.00105 0.00109 0.00645 0.00133 0.00047 0.00410 0.00109 0.00053 0.00356 0.00114 0.00092 0.00159 0.00056 0.00105 0.00123 0.00192 0.00042 0.00186 0.00106 0.00058 0.00148 0.00106 0.00090 0.00121 0.00138 0.00048 0.00144 0.00162 0.00072 0.00052

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

48,979.3 49,756.5 48,539.4 50,478.4 50,760.6 50,769.0 47,504.7 49,119.1 48,932.9 49,866.3 47,435.3 49,314.5 50,962.3 48,228.2 47,283.3 49,394.6 48,686.2 49,221.5 49,066.4 50,319.9 46,631.7 50,649.5 50,459.1 50,147.3 48,269.4 48,981.4 48,785.7 49,593.5 49,094.3 49,542.4 49,826.5 49,386.9 49,040.1 48,597.6 49,564.4 49,185.4

47,165.7 47,394.0 47,011.3 44,448.6 48,720.9 47,511.0 44,446.9 47,176.4 48,194.7 47,305.7 44,857.6 48,126.7 47,176.4 45,138.7 44,090.3 47,019.8 47,165.7 47,640.1 44,448.6 46,875.7 44,090.3 47,415.7 47,176.4 49,177.8 44,448.6 44,090.3 46,875.7 46,440.8 47,394.0 47,512.0 47,305.7 47,605.0 48,179.7 45,466.2 45,259.1 44,773.7

51,967.7 52,291.8 51,386.3 52,423.7 52,425.8 52,404.8 52,423.7 52,291.8 50,460.3 52,402.7 52,039.8 51,774.8 52,403.8 51,731.8 51,387.7 51,681.8 51,246.6 51,386.3 51,702.8 51,716.8 52,340.8 52,418.8 52,403.8 51,841.6 51,625.8 52,403.8 52,039.8 52,423.7 52,403.8 52,039.8 52,403.8 51,716.8 51,732.8 52,039.8 52,425.8 52,403.8

426 36 13 1162 989 651 1336 33 12 309 1273 24 352 738 176 20 28 20 91 32 473 456 13 12 1265 103 135 1689 20 18 424 46 26 17 2197 551

22792 414 201 24374 112142 5045 27489 383 80 2760 43231 294 4636 23277 5104 439 247 95 1233 389 14193 3917 470 119 28146 6197 771 27331 331 296 4614 706 403 346 54569 14562

TABLE 3—Continued

Designation

3603

ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF

J074533.0+101112................ J074554.0
004417............... J092129.3
261843............... J092703.0+390220................ J095533.1+690355................ J095649.8+251516................ J100741.4+135629................ J102429.5
005255............... J103716.0
293402............... J104806.6
190935............... J105829.6+013358................ J110708.6
444907............... J112553.7+261019................ J114658.2+395834................ J122906.6+020308................ J125611.1
054721............... J130252.4+574837................ J133739.7
125724............... J135704.4+191907................ J140700.3+282714................ J141558.8+132023................ J142756.2
420619............... J145907.5+714019................ J150424.9+102939................ J151250.5
090559............... J155035.2+052710................ J160913.3+264129................ J161749.2
771718............... J162546.8
252738............... J163515.4+380804................ J164029.6+394646................ J164258.8+394836................ J171913.0+174506................ J173302.7
130449............... J174358.8
035004............... J175342.4+284804................ J181945.3
552120...............

 (J2000.0)

Source 0742+103 0743
006 0919
260 0923+392 0951+693 0953+254 1004+141 1021
006 1034
293 1045
188 1055+018 1104
445 1123+264 1144+402 1226+023 1253
055 1300+580 1334
127 1354+195 1404+286 1413+135 1424
418 1458+718 1502+106 1510
089 1548+056 1607+268 1610
771 1622
253 1633+382 1638+398 1641+399 1717+178 1730
130 1741
038 1751+288 1815
553

07 07 09 09 09 09 10 10 10 10 10 11 11 11 12 12 13 13 13 14 14 14 14 15 15 15 16 16 16 16 16 16 17 17 17 17 18

