USNO Circular 179 - U.S. Naval Observatory

20 CELESTIAL REFERENCE SYSTEM
3.1 The ICRS, the ICRF, and the HCRF
The fundamental celestial reference system for astronomical applications is now the International
Celestial Reference System (ICRS), as provided in res. B2 of 1997. The ICRS is a coordinate
system with its origin at the solar system barycenter and axis directions that are fixed with respect
to distant objects in the universe; it is to be used to express the positions and motions of stars,
planets, and other celestial objects. Its relativistic basis is defined by res. B1.3 of 2000; in the
words of that resolution, it is a Barycentric Celestial Reference System (BCRS), and as such its
axes are kinematically non-rotating (see Chapter 1). To establish the ICRS as a practical system,
the IAU specified a set of distant benchmark objects, observable at radio wavelengths, whose
adopted coordinates effectively define the directions of the ICRS axes. This “realization” of the
ICRS, called the International Celestial Reference Frame (ICRF), is a set of high accuracy positions
of extragalactic radio sources measured by very long baseline interferometry (Ma & Feissel 1997;
Ma et al. 1998). The ICRS is realized at optical wavelengths — but at lower accuracy — by the
Hipparcos Celestial Reference Frame (HCRF), consisting of the Hipparcos Catalogue (ESA 1997)
of star positions and motions, with certain exclusions (res. B1.2 of 2000). The coordinates of the
ICRF radio sources and HCRF stars are given relative to the ICRS origin at the solar system
barycenter, and a number of transformations are required to obtain the coordinates that would be
observed from a given location on Earth at a specific date and time.
Although the directions of the ICRS coordinate axes are not defined by the kinematics of the
Earth, the ICRS axes (as implemented by the ICRF and HCRF) closely approximate the axes
that would be defined by the mean Earth equator and equinox of J2000.0 (to within about 0.02
arcsecond), if the latter is considered to be a barycentric system. Because the ICRS axes are
meant to be “space fixed”, i.e., kinematically non-rotating, there is no date associated with the
ICRS. Furthermore, since the defining radio sources are assumed to be so distant that their angular
motions, seen from Earth, are negligible, there is no epoch associated with the ICRF. It is technically
incorrect, then, to say that the ICRS is a “J2000.0 system”, even though for many current data
sources, the directions in space defined by the equator and equinox of J2000.0 and the ICRS axes
are the same to within the errors of the data.
The ICRS, with its origin at the solar system barycenter and “space fixed” axis directions,
is meant to represent the most appropriate coordinate system currently available for expressing
reference data on the positions and motions of celestial objects.
The IAU Working Group on Nomenclature for Fundamental Astronomy has recommended the
following definitions for the ICRS and ICRF:
International Celestial Reference System (ICRS): The idealized barycentric coordinate
system to which celestial positions are referred. It is kinematically non-rotating
with respect to the ensemble of distant extragalactic objects. It has no intrinsic orientation
but was aligned close to the mean equator and dynamical equinox of J2000.0
for continuity with previous fundamental reference systems. Its orientation is independent
of epoch, ecliptic or equator and is realized by a list of adopted coordinates of
extragalactic sources.
International Celestial Reference Frame (ICRF): A set of extragalactic objects
whose adopted positions and uncertainties realize the ICRS axes and give the uncertainties
of the axes. It is also the name of the radio catalogue whose 212 defining sources
are currently the most accurate realization of the ICRS. Note that the orientation of the
ICRF catalogue was carried over from earlier IERS radio catalogs and was within the