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I/251 VLBI International Celestial Reference Frame (ICRF) (Ma+, 1997)
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Definition and Realization of the International Celestial Reference System
by VLBI Astrometry of Extragalactic Objects
Ma C., Feissel M.
<IERS Tech. Note 23 (1997)>
Realization of the International Celestial Reference Frame
from VLBI Astrometry
Ma C.
<Bull. American Astron. Soc., 191, #16.13 (1997)>
=1997AAS...191.1613M
The international celestial reference frame as realized by very long
baseline interferometry.
Ma C., Arias E.F., Eubanks T.M., Fey A.L., Gontier A.-M., Jacobs C.S.,
Sovers O.J., Archinal B.A., Charlot P.
<Astron. J. 116, 516 (1998)>
=1998AJ....116..516M
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ADC_Keywords: Fundamental catalog ; Positional data; VLBI
Keywords: astrometry - catalogs - quasars: general - radio continuum -
reference systems - techniques: interferometric
Description:
The International Celestial Reference Frame (ICRF), is intended for
adoption be the International Astronomical Union (IAU) as the
conventional celestial system, under the name International Celestial
Reference System (ICRS).
(See http://maia.usno.navy.mil/iauc19/iaures.html#B5).
A Working Group on Reference Frames (WGRF), with the participation of
International Earth Rotation Service (IERS), was appointed by the IAU
to accomplish the task of constructing the ICRF. The WGRF set up a
list of 608 extragalactic radio sources uniformly distributed on the
sky and evaluated their J2000.0 Very Long Baseline Interferometry
(VLBI) coordinates in the ICRS by applying a no rotation constraint
with respect to an earlier realization of the IERS Celestial reference
system, RSC(IERS) 95 C 02 .
The first realization of the ICRF has J2000.0 coordinates of 608
objects and is named RSC(WGRF)95 R 01. Radio sources in RSC(WGRF)95 R
01 are divided into three categories (see file "guide.txt"): defining,
candidate and other sources.
In order to provide radio source coordinates to link to the Hipparcos
stellar reference frame to the ICRS, the WGRF issued a shorter version
of the ICRF, RSC(WGRF) 95 R 02. It is a subset of RSC(WGRF) 95 R 01
that contains J2000.0 coordinates of 253 radio sources covering the
declination range -81 degrees to +85 degrees. The 212 defining sources
of RSC(WGRF)95 R 01 included 17 Hipparcos link objects. An additional
41 other radio sources had to be added RSC(WGRF) 95 R 02 for the link.
The median positional accuracy of RSC(WGRF) 95 R 02 is 0.4 mas. The
other 357 sources of RSC(WGRF) 95 R 01 have coordinates with lower
precision (median= 0.8 mas) but they are still consistent with the
ICRS.
The direction of the axes of the ICRF are consistent with those of the
FK5 system within the uncertainties of the latter. On the basis of
Fricke's (1982A&A...107L..13F) and Schwan (1988) considerations, the
uncertainty of the pole position relative to the mean pole at J2000.0
is +/-50 mas. Studies by Morrison et al. (1990A&A...240..173M) and
Lindegren et al. (1995A&A...304...44L) have shown that the FK5 origin
of right ascensions has an uncertainty of the order of +/-100 mas.
The accuracy of the tie of the Hipparcos stellar frame to ICRS is
+/-0.6 mas at the Hipparcos mean epoch of observation (1991.25) and
+/-0.25 mas/year for the time evolution. The tie between the dynamical
planetary frame and ICRS is known within +/-3 mas (Folkner et al.
1994A&A...287..279F).