45 45 21 27 55 56 07 24 37 48 58 07 25 46 29 56 02 37 57 07 15 27 59 04 12 50 09 17 25 35 40 42 19 33 43 53 19

33.059522 54.082321 29.353855 03.013928 33.173056 49.875370 41.498080 29.586661 16.079727 06.620601 29.605205 08.694131 53.711923 58.297910 06.699730 11.166572 52.465277 39.782774 04.436646 00.394412 58.817505 56.297556 07.583892 24.979778 50.532931 35.269235 13.320747 49.276407 46.891640 15.492972 29.632768 58.809956 13.048490 02.705784 58.856132 42.473630 45.399524


(J2000.0) 10 11
00 44
26 18 39 02 69 03 25 15 13 56
00 52
29 34
19 09 01 33
44 49 26 10 39 58 02 03
05 47 57 48
12 57 19 19 28 27 13 20
42 06 71 40 10 29
09 05 05 27 26 41
77 17
25 27 38 08 39 46 39 48 17 45
13 04
03 50 28 48
55 21

12.69229 17.53987 43.38619 20.85193 55.06096 16.04990 29.60093 55.49707 02.81318 35.72673 58.82382 07.61841 19.97873 34.30460 08.59824 21.52469 37.60937 24.69322 07.37244 14.69029 23.71298 19.43761 19.86709 39.19870 59.82947 10.44846 29.03626 18.46747 38.32685 04.50061 46.02855 36.99397 06.43711 49.54824 04.61660 04.93911 20.74548

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000033 0.000061 0.000066 0.000035 0.000118 0.000047 0.000039 0.000110 0.000073 0.000025 0.000036 0.000141 0.000057 0.000038 0.000106 0.000069 0.000026 0.000036 0.000049 0.000033 0.000035 0.000078 0.000064 0.000050 0.000056 0.000039 0.000087 0.000206 0.000058 0.000055 0.000024 0.000084 0.000025 0.000072 0.000029 0.000024 0.000285

0.00072 0.00169 0.00212 0.00035 0.00062 0.00067 0.00083 0.00192 0.00147 0.00060 0.00095 0.00206 0.00086 0.00051 0.00249 0.00135 0.00030 0.00083 0.00089 0.00073 0.00083 0.00151 0.00088 0.00127 0.00153 0.00093 0.00242 0.00085 0.00127 0.00062 0.00034 0.00094 0.00047 0.00180 0.00064 0.00048 0.00173

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

48,247.7 48,509.8 50,196.5 48,594.9 50,693.1 50,345.4 50,948.2 49,526.2 49,896.3 50,343.2 48,257.1 49,411.6 48,641.1 48,292.3 46,802.2 47,804.4 51,344.7 49,782.0 48,823.2 47,389.5 49,307.7 50,111.4 49,772.7 48,161.1 48,721.7 48,129.4 48,329.1 49,904.6 49,973.0 48,090.3 50,611.7 46,808.0 48,865.0 47,942.6 49,481.9 48,899.0 50,124.4

44,773.7 45,997.7 46,840.7 44,090.3 49,141.8 44,446.9 47,011.3 48,664.7 46,440.8 47,176.4 44,773.7 47,626.4 46,977.8 45,138.7 44,090.3 44,090.3 49,422.9 46,840.7 44,446.9 44,342.0 45,138.7 47,305.7 48,194.7 44,446.9 44,773.7 44,773.7 44,090.3 47,626.4 46,840.7 44,446.9 44,448.6 44,090.3 47,011.3 45,259.1 44,773.7 47,005.7 47,626.4

52,403.8 51,074.2 52,403.8 52,425.8 51,870.8 52,423.7 52,423.7 51,617.6 52,425.8 51,883.6 51,939.6 52,402.7 51,687.6 52,039.8 51,387.7 51,513.7 52,423.7 52,425.8 52,017.8 52,425.8 51,870.8 52,403.8 52,039.8 51,774.8 51,723.8 51,939.6 49,820.4 52,257.6 52,425.8 51,702.8 52,423.7 51,387.7 51,883.6 51,731.8 52,425.8 52,403.8 52,403.8

