USNO Circular 179 - U.S. Naval Observatory

Chapter 3
The Fundamental Celestial Reference
System
Relevant IAU resolutions: A4.VI, A4.VII of 1991; B5 of 1994; B2 of 1997; B1.2 of 2000
Summary Reference data for positional astronomy, such as the data in astrometric
star catalogs or barycentric planetary ephemerides, are now specified within the International
Celestial Reference System (ICRS). The ICRS is a coordinate system whose
origin is at the solar system barycenter and whose axis directions are effectively defined
by the adopted coordinates of 212 extragalactic radio sources observed by VLBI. These
radio sources (quasars and active galactic nuclei) are assumed to have no observable
intrinsic angular motions. Thus, the ICRS is a “space-fixed” system (more precisely, a
kinematically non-rotating system) without an associated epoch. However, the ICRS
closely matches the conventional dynamical system defined by the Earth’s mean equator
and equinox of J2000.0; the alignment difference is at the 0.02 arcsecond level, negligible
for many applications.
Strictly speaking, the ICRS is somewhat of an abstraction, a coordinate system that
perfectly satisfies a list of criteria. The list of radio source positions that define it for
practical purposes is called the International Celestial Reference Frame (ICRF). In the
terminology that is now commonly used, a reference system like the ICRS is “realized”
by a reference frame like the ICRF, and there can be more than one such realization.
In the case of the ICRS, there is, in fact, a second, lower-accuracy realization for work
at optical wavelengths, called the Hipparcos Celestial Reference Frame (HCRF). The
HCRF is composed of the positions and proper motions of the astrometrically wellbehaved
stars in the Hipparcos catalog.
Astrometric data referred to the ICRS is becoming more common, with new catalogs
now available in the optical, infrared, and radio.
The ICRS is itself a specific example of a Barycentric Celestial Reference System, incorporating
the relativistic metric specified in res. B1.3 of 2000 for solar system barycentric
coordinate systems. In other words, the ICRS provides the orientation of the BCRS
axes.
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