Abstract of the 1998AJ....116..516M paper:
A quasi-inertial reference frame is defined based on the radio
positions of 212 extragalactic sources distributed over the entire
sky. The positional accuracy of these sources is better than about
1mas in both coordinates. The radio positions are based upon a general
solution for all applicable dual-frequency 2.3 and 8.4GHz Mark III
very long baseline interferometry data available through the middle of
1995, consisting of 1.6 million pairs of group delay and phase delay
rate observations. Positions and details are also given for an
additional 396 objects that either need further observation or are
currently unsuitable for the definition of a high-accuracy reference
frame. The final orientation of the frame axes has been obtained by a
rotation of the positions into the system of the International
Celestial Reference System and is consistent with the FK5 J2000.0
optical system, within the limits of the link accuracy. The resulting
International Celestial Reference Frame has been adopted by the
International Astronomical Union as the fundamental celestial
reference frame, replacing the FK5 optical frame as of 1998 January 1.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
guide.txt 82 380 Detailed description of the RSC(WGRF)95 R 01
rsc95def.dat 144 212 ICRF RSC(WGRF)95 R 01 defining sources
(Table 3 of 1998AJ....116..516M paper)
rsc95can.dat 144 294 ICRF RSC(WGRF)95 R 01 candidate sources
(Table 4 of 1998AJ....116..516M paper)
rsc95oth.dat 144 102 ICRF RSC(WGRF)95 R 01 other sources
(Table 5 of 1998AJ....116..516M paper)
rsc95r01.dat 144 608 ICRF RSC(WGRF)95 R 01 all sources
rsc95r02.dat 91 253 ICRF RSC(WGRF)95 R 02 subset
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See also:
I/218 : New Reference Frame defined by Extragalactic Radio Source
(IAU WG, 1995)
I/239 : The Hipparcos and Tycho Catalogues (ESA 1997)
http://hpiers.obspm.fr/webiers/results/icrf/README.html
http://aa.usno.navy.mil/AA/data/docs/ICRS_links.html
http://hpiers.obspm.fr/webiers/results/icrf/Updating.html
Table 1: Summary of catalog differences (from 1998AJ....116..516M)
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Rotation angles RMS residuals Internal trends
Catalog pair A1 A2 A3 ki2 RAcosDE DE Arc Length Dmax Dmax/sigma
(mas) (mas) mas/100deg
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WGRF vs. IERS95 0.1 -0.4 0.0 2.92 0.23 0.39 0.55 0.32 14
WGRF vs. RORF -0.2 -0.5 0.0 2.77 0.10 0.38 0.42 0.32 15
IERS95 vs. IERS94 0.0 0.0 0.0 2.85 0.30 0.31 0.52 0.29 12
WGRF vs. GSFC -0.1 -0.1 -0.0 1.68 0.09 0.29 0.39 0.15 7
WGRF vs. JPL 0.1 -0.3 -0.2 2.84 0.26 0.44 0.51 0.66 15
GSFC vs. JPL 0.2 -0.3 0.2 3.42 0.30 0.36 0.58 0.38 14
WGRF vs. WGRFna 0.2 0.0 0.0 0.18 0.05 0.06 0.12 0.07 2
WGRF vs. WGRFel 0.0 0.0 0.0 0.04 0.01 0.02 0.04 0.02 2
WGRF vs. WGRFng 0.0 -0.1 0.0 0.99 0.03 0.12 0.22 0.19 8
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Table 2: Alignment of the ICRF axes with the ICRS (from 1998AJ....116..516M)
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Parameter Value
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A_1_ (mas) -0.006+/-0.018
A_2_ (mas) 0.007+/-0.018
A_3_ (mas) 0.005+/-0.021
D_DE_ (mas/100{deg}) 0.0
D_DE_ (mas/100{deg}) 0.2
B_DE_ (mas/100{deg}) -0.28 +/-0.02
wrms_RAcosDE_ (mas) 0.14
wrms_DE_ (mas) 0.20
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Byte-by-byte Description of file: rsc95can.dat, rsc95def.dat, rsc95oth.dat
Byte-by-byte Description of file: rsc95r01.dat
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Bytes Format Units Label Explanations
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2- 22 A21 --- ICRF *ICRF designation
26- 33 A8 --- IERS *IERS designation
35 A1 --- c *[DCO] Defining / Candidate / Other
category of source
36 I1 --- X ? X band structure index
38 I1 --- S ? S band structure index
40 A1 --- Hip *[* ] Link to Hipparcos reference frame
43- 44 I2 h RAh Right Ascension (J2000) hours
46- 47 I2 min RAm Right Ascension (J2000) minutes
49- 57 F9.6 s RAs Right Ascension (J2000) seconds
60 A1 --- DE- Declination (J2000) sign
61- 62 I2 deg DEd Declination (J2000) degrees
64- 65 I2 arcmin DEm Declination (J2000) arcminutes
67- 74 F8.5 arcsec DEs Declination (J2000) arcseconds
78- 85 F8.6 s e_RAs Uncertainty in Right Ascension
88- 94 F7.5 arcsec e_DEs Uncertainty in Declination
96-101 F6.3 --- Corr [-1,1]? RA/Dec correlation
103-111 F9.1 d Ep-MJD Mean date of observation, expressed in
Modified Julian Date MJD = JD-2400000.5
113-121 F9.1 d O1-MJD First date of observation (MJD)
123-131 F9.1 d Ol-MJD Last Julian date of observation (MJD)
134-137 I4 --- Nsess Number of observing sessions
139-144 I6 --- Ndel Number of delay, delay rate obs. pairs
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Note on ICRF:
ICRF Designations, constructed from J2000.0 coordinates with the format
ICRF JHHMMSS.s+DDMMSS or ICRF JHHMMSS.s-DDMMSS
They follow the recommendations of the IAU Task Group on Designations.