311 29 388 2819 66 620 297 11 1232 12 296 256 148 151 1073 231 253 1821 132 1203 43 383 18 598 336 251 10 277 1411 457 880 1094 16 623 2276 25 184

9877 283 5929 154312 1357 13053 3880 220 10869 288 5426 1629 1374 3296 26111 4905 13933 40920 2038 41503 682 2653 1250 12328 4024 6115 246 2970 17577 13410 65477 44217 510 14254 72181 515 844

TABLE 3—Continued

Designation ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF ICRF a

J182314.1+793849................ J190255.9+315941................ J192451.0
291430............... J192748.4+735801................ J194025.5
690756............... J195510.7
611519............... J200057.0
174857............... J200530.9+775243................ J200617.6+642445................ J200744.9+402948................ J201115.7
154640 ............... J202206.6+613658................ J212344.5+053522................ J213032.8+050217................ J213135.2
120704............... J213638.5+004154................ J215155.5
302753............... J215806.2
150109............... J220243.2+421639................ J220314.9+314538................ J221852.0
033536............... J223236.4+114350................ J224618.2
120651............... J225357.7+160853................ J225805.9
275821............... J234029.0+264156................ J234802.6
163112............... J235421.6+455304................ J235509.4+495008................

Source

 (J2000.0)

1826+796 1901+319 1921
293 1928+738 1935
692 1950
613 1958
179 2007+777 2005+642 2005+403 2008
159 2021+614 2121+053 2128+048 2128
123 2134+004 2149
307 2155
152 2200+420 2201+315 2216
038 2230+114 2243
123 2251+158 2255
282 2337+264 2345
167 2351+456 2352+495

18 23 14.108466 19 02 55.938867 19 24 51.055957 19 27 48.495167 19 40 25.528172 19 55 10.770541 20 00 57.090444 20 05 30.998511 20 06 17.694594 20 07 44.944838 20 11 15.710929 20 22 06.681732 21 23 44.517408 21 30 32.877530 21 31 35.261751 21 36 38.586308 21 51 55.523981 21 58 06.281904 22 02 43.291371 22 03 14.975790 22 18 52.037725 22 32 36.408903 22 46 18.231976 22 53 57.747931 22 58 05.962885 23 40 29.029456 23 48 02.608533 23 54 21.680254 23 55 09.458150

Includes all sessions in which a source was observed.


(J2000.0) 79 31
29 73
69
61
17 77 64 40
15 61 05 05
12 00
30
15 42 31
03 11
12 16
27 26
16 45 49

38 59 14 58 07 15 48 52 24 29 46 36 35 02 07 41 27 01 16 45 35 43 06 08 58 41 31 53 50

49.00188 41.70203 30.12115 01.57010 56.97179 19.14034 57.67247 43.24763 45.41794 48.60402 40.25375 58.80460 22.09317 17.47481 04.79593 54.21310 53.69828 09.32772 39.97995 38.27011 36.87932 50.90411 51.27759 53.56093 21.25663 56.80478 12.02236 04.23658 08.33954

Epoch of Observation (MJD)

 (s)


(arcsec)

C -


Mean

First

Last

Nexpa

Nobs

0.000396 0.000039 0.000063 0.000092 0.000311 0.000796 0.000054 0.000115 0.000107 0.000180 0.000038 0.000062 0.000047 0.000308 0.000068 0.000085 0.000041 0.000056 0.000028 0.000029 0.000051 0.000035 0.000030 0.000073 0.000080 0.000042 0.000064 0.000052 0.000114

0.00080 0.00050 0.00147 0.00052 0.00231 0.00297 0.00099 0.00035 0.00037 0.00240 0.00104 0.00054 0.00071 0.00356 0.00147 0.00243 0.00111 0.00128 0.00052 0.00053 0.00128 0.00078 0.00096 0.00133 0.00204 0.00063 0.00196 0.00074 0.00164

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0. 0.000 0.000 0.000 0.000 0.000 0.000 0.000

