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\write 0 {1.  CONVENTIONAL CELESTIAL REFERENCE SYSTEM
\noexpand\dotfill\the\pageno\noexpand\break}
\noindent{\large CHAPTER 1} {\bf CONVENTIONAL CELESTIAL REFERENCE
SYSTEM}
\bigskip

     In 1991 the International Astronomical Union (IAU) decided
that the IAU celestial reference system would be realized by a
celestial reference frame defined by the precise coordinates of
extragalactic radio sources.  The related IAU recommendations
(see McCarthy, 1992) specify that the origin is to be at the
barycenter of the solar system and the directions of the axes are
to be fixed with respect to the quasars.  In compliance with this
recommendation, the IERS Celestial Reference System (ICRS) is
realized by the IERS Celestial Reference Frame (ICRF) defined
by the J2000.0 equatorial coordinates of extragalactic objects
determined from Very Long Baseline Interferometry (VLBI)
observations.  It is a frame whose directions are consistent with
those of the FK5 (Fricke {\it et al}., 1988).  The origin is
located at the barycenter of the solar system through 
appropriate modeling of observations in the framework of General 
Relativity (see Chapters 2 and 12). The rotational stability of
the frame is based on the assumption that the sources have no 
proper motions.  Checks are performed regularly to ensure the 
validity of this constraint (Ma and Shaffer, 1991;  Eubanks {\it et
al.}, 1994). The HIPPARCOS reference frame is planned to be linked 
astrometrically to the ICRF to unify the radio and optical
coordinate systems at the level of $\pm0\qd0005$ in direction and
$\pm 0\qd0005$/year in rotation (Lindegren and Kovalevsky, 1995).
The ICRS is recommended for the IAU conventional celestial system
by the IAU Working Group on Reference Frames under the name 
``International Celestial Reference System." See Arias {\it et al}. (1995).

\write 0 {\indent Equator \noexpand\dotfill\the\pageno\noexpand\break}
\noindent{\bf Equator}

\nobreak

     The IAU recommendations call for the principal plane of the
conventional reference frame to be close to the mean equator at
J2000.0.  The VLBI observations which are used to establish the
extragalactic reference frame also provide the monitoring of the
motion of the celestial pole in the sky (precession and
nutation).  In this way, the VLBI analyses provide corrections to
the conventional IAU models for precession and nutation (Lieske
{\it et al.}, 1977;  Seidelmann, 1982) and the accurate estimation 
of the shift of the mean pole at J2000.0 relative to its
conventional one, to which the pole of the ICRS is attached. 
Based  on the VLBI observations available in early 1995 and the
IERS correction model for precession and nutation (see Chapters
4 and 5), one can estimate the shift of the pole at J2000.0
relative to the IERS celestial pole to be 17.3 $\pm$ 0.2 mas in the
direction $12^{\rm h}$ and 5.1 $\pm$ 0.2 mas in the direction $18
^{\rm h}$. 

     On the other hand, the IAU recommendations stipulate that 
the direction of the new conventional celestial pole be
consistent with that of the FK5.  The uncertainty in the direction of
the FK5 pole can be estimated by considering (1) that the
systematic part is dominated by a correction of $-0\qd25$/cy to the
precession constant imbedded in the FK5 System, and (2) by
adopting Fricke's (1982) estimation of the accuracy of the FK5
equator ($\pm0\qd02$), and Schwan's (1988) estimation of the limit
of the residual rotation ($\pm0\qd07$/cy), taking the epochs of
observations from Fricke {\it et al.} (1988).  Assuming that the error
in the precession rate is absorbed by the proper motions of
stars, the uncertainty in the FK5 pole position relative to the
mean pole at J2000.0 estimated in this way is $\pm$ 50 mas.  The
IERS celestial pole is therefore consistent with that of the FK5
within the uncertainty of the latter.

\write 0 {\indent Origin of Right Ascension \noexpand\dotfill\the
\pageno\noexpand\break}
\noindent{\bf Origin of Right Ascension}

\nobreak

     The IAU recommends that the origin of right ascensions of 
the new celestial reference system be close to the dynamical 
equinox at J2000.0.  The x axis of the IERS celestial system was
implicitly  defined in the initial realization (Arias {\it et
al}., 1988) by adopting the mean right ascension of 23 radio
sources in a group of catalogs that were compiled by fixing the
right ascension of 3C 273B to the usual (Hazard {\it et al}.,
1971) conventional FK5 value ($12^{\rm{h}} 29^{\rm{m}}
6^{\rm{s}}{\kern-.13cm{.}\kern.01cm}6997$ at J2000.0) (Kaplan
{\it et al}., 1982).