Note on IERS:
IERS Designations, previously constructed from B1950 coordinates.
The complete format, including acronym and epoch in addition to the
coordinates, is IERS BHHMM+DDd or IERS BHHMM-DDd
Note on c:
Category of source, present in file rsc95r01.dat only:
D = defining source (all sources of files rsc95def.dat)
C = candidate source (all sources of files rsc95can.dat)
O = other source (all sources of files rsc95oth.dat)
Note on Hip:
* = the source serves to link the Hipparcos stellar reference frame
to the ICRS
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Byte-by-byte Description of file: rsc95r02.dat
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Bytes Format Units Label Explanations
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2- 22 A21 --- ICRF *ICRF designation
26- 33 A8 --- IERS *IERS designation
36- 37 A2 --- status *[DL ] Status of source
40- 41 I2 h RAh Right Ascension (J2000) hours
43- 44 I2 min RAm Right Ascension (J2000) minutes
46- 54 F9.6 s RAs Right Ascension (J2000) seconds
57 A1 --- DE- Declination (J2000) sign
58- 59 I2 deg DEd Declination (J2000) degrees
61- 62 I2 arcmin DEm Declination (J2000) arcminutes
64- 71 F8.5 arcsec DEs Declination (J2000) arcseconds
75- 82 F8.6 s e_RAs Uncertainty in Right Ascension
85- 91 F7.5 arcsec e_DEs Uncertainty in Declination
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Note on ICRF:
ICRF Designations, constructed from J2000.0 coordinates with the format
ICRF JHHMMSS.s+DDMMSS or ICRF JHHMMSS.s-DDMMSS
They follow the recommendations of the IAU Task Group on Designations.
Note on IERS:
IERS Designations, previously constructed from B1950 coordinates.
The complete format, including acronym and epoch in addition to the
coordinates, is IERS BHHMM+DDd or IERS BHHMM-DDd
Note on status:
D = Defining source not contributing to the link
L = Source included in the frame only because they serve the Hipparcos link
DL= Defining source which are also Hipparcos link object
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History:
* 17-Jul-1998 (Julie Anne Watko, James E. Gass [SSDOO/ADC]):
The RSC(WGRF) 95 R 01 files were provided by the author. ADC has added
the table of RSC(WGRF) 95 R 02 as found on the IERS Web site as a
convenience to users.
* 04-Dec-1998 (Francois Ochsenbein, Patricia Bauer [CDS]):
Added in this ReadMe extracts from the 1998AJ....116..516M paper.
References:
Folkner W.M, et al.,1994, A&A, 287, 279 =1994A&A...287..279F
Fricke, W. 1982, A&A, 107, L13 =1982A&A...107L..13F
Kovalevsky J. et al., 1997, A&A, 323, 620 =1997A&A...323..620K
Lindegren L., et al., 1995, A&A, 304, 44 =1995A&A...304...44L
Ma C. and Feissel M., eds. June 1997. IERS Technical Note 23 (Paris:
Observatoire de Paris).
Morrison L.U, et al., 1990, A&A, 240, 173 =1990A&A...240..173M
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(End) Julie Anne Watko, James E. Gass [SSDOO/ADC] 17-Jul-1998