49,107.9 49,984.7 49,319.4 48,607.9 49,216.3 49,426.6 49,783.6 49,124.9 50,419.4 48,615.8 50,115.2 49,125.6 49,035.2 48,626.2 49,830.9 47,342.1 49,202.2 49,637.3 47,501.8 50,255.0 48,007.9 48,585.4 50,456.7 46,799.0 49,563.2 49,480.3 48,365.0 48,700.8 48,225.6

47,019.8 48,103.5 45,259.1 44,772.6 47,626.4 48,766.8 46,875.7 45,997.7 49,422.9 44,773.7 46,840.7 47,165.7 44,448.6 47,288.7 45,466.2 44,090.3 47,640.1 46,835.7 44,090.3 44,448.6 44,773.7 45,997.7 44,773.7 44,090.3 46,875.7 48,357.8 46,440.8 47,011.3 47,019.8

51,883.6 51,702.8 52,403.8 51,870.8 50,259.6 50,182.6 52,402.7 51,562.8 51,967.7 50,459.3 52,017.8 52,340.8 52,423.7 51,288.7 52,423.7 52,423.7 51,981.6 51,751.8 52,403.8 52,425.8 52,340.8 52,418.8 52,425.8 51,387.7 52,423.7 51,927.8 52,340.8 51,120.7 51,246.6

23 37 1435 135 14 3 904 241 5 10 95 32 935 24 779 960 17 76 972 354 452 170 459 1076 1010 15 160 32 15

507 1380 23255 2718 52 15 11368 11922 80 110 905 2421 26086 81 7393 18440 178 574 45941 15342 9662 3106 8644 30488 10339 619 1182 371 251

ICRF EXTENSION 2 TABLE 4 Structure Index by ICRF Category ICRF Categoryb

Structure Indexa

Defining

Candidate

Other

1............................................... 2............................................... 3............................................... 4...............................................

32 77 39 16

26 59 45 15

17 18 24 20

a b

Structure index at X band from Fey & Charlot 1997, 2000. ICRF category from Ma et al. 1998.

for these sources. At S band, intrinsic structural effects were found to be less significant (about 90% of the sources had a structure index value of either 1 or 2), a consequence of the fact that the S-band structure corrections are scaled down as a result of the dual-frequency delay calibration. The analysis of Fey & Charlot (1997, 2000) also revealed that, despite the stringency of the initial selection criteria, about one-third of the ICRF defining sources have a structure index of either 3 or 4, indicating that they are somewhat spatially extended and thus may not be appropriate for defining the celestial frame with the highest level of accuracy (the structure index distribution for each ICRF source category is listed in Table 4). This suggests that revision of source categories would be mandatory upon realization of a new ICRF. 5.1.3. Source Position Stability Based on Time Series Analysis

In order to address the question of finding an improved method of selecting stable and potential defining sources in a subsequent ICRF realization, Gontier et al. (2001) investigated the time stability of source positions using data obtained during the time interval 1987–1999. Using position time series computed in parallel with the ICRF-Ext.1 analysis, Gontier et al. (2001) showed that the correspondence of the set of 212 ICRF defining sources with those that appeared to be stable from the time series analysis was not very strong. Using position time series computed in parallel with the ICRF-Ext.2 analysis, Feissel-Vernier (2003) derived a global source stability index based on the repeatability of the source positions from epoch to epoch. Table 5 gives the relationship of this stability index to the ICRF source category, that is, defining, candidate, or other, as specified in Ma et al. (1998). The numbers of objects for each pair of categories are given. Stability index values of 1 and 2 point to stable sources, index value 3 points to unstable, and index value 4 points to drifting sources. Also listed are the structure index values from Fey &

3605

Charlot (1997, 2000). As stated previously, structure index values of 1 and 2 indicate excellent and good astrometric suitability, respectively, while values of 3 and 4 suggest poor suitability. The analysis of Feissel-Vernier (2003) shows that the ICRF defining sources are not all as good over the longer time span as could be hoped and identifies a set of sources that is potentially better and which could be used to improve the next realization of the celestial frame. Initial analysis shows an improvement in the stability of the direction of the ICRF axes that is better by about a factor of 4 or 5 over that of the current 212 ICRF defining sources. This ability to better define the reference frame was confirmed by Arias & Bouquillon (2004). Other tests of this method of source selection showed improved performance for the determination of universal time and polar motion, as well as nutation (Dehant et al. 2003). 5.2. Observational Improvements 5.2.1. Observing Strategy

The source stability analysis of Feissel-Vernier (2003) found that only about half of the available sources had sufficient data for a time series analysis. The lack of data for stability analysis is most pronounced in the southern hemisphere. Indeed, fewer stable sources were identified in the southern hemisphere than the corresponding number of ICRF defining sources. As discussed previously, the ICRF suffers from a deficit of sources, particularly in the southern hemisphere. The nonuniform distribution of ICRF sources between the northern and southern sky is due primarily to the fact that astrometric and geodetic observations have historically concentrated on northern hemisphere sources. The organization of global geodetic and astrometric observing is now closely coordinated by the IVS, and special emphasis is planned for observations of identified stable and potentially stable sources. Consequently, it should be possible to fill the gaps in temporal and spatial coverage in the next few years. Further, as stated previously, close attention is also being given to designing observing schedules to achieve optimal geometry and temporal resolution for estimating troposphere parameters. 5.2.2. Data Selection

The observational data for the ICRF spanned the time period from mid-1979 to mid-1995. All available geodetic and astrometric VLBI sessions were included. Because of sensitivity limitations of the early VLBI systems, early sessions were limited to observing only 12–15 of the strongest radio sources in a 24 hr session. Improvements in data acquisition

TABLE 5 Stability Index by ICRF Category ICRF Categoryb

Structure Indexc

Stability Indexa

Defining

Candidate

Other

1

2

3, 4

1....................................................... 2....................................................... 3....................................................... 4.......................................................

33 48 2 58

26 42 1 61

15 34 4 34

46 76 6 70

11 26 2 26

7 6 0 13

a b c

Stability index from Feissel-Vernier 2003. ICRF category from Ma et al. 1998. Structure index at X band from Fey & Charlot 1997, 2000.

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technology and the consequent increases in sensitivity allowed the observing profile to evolve over time to the point where 20–40 sources could be routinely observed in a typical 24 hr session. In more recent years (i.e., after 1995), the number of sources observed in a 24 hr session has risen to about 40–50. Gontier et al. (2001) showed that the stability of source positions derived from yearly subsets of the data improved significantly around 1990, largely as a result of changes in observing strategy and observing networks (the radio telescopes used). Consequently, a considerable improvement can be expected in any new realization of the ICRF by simply using only the data acquired since 1990. This selection would also exclude the data acquired with mobile VLBI systems in small networks. It is also worth noting that the time span of higher quality VLBI data as determined by Gontier et al. (2001) has more than doubled between that of the ICRF and the current ICRF-Ext.2 (1990 to mid-1995 for the ICRF and 1990 to mid-2002 for ICRF-Ext.2), which, coupled with a rigorous time series analysis to find the sources with the most stable positions, should lead to further improvements. 5.2.3. Data Acquisition Technology

Several technical developments promise to improve the quality of the astrometric data set over the next few years. While the Mark IV VLBI system (Whitney 1993) has a maximum recording rate of 1 Gbit s
1, in practice this potential sensitivity has not been used in the past because of limitations in other parts of the data processing chain, for example, the recording media and the tape drive speed. As the disk-based Mark V recorders (Whitney 2002a) are deployed and the remaining Mark IV gaps are filled, much higher sensitivity observations will become the norm, increasing the number of observations and increasing the number of observable radio sources. The supply of disk media may still be a limitation in the immediate future, however. Another development in the foreseeable future is e-VLBI, the transfer of raw VLBI data bits from the stations directly to the correlators over high-bandwidth optical fiber networks. This avenue is being actively developed, and several demonstrations show that the concept is feasible (Whitney 2002b). If e-VLBI becomes standard practice, then the supply of disk media needed to accommodate shipping and correlator queuing will be eliminated. Some media would still be needed for recording at the stations and buffering at the correlators, since true realtime e-VLBI is not now contemplated. The barriers to widespread use of e-VLBI include obtaining optical fiber links from stations and correlators to optical fiber networks, tuning the optical fiber networks for Gbit s
1 data, and the development of a network protocol to allow VLBI data to flow in the background without interfering with other high-speed data streams of limited duration. Under the current situation of extensive unused network bandwidth, the cost of e-VLBI data transmission is not a significant factor. These technical developments along with specific efforts by the IVS should provide increased observations on stable and potentially stable sources. 5.3. Extending the ICRF to Higher Radio Frequencies VLBA observations to extend the ICRF to radio frequencies of 24 GHz (K band) and 43 GHz (Q band) began in 2002 May (Jacobs et al. 2002). The use of this frequency pair was motivated by the NASA decision to move future spacecraft telemetry from the current X band to 32 GHz (Ka band, which

