International Celestial Reference System (ICRS)
The International Celestial Reference System (ICRS) is the fundamental
celestial reference system adopted by the
International Astronomical
Union
(IAU) for high-precision positional astronomy. The
ICRS, with its origin at the solar system barycenter and "space fixed"
axis directions, is meant to represent the most appropriate coordinate
system for expressing reference data on the positions and motions of
celestial objects.
Background
The ICRS was established by a set of specifications agreed to by the international
astronomical community from 1997 to 2006. The origin of the ICRS is at the
barycenter of the solar system and the orientation of its axes is "space fixed" (kinematically non-rotating)
with respect to distant objects in the universe. Other specifications include a metric tensor,
a prescription for establishing and maintaining the axis directions,
a list of benchmark objects with precise coordinates for each one,
and standard models and algorithms that allow these coordinates to be transformed
into observable quantities for any location and time.
In this context it is helpful to distinguish between a reference system and a
reference frame as used in astronomy. A reference system
is the complete specification of how a celestial coordinate system is to be
formed. It defines the origin and fundamental planes (or axes) of the
coordinate system. It also specifies all of the constants, models, and
algorithms used to transform between observable quantities and reference
data that conform to the system. A reference frame consists of a set of
identifiable fiducial points on the sky (specific astronomical objects), along with
their coordinates, that serves as the practical realization of a reference system.
For example, the fundamental plane of an astronomical reference system
has conventionally been the extension of the Earth's equatorial
plane, at some date, to infinity. The declination of a star or
other object is its angular distance north or south of this plane. The
right ascension of an object is its angular distance measured
eastward along the equator from some defined reference point where the right
ascension value is set to zero. This
reference point, the origin of right ascension, has traditionally been
the equinox: the point at which the Sun, in its yearly circuit of the
celestial sphere, crosses the equatorial plane moving from south to
north. The Sun's apparent yearly motion lies in the ecliptic,
the plane of the Earth's orbit. The equinox, therefore, is a direction in
space along the nodal line defined by the intersection of the ecliptic and
equatorial planes; equivalently, on the celestial sphere, the equinox is at
one of the two intersections of the great circles representing these planes.
Because both of these planes are moving, the coordinate systems
that they define must have a date associated with them; such a reference
system must be therefore specified as "the equator and equinox of [some
date]".
Of course, such a reference system is an idealization, because the
theories of motion of the Earth that define how the two planes move are
imperfect. In fact, the very definitions of these planes are problematic for
high-precision work. Even if the fundamental planes are defined without
any reference to the motions of the Earth, there is no way to magically
paint them on the celestial sphere at any particular time. Therefore,
in practice, we use a specific reference frame—a set of fiducial
objects with assigned coordinates—as the practical representation of an
astronomical reference system. The scheme is completely analogous to
how terrestrial reference systems are established using survey control stations
(geodetic reference points) on the Earth's surface.
Most commonly, a reference frame consists of a catalog of precise
positions (and motions, if measurable) of stars or extragalactic objects
as seen from the solar system barycenter at a specific epoch (now usually
"J2000.0", which is 12h TT
on 1 January 2000). Each object's instantaneous position, expressed
as right ascension and declination, indicates the object's angular distance
from the catalog's equator and origin of right ascension.
(A catalog's right ascension origin was formerly referred
to as the catalog equinox, a now-obsolete term.) Any two such
objects in the catalog therefore uniquely orient a spherical coordinate
system on the sky—a reference frame.
A modern astrometric catalog contains data on a large number of
objects (N), so the coordinate system is vastly overdetermined. The
quality of the reference frame defined by a catalog depends on the
extent to which the coordinates of all possible pairs of objects
(approx. N2/2) serve to define the identical equator and right
ascension origin, within the expected random errors. Typically, every
catalog contains systematic errors, that is, errors in position
that are similar in direction and magnitude for objects that are in the
same area of the sky, or are of the same magnitude (flux) or color
(spectral index). Systematic errors mean that the reference frame is
warped, or is effectively different for different classes of objects.
Obviously, minimizing systematic errors when a catalog is constructed
is as important (if not more so) than minimizing the random errors.
To be useful, a reference frame must be implemented at the time of
actual observations, and this requires the computation of the geocentric
coordinates of the catalog objects at arbitrary dates and times. The
accuracy with which we know the motions of the objects (unless they are assumed
zero) is an essential factor in this computation. Astrometric star
catalogs list proper motions, which are the projection of each
star's space motion onto the celestial sphere, expressed as an angular
rate in right ascension and declination per unit time. Because the
tabulated proper motions are never perfect (even if assumed zero), any
celestial reference frame deteriorates with time. Moreover, systematic
errors in the proper motions can produce time-dependent warpings and
spurious rotations in the frame. Therefore, the accuracy and
consistency of the proper motions are critical to the overall quality,
utility, and longevity of reference frames defined by stars.
The positions of solar system objects can also be used to define a
reference frame. For each solar system body involved, an
ephemeris (pl. ephemerides) is used, which is simply a
table or file of the celestial coordinates of the body as a function of time (or
an algorithm that yields such a table). A reference frame defined by
the ephemerides of one or more solar system bodies is called a
dynamical reference frame. Because the ephemerides used
incorporate the theories of motion of the Earth as well as that
of the other solar system bodies, dynamical reference
frames embody in a very fundamental way the moving equator
and ecliptic, hence the equinox. They have, therefore, been used to align
star catalog reference frames properly (the star positions were systematically
adjusted) on the basis of simultaneous observations of stars and planets.
