5.4 Space-Time Reference Based on General Relativity
327
reference frame with the extragalactic radio sources. Moreover, the VLBI can also
be used to adjust the correction value of the IAU axial-precession and nutation model
and the drift of the mean celestial pole at the J2000.0 epoch relative to the celestial
reference pole defined by the ICRS. Generally, two nutation models, IERS1996 and
MBH2000, are used to evaluate the deviation of the J2000.0 mean celestial pole relative to the ICRS’s celestial pole. Using the IERS1996, the deviation of the J2000.0
mean celestial pole is +17.1 milliarcseconds at 12-h direction, and +5.0 milliarcseconds at 18-h direction. Using the MBH2000, the deviation is +16.6 milliarcseconds
at 12-h direction and +6.8 milliarcseconds at 18-h direction. It is shown that the
distance between the ICRS and J2000.0 poles is within 20 milliarcseconds.
The vernal equinox of the ICRS is determined by the mean value of the right
ascensions of 23 extragalactic radio sources at the J2000.0 epoch, selected from
three radio source catalogues provided by the GSFC, JPL and NGA. In the three
radio source catalogues, the origins of right ascensions are determined by using right
ascension 12 h 19 min 6.6997 s of quasar 3C273B in the Fifth Fundamental Catalogue
(FK5), and thus the origin of right ascension of ICRS is determined. As a result, it
can be deduced that the distance of the origin of ICRS’s right ascension relative to
the dynamical equinox at epoch J2000.0 is 78 ± 10 milliarcseconds.
5.4.5.3 International Celestial Reference Frame
The ICRS is realized by the International Celestial Reference Frame (ICRF) calculated by the IERS using the global VLBI observations. The ICRF is composed of
a group of extragalactic radio sources with precise position coordinates, including
defining, candidate and other sources. The defining sources refer to the sources with
more observation numbers over a long time span, which is mainly used to maintain
the stability of the coordinate axes of ICRS. The candidate sources refer to the
sources with short observation time and less observation numbers, but can be used as
the defining sources in the future. The other sources refer to the sources with very
few observation numbers, but can be used for connection between different reference
frames. In 1995, using arc-parameter elimination method, the WGRF carried out the
adjustment processing of a total of 1.6 million pairs of delays and their rates from
the global VLBI observation data, and then it was obtained that the Root-MeanSquares (RMS) of the delays and their rates are, respectively, 32.6 picoseconds and
104.2 picoseconds per second. A total of 1305 global parameters and 650,000 arcparameters were solved, and the WGRF radio source catalogue was gotten finally.
Using 133 co-sources, the WGRF radio source catalogue was compared with the
IERS integrated-radio catalogue in 1995, i.e., RSC (ICRS) 95C02. In this way, the
WGRF radio catalogue was rotated to the RCS (ICRS) 95C02, and finally the ICRF
was obtained, which was published at the 23rd IAU general assembly in 1997.
The ICRF has 608 extragalactic radio sources, including 212 defining sources,
294 candidate sources and 102 other sources, and their angular position accuracies
reach 0.25 milliarcseconds. In the data processing, the basic principles of selecting
the sources are the accuracies of the sources’ angular position are better than 1
327
reference frame with the extragalactic radio sources. Moreover, the VLBI can also
be used to adjust the correction value of the IAU axial-precession and nutation model
and the drift of the mean celestial pole at the J2000.0 epoch relative to the celestial
reference pole defined by the ICRS. Generally, two nutation models, IERS1996 and
MBH2000, are used to evaluate the deviation of the J2000.0 mean celestial pole relative to the ICRS’s celestial pole. Using the IERS1996, the deviation of the J2000.0
mean celestial pole is +17.1 milliarcseconds at 12-h direction, and +5.0 milliarcseconds at 18-h direction. Using the MBH2000, the deviation is +16.6 milliarcseconds
at 12-h direction and +6.8 milliarcseconds at 18-h direction. It is shown that the
distance between the ICRS and J2000.0 poles is within 20 milliarcseconds.
The vernal equinox of the ICRS is determined by the mean value of the right
ascensions of 23 extragalactic radio sources at the J2000.0 epoch, selected from
three radio source catalogues provided by the GSFC, JPL and NGA. In the three
radio source catalogues, the origins of right ascensions are determined by using right
ascension 12 h 19 min 6.6997 s of quasar 3C273B in the Fifth Fundamental Catalogue
(FK5), and thus the origin of right ascension of ICRS is determined. As a result, it
can be deduced that the distance of the origin of ICRS’s right ascension relative to
the dynamical equinox at epoch J2000.0 is 78 ± 10 milliarcseconds.
5.4.5.3 International Celestial Reference Frame
The ICRS is realized by the International Celestial Reference Frame (ICRF) calculated by the IERS using the global VLBI observations. The ICRF is composed of
a group of extragalactic radio sources with precise position coordinates, including
defining, candidate and other sources. The defining sources refer to the sources with
more observation numbers over a long time span, which is mainly used to maintain
the stability of the coordinate axes of ICRS. The candidate sources refer to the
sources with short observation time and less observation numbers, but can be used as
the defining sources in the future. The other sources refer to the sources with very
few observation numbers, but can be used for connection between different reference
frames. In 1995, using arc-parameter elimination method, the WGRF carried out the
adjustment processing of a total of 1.6 million pairs of delays and their rates from
the global VLBI observation data, and then it was obtained that the Root-MeanSquares (RMS) of the delays and their rates are, respectively, 32.6 picoseconds and
104.2 picoseconds per second. A total of 1305 global parameters and 650,000 arcparameters were solved, and the WGRF radio source catalogue was gotten finally.
Using 133 co-sources, the WGRF radio source catalogue was compared with the
IERS integrated-radio catalogue in 1995, i.e., RSC (ICRS) 95C02. In this way, the
WGRF radio catalogue was rotated to the RCS (ICRS) 95C02, and finally the ICRF
was obtained, which was published at the 23rd IAU general assembly in 1997.
The ICRF has 608 extragalactic radio sources, including 212 defining sources,
294 candidate sources and 102 other sources, and their angular position accuracies
reach 0.25 milliarcseconds. In the data processing, the basic principles of selecting
the sources are the accuracies of the sources’ angular position are better than 1