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lies roughly midway between K band and Q band) and the availability of K-band and Q-band receivers on the VLBA. One of the goals of these observations was to study whether the sources were more compact at K band and Q band, in order to improve the astrometric accuracy at these frequencies. Initial imaging results (Fey et al. 2004) show that sources are indeed more spatially compact at these frequencies than at the current standard X band. The initial reference frame derived from these data (Jacobs et al. 2004) shows agreement with the ICRF to roughly the 0.3 mas level, with zonal errors dominating the differences. These observations are part of a joint program between NASA, USNO, NRAO, and Bordeaux Observatory. 5.4. Space-based Optical Astrometric Satellites In the coming decades, there will be significant advances in the area of space-based optical astrometry. Proposed and scheduled missions such as the NASA Space Interferometry Mission (SIM ) and the European Space Agency’s Global Astrometric Interferometer for Astrophysics (GAIA) mission will achieve positional accuracies well beyond that presently obtained by any ground-based radio interferometric measurements. SIM will be a space-based optical interferometer operating in an Earth-trailing solar orbit (Shao 1998). It is planned that SIM will be able to determine the positions of stars with a precision approaching the microarcsecond level. SIM is planned as a pointed mission with a limited number of target objects and limited sensitivity. Nonetheless, SIM mission plans call for observation of a full-sky set of approximately 1300 reference objects, which will largely consist of red giant stars within the Galaxy. In order to remove the global rotation of the stellar frame, the SIM mission currently has plans to observe on the order of 100 extragalactic objects. The SIM astrometric grid, which will consist almost entirely of stellar sources, will ultimately be more accurate than the current ICRF but will not be quasi-inertial, as the stars that will be observed are nearby objects, in comparison with the quasars that make up the radio frame. The accuracy goal for the resulting SIM reference frame is 4 as. GAIA, on the other hand, is planned as a survey mission and will make observations of on the order of 109 objects and, with a limiting magnitude of mv  20, will be able to observe almost all known extragalactic reference sources. Because of the large number of extragalactic objects accessible by GAIA, the astrometric grid defined by GAIA can be constructed in such a way as to be quasi-inertial. If the projected accuracies for GAIA are realized, the GAIA astrometric grid will be serious competition for the radio realization of the ICRF and must prompt reevaluation of the spectral regime at which the ICRF is defined. 6. DISCUSSION What does the future hold for the ICRF? Based on our past experience and looking forward from our present vantage point, several developments seem reasonable. Modeling and analysis capability have advanced significantly since the ICRF was defined, particularly for handling the troposphere and source structure. Refinements in modeling the motion of the stations, especially in various loading effects, should permit the unification of analysis for the celestial reference frame, the terrestrial reference frame, and Earth orientation parameters, which were separated in the ICRF analysis and the analysis presented here.