     The uncertainty in the FK5 origin of right ascensions can be 
derived from the quadratic sum of the accuracies given by Fricke 
(1982) and Schwan (1988), considering a mean epoch of 1955 for 
the proper motions in right ascension.  The uncertainty thus 
obtained is $\pm$ 80 mas.  Folkner {\it et al.} (1994), comparing 
VLBI and LLR Earth orientation and terrestrial frames and the
estimated frame tie of planetary ephemerides, show that the mean
equinox of J2000.0 is shifted from the right ascension origin of
the IERS system by 78 $\pm$ 10 mas (direct rotation around the
polar axis).  This shows that the ICRS origin of right
ascension complies with the IAU  recommendations.

\write 0 {\indent Precision and Accuracy \noexpand\dotfill\the
\pageno\noexpand\break}
\noindent{\bf Precision and Accuracy}

\nobreak

     The estimation of coordinates performed by the Central
Bureau of the IERS is based on individual frames contributed by
the IERS Analysis Centers.   Several extragalactic frames are
produced each year by independent VLBI groups.  Selected
realizations are used to form the ICRF.  The algorithm
used for the compilation is designed primarily to maintain the
three directions of axes fixed for successive realizations while
the precision of coordinates of individual sources is improved.
Successive realizations produced up to now have maintained the
initial definition of the axes within $\pm 0\qd0001$.

     The inaccuracy of the conventional IAU 1976 Precession and
IAU 1980 Theory of Nutation would give rise to systematic errors
in the source positions and to misorientation of the axes of the
frames, both at the level of a few  milliarcseconds, in the
analysis of VLBI observations.  Therefore, the usual practice in
catalog work is to estimate additional parameters which
describe the motion of the celestial pole relative to its
conventional position (see Sovers, 1991).  Another type of
possible systematic error is related to low  elevation
observations in cases where the observing network has a  modest
north-south extension.  This effect can be modelled as a linear
dependence of declination errors on declination which can reach 
0.01 mas per degree and create a tilt of the frame equator of up
to 0.6 mas (Feissel {\it et al.}, 1995).  However, for recent
global analyses these systematic differences are at the level of
0.001 mas per degree and 0.1  mas respectively (IERS, 1995). 
No other type of systematic differences can be found above this
level in the present-day VLBI celestial reference frames (Feissel
{\it et al.,} 1995, Eubanks {\it et al.}, 1994).  After taking
into account the small systematic errors  mentioned above, the
precision of the source coordinates is a  white noise function
of the number of observations, with a typical value of $\pm$ 0.2
mas for 100 observations (Arias {\it et al.}, 1995). 

\write 0 {\indent Availability of the Frame \noexpand\dotfill\the
\pageno\noexpand\break}
\noindent{\bf Availability of the Frame}

\nobreak

     The catalog of source coordinates published in the 1994 
IERS Annual Report (July 1995) provides access to the
ICRS.  It includes a total of 608 objects, among which 236 with 
good observational histories attach
the frame to the ICRS.  In the future, based on new 
observations and new analyses, the stability of the source 
coordinates will be monitored, and the appropriate warnings 
and updates will appear in IERS publications.

     The direct access to the quasars is most precise through 
VLBI observations, a technique which is not widely available to 
users.  Therefore, while VLBI is used for the maintenance of the 
frame, the tie of the ICRF to the major practical reference
frames may be obtained through use of the IERS Terrestrial
Reference Frame (ITRF, see Chapter 3), the HIPPARCOS Galactic 
Reference Frame, and the JPL ephemerides of the solar system (see 
Chapter 2).

     The principles on which the ITRF is established and 
maintained are described in Chapter 3.  The IERS Earth
orientation parameters provide the permanent tie of the ICRF to
the ITRF.  They describe the orientation of the Celestial
Ephemeris Pole in the terrestrial system and in the celestial
system (polar coordinates x,  y;  nutation offsets d$\psi$,
d$\epsilon$) and the orientation of the Earth around this axis
(UT1-TAI), as a function of time.  This tie is available daily 
with an accuracy of $\pm$ 0.5 mas in the IERS publications.

     The other ties to major celestial frames are established
by differential VLBI observations of solar system probes,
galactic stars relative to quasars and other ground- or 
space-based astrometry projects.  The tie of the solar system
ephemerides of the Jet Propulsion Laboratory (JPL) is described
by Standish {\it et al.} (1995).  Its estimated precision is $\pm$ 
3 mas, according to Folkner {\it et al.} (1994).  The tie of the 
galactic frame to ICRS is a part of the  HIPPARCOS project. It is
described in some detail by Lindegren and Kovalevsky (1995).  Its 
expected accuracy is $\pm$ 0.5 mas at the HIPPARCOS mean epoch of
observation (1991.25) and $\pm$ 0.5 mas/year for the time evolution.


\noindent{\bf References}

\nobreak

\item {}\kern-2pc
Arias E. F., Charlot P., Feissel M., Lestrade J.-F., 1995,
     ``The Extragalactic Reference System of the
     International Earth Rotation Service, ICRS," {\it Astron. 
     Astrophys.}, {\bf 303}, pp. 604--608.