However, dynamical reference frames are not very
practical for establishing a coordinate system for day-to-day
astronomical observations. The ICRS does not involve a dynamical
reference frame.
Descriptions of reference frames and reference systems often refer
to three coordinate axes, which are simply the set of right-handed
Cartesian axes that correspond to the usual celestial spherical
coordinate system. The xy-plane is the equator, the z-axis points
toward the north celestial pole, and the x-axis points toward the origin
of right ascension. Although in principle this allows us to specify the
position of any celestial object in rectangular coordinates, the
distance scale is not established to high precision beyond the solar
system. What an astronomical reference system actually defines is the way in
which the two conventional astronomical angular coordinates, right
ascension and declination, overlay real observable points in
the sky.
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Late 20th Century Developments
The establishment of celestial reference systems is coordinated by the
International Astronomical Union
(IAU).
The previous astronomical reference system was based on
the equator and equinox of J2000.0 determined from observations of planetary
motions, together with the IAU (1976) System of Astronomical Constants and
related algorithms (Kaplan 1982).
The reference frame that embodied this system for practical purposes was the
Fifth Fundamental Catalogue (FK5) (Fricke et al. 1988).
The FK5 is a catalog of 1535 bright stars (to magnitude 7.5), supplemented by
a fainter extension of 3117 additional stars (to magnitude 9.5). The FK5 was
the successor to the FK3 and FK4 catalogs, all compiled from catalogs of
meridian observations taken in the visual band (many such observations were,
in fact, taken by eye). The formal uncertainties in the star positions of
the FK5 at the time of its publication in 1988 were about
30-40 milliarcseconds over most of the sky, but the errors are considerably
worse when systematic trends are taken into account.
Beginning in the 1970s, the most precise wide-angle (all-sky) astrometry was
conducted not in the optical regime but at radio wavelengths, involving the
techniques of
Very
Long Baseline Interferometry (VLBI)
and pulsar timing. Uncertainties of radio
source positions listed in all-sky VLBI catalogs are now typically less than
one milliarcsecond (5 × 10–9 radian), and often a factor of ten better.
Furthermore, because these radio sources are very distant extragalactic objects (mostly
quasars) that are not expected to show measurable intrinsic motion, a reference frame
defined by VLBI positions should be "more inertial" (less subject to spurious
rotation) than a reference frame defined by galactic objects, such as stars or
pulsars. The VLBI catalogs do have the disadvantage that their origin of
right ascension is somewhat arbitrary; there is no real equinox in VLBI
catalogs, since VLBI has little sensitivity to the ecliptic plane. The VLBI
origin of right ascension has effectively been carried over from one catalog
to the next; it was originally based on the right ascension of the radio source
3C 273B
measured using lunar occultations.
Because of the accuracy and stability of radio reference frames, since the mid 1980s,
astronomical measurements of the Earth's rotation—from which astronomical time is
determined—have depended heavily on VLBI, with classical methods based on
star transits phased out. Hence the situation evolved to where the
definition of the fundamental astronomical reference frame (the FK5) became
irrelevant to some of the most precise and important astrometric measurements.
VLBI revealed, in addition, that the models of the Earth's precession and
nutation that were part of the old system were inadequate for modern
astrometric precision. In particular, the "constant of precession"—a
measurement of the long-term rate of change of the orientation of the Earth's
axis in space—had been overestimated by about 0.3 arcseconds per century.
Moreover, the success of the European Space Agency
Hipparcos
astrometric satellite
, launched in 1989, promised to provide a new, very
accurate set of star coordinates in the optical regime.
Thus, beginning in 1988, a number of IAU working groups began considering the
requirements for a new fundamental astronomical reference system
(Lieske & Abalakin 1990,
Hughes et al. 1991). The resulting
series of
IAU resolutions, passed in 1991, 1994, 1997, and 2000
(IAU 1992,
1996,
1999,
2001), effectively form the
specifications for the ICRS. The axes of the ICRS are defined by the adopted
positions of a specific set of extragalactic objects, which are assumed to have
no measurable proper motions. The ICRS axes are consistent, to better than 0.1
arcsecond, with the equator and equinox of J2000.0 defined by the dynamics of
the Earth. However, the ICRS axes are meant to be regarded as fixed directions
in space that have an existence independent of the dynamics of the Earth or the
particular set of objects used to define them at any given time.
The promotion, maintenance, extension, and use of the ICRS are the
responsibilities of
IAU Division A
(Fundamental Astronomy)
, especially
Commission A1 (Astrometry)
and
Commission A2 (Rotation of the Earth)
.
The
International Earth Rotation and Reference System Service (IERS)
, which was established by the IAU and
International Union of Geodesy and Geophysics (IUGG), is also involved.
The IERS generates VLBI-based science products for astrometry and geodesy, and the IAU
entities provide a framework within the astronomical community
for international collaboration, overall guidance for the work, and evaluation and
endorsement of results.
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ICRS Implementation
The Defining Extragalactic Frame
The International Celestial Reference Frame (ICRF or ICRF1) is a catalog of adopted
positions of 608 extragalactic radio sources observed with VLBI, all strong (greater than 0.1 Jy)
at S and X bands (wavelengths 13 and 3.6 cm) (Ma & Feissel 1997).
Most have faint optical counterparts (typically with visual magnitudes fainter than 18) and the
majority are quasars. Of these objects, 212 are defining sources that