No. 6, 2004

ICRF EXTENSION 2

On the observing side of the equation, the institution of the IVS has significantly improved the organization and coordination of global geodetic and astrometric VLBI observations. As a consequence of changes in observing strategy and networks over the lifetime of astrometric and geodetic VLBI, the stability of source positions derived from individualyear data sets has improved significantly since about 1990 (Gontier et al. 2001). A considerable improvement in the ICRF can be expected by using only the data obtained after 1990. Technical innovations now allow the use of both wider spanned bandwidths and higher recorded data rates. The consequent increase in sensitivity allows observation of weaker sources and hence increases the available pool of new sources. The introduction of e-VLBI and the use of optical fiber networks should provide increased observations on stable and potentially stable sources. Special emphasis by the IVS, especially in the southern hemisphere, is now being placed on observations of the stable and potentially stable sources identified as possible defining sources for a next realization of the ICRF. We also anticipate a trend toward observing at higher radio frequencies such as at K band and Q band, where the radio sources are intrinsically more compact than at X band (Fey et al. 2004). Further, the detected source components at these higher frequencies are expected to remain closer to the central engine of the active galactic nuclei and thus be more astrometrically stable in the long term. Observations at these higher frequencies also have the advantage that the charged-particle propagation media effects (e.g., due to Earth’s ionosphere) are greatly reduced and should allow, among other things, observation of background sources closer to the Sun, where solar plasma effects currently make observing impossible because of lack of phase coherence. Within the next 10–15 years, optical astrometric satellites will present serious competition to the radio-based ICRF. Reevaluation of the spectral regime at which the ICRF is

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defined will then be necessary. This raises the question whether a radio-based ICRF will still be needed in the future. The answer is, of course, yes, primarily because the ICRF forms the stable reference points from which the orientation of Earth in three-dimensional space can be measured. The technique of VLBI provides the unique tie to the inertial reference frame realized by the ICRF. The ICRF is also used for navigation and positioning applications on Earth and in space. In addition, ICRF sources will continue to be used as fiducial reference points for astrophysical observations. 7. SUMMARY We have used very long baseline interferometry data obtained between mid-1995 and the end of 2002 May together with older data to extend and revise the ICRF. To maintain consistency with the ICRF, the configuration of the analysis followed that of the ICRF. Revised positions of ICRF candidate and ‘‘other’’ sources, based on inclusion of the additional data, are presented. Positions, in the frame of the ICRF, for an additional 109 new sources are also presented. All but four of the new sources are located north of
=
30 . Positions of the ICRF defining sources remain unchanged.

VLBI is a collaborative and cooperative activity. Without the sustained efforts of many individuals and institutions located around the world over an extended period of time, the new celestial reference frame would not have been possible. We wish to recognize and thank the designers and fabricators of VLBI instrumentation, from masers to receivers to data acquisition terminals to correlators; the operating personnel at observatories and correlator facilities; the schedule makers and coordinators; the generations of model builders, software developers, and analysts; and the farsighted visionaries and funding agencies who thought the job could be done.