\item {}\kern-2pc
Arias, E. F., Feissel, M., and Lestrade, J.-F., 1988, ``An 
     Extragalactic Celestial Reference Frame Consistent with the 
     BIH Terrestrial System (1987)," {\it BIH Annual Rep. for 1987}, 
     pp. D-113--D-121.

\item{}\kern-2pc
Eubanks, T. M., Matsakis, D. N., Josties, F. J.,Archinal, B. A.,
      Kingham, K. A., Martin, J. O., McCarthy, D. D., Klioner, S. A.,
      Herring, T. A., 1994, ``Secular motions of extragalactic
      radio sources and the stability of the radio reference frame,"
      {\it Proc. IAU Symposium 166}, J. Kovalevsky (ed.). 

\item{}\kern-2pc
Feissel, M., Ma, C., Sovers, O., and Jacobs, C., 1995, 
     ``Sub-milliarcsecond VLBI celestial frames: problem areas,"
      (unpublished contribution).

\item{}\kern-2pc
Folkner, W. M., Charlot, P., Finger, M. H., Williams, J. G., Sovers,
     O. J., Newhall, X X, and Standish, E. M., 1994, ``Determination 
     of the extragalactic-planetary frame tie from joint analysis 
     of radio interferometric and lunar laser ranging measurements,"
     {\it Astron. Astrophys.}, {\bf 287}, pp. 279--289. 

\item{}\kern-2pc
Fricke, W., 1982, ``Determination of the Equinox and Equator of
     the FK5," {\it Astron. Astrophys.}, {\bf 107}, p. L13--16.

\item{}\kern-2pc
Fricke, W., Schwan, H., and Lederle, T., 1988, {\it Fifth
     Fundamental Catalogue, Part I. Veroff. Astron. Rechen Inst.},
     Heidelberg.

\item{}\kern-2pc
Hazard, C., Sutton, J., Argue, A. N., Kenworthy, C. N., Morrison,
     L. V., and Murray, C. A., 1971, ``Accurate radio and optical
     positions of 3C273B," {\it Nature Phys. Sci.}, {\bf 233}, p. 89.

\item{}\kern-2pc
IERS, 1995, 1994 International Earth Rotation Service Annual 
     Report, Observatoire de Paris, Paris.

\item{}\kern-2pc
Kaplan, G. H., Josties, F. J., Angerhofer, P. E., Johnston, K. J.,
     and Spencer, J. H., 1982, ``Precise Radio Source Positions
     from Interferometric Observations," {\it Astron. J.}, {\bf 87},
     pp. 570--576.

\item{}\kern-2pc
Lieske, J. H., Lederle, T., Fricke, W., and Morando, B., 1977, 
     ``Expression for the Precession Quantities Based upon the IAU
     (1976) System of Astronomical Constants," {\it Astron. 
     Astrophys.}, {\bf 58}, pp. 1--16.

\item{}\kern-2pc
Lindegren, L. and Kovalevsky, J., 1995, ``Linking the Hipparcos
     catalogue to the extragalactic reference frame," {\it
     Astron. Astrophys.}, {\bf 304}, pp. 189--201.

\item{}\kern-2pc
Ma, C. and Shaffer, D. B., 1991, ``Stability of the extragalactic
     reference frame realized by VLBI," in {\it Reference Systems}, 
     J. A. Hughes, C. A. Smith, and G. H. Kaplan (eds), U. S. Naval
     Observatory, Washington, pp. 135--144.

\item{}\kern-2pc
McCarthy D.D. (ed.), 1992, ``IERS Standards," {\it IERS Technical 
     Note 13}, Observatoire de Paris, Paris.

\item{}\kern-2pc
Schwan, H., 1988, ``Precession and galactic rotation in the system 
     of FK5," {\it Astron. Astrophys.}, {\bf 198}, pp. 116--124.

\item{}\kern-2pc
Seidelmann, P. K., 1982, ``1980 IAU Nutation:  The Final Report of 
     the IAU Working Group on Nutation," {\it Celest. Mech.}, {\bf 
     27}, pp. 79--106.

\item{}\kern-2pc
Sovers, O. J., 1991, ``Consistency of Nutation Modeling in Radio 
     Source Position Comparisons," in {\it Reference Systems}, 
     J. A. Hughes, C. A. Smith, and G. H. Kaplan (eds), U. S. Naval
     Observatory, Washington, pp. 130--134.

\item{}\kern-2pc
Standish, E. M., Newhall, X X, Williams, J. G., and Folkner, W. F.,
     1995, ``JPL Planetary and Lunar Ephemerides, DE403/LE403,"
     JPL IOM 314.10-127, to be submitted to {\it Astron. Astrophys.}

\vfil\eject