REFERENCES Arias, E. F., & Bouquillon, S. 2004, A&A, in press Fomalont, E. B., Petrov, L., MacMillan, D. S., Gordon, D., & Ma, C. 2003, AJ, Arias, E. F., Charlot, P., Feissel, M., & Lestrade, J.-F. 1995, A&A, 303, 604 126, 2562 Beasley, A. J., Gordon, D., Peck, A. B., Petrov, L., MacMillan, D. S., Fomalont, Fricke, W., et al. 1988, Vero¨A. Astron. Rechen-Inst. Heidelberg, No. 32 E. B., & Ma, C. 2002, ApJS, 141, 13 Gontier, A.-M., Le Bail, K., Feissel, M., & Eubanks, T. M. 2001, A&A, Caprette, D. S., Ma, C., & Ryan, J. W. 1990, Crustal Dynamics Project Data 375, 661 Analysis: 1990 (NASA Tech. Memo. 100765) (Greenbelt, MD: GSFC) IERS. 2000, IERS Annual Report 1999, ed. D. Gambis (Frankfurt: Verlag Charlot, P. 2002, in International VLBI Service for Geodesy and Astrometry: Bundesamts Kartogr. Geod.), chap. 6 General Meeting Proceedings, ed. N. R. Vandenberg & K. D. Baver (NASA Jacobs, C. S. et al. 2002, in International VLBI Service for Geodesy and CP-2002-210002) (Greenbelt, MD: GSFC), 233 Astrometry: General Meeting Proceedings, ed. N. R. Vandenberg & Charlot, P., Viateau, B., Baudry, A., Ma, C., Fey, A., Eubanks, M., Jacobs, C., K. D. Baver (NASA CP-2002-210002) (Greenbelt, MD: GSFC), 350 & Sovers, O. 2000, in International VLBI Service for Geodesy and Astrom———. 2004, in ASP Conf. Ser., Future Directions in High Resolution etry: 2000 General Meeting Proceedings, ed. N. R. Vandenberg & K. D. Astronomy, ed. J. D. Romney & M. J. Reid (San Francisco: ASP), in press Baver (NASA CP-2000-209893) (Greenbelt, MD: GSFC), 168 Kovalevsky, J., et al. 1997, A&A, 323, 620 Clark, T. A., et al. 1985, IEEE Trans. Geosci. Remote Sens., 23, 438 Ma, C., et al. 1998, AJ, 116, 516 Dehant, V., Feissel-Vernier, M., de Viron, O., Ma, C., Yseboodt, & ———. 1986, AJ, 92, 1020 M. Bizouard, C. 2003, J. Geophys. Res., 108(B5), No. 2275 Ma, C., Sauber, J. M., Bell, L. J., Clark, T. A., Gordon, D., Himwich, W. E., & Feissel-Vernier, M. 2003, A&A, 403, 105 Ryan, J. W. 1990, J. Geophys. Res., 95, 21991 (erratum 96, 14633 [1991]) Fey, A. L., & Boboltz, D. A. 2002, in International VLBI Service for Geodesy MacMillan, D. S., & Ma, C. 1997, Geophys. Res. Lett., 24, 453 and Astrometry: General Meeting Proceedings, ed. N. R. Vandenberg & McCarthy, D. D., ed. 1996, IERS Conventions (1996) (IERS Tech. Note 21) K. D. Baver (NASA CP-2002-210002) (Greenbelt, MD: GSFC), 282 (Paris: Obs. Paris) Fey, A. L., et al. 2004, in ASP Conf. Ser., Future Directions in High Resolution Niell, A., & Tang, J. 2002, in International VLBI Service for Geodesy and Astronomy, ed. J. D. Romney & M. J. Reid (San Francisco: ASP), in press Astrometry: General Meeting Proceedings, ed. N. R. Vandenberg & Fey, A. L., Boboltz, D. A., Gaume, R. A., Eubanks, T. M., & Johnston, K. J. K. D. Baver (NASA CP-2002-210002) (Greenbelt, MD: GSFC), 215 2001, AJ, 121, 1741 Niell, A. E. 1996, J. Geophys. Res., 101, 3227 Fey, A. L., & Charlot, P. 1997, ApJS, 111, 95 Perryman, M. A. C., et al. 1997, A&A, 323, L49 ———. 2000, ApJS, 128, 17 Rogers, A. E. E. 1970, Rad. Sci., 5, 1239 Fey, A. L., Clegg, A. W., & Fomalont, E. B. 1996, ApJS, 105, 299 Ryan, J. W., Ma, C., & Caprette, D. S. 1993, NASA Space Geodesy Program: Fey, A. L., et al. 2000, in International VLBI Service for Geodesy and GSFC Data Analysis, 1992: Final Report of the Crustal Dynamics Project Astrometry: 2000 General Meeting Proceedings, ed. N. R. Vandenberg & VLBI Geodetic Results, 1979–1991 (NASA Tech. Memo. 104572) (GreenK. D. Baver (NASA CP-2000-209893) (Greenbelt, MD: GSFC), 164 belt, MD: GSFC)

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Whitney, A. R. 2002a, in International VLBI Service for Geodesy and Astrometry: General Meeting Proceedings, ed. N. R. Vandenberg & K. D. Baver (NASA CP-2002-210002) (Greenbelt, MD: GSFC), 132 ———. 2002b, in International VLBI Service for Geodesy and Astrometry: General Meeting Proceedings, ed. N. R. Vandenberg & K. D. Baver (NASA CP-2002-210002) (Greenbelt, MD: GSFC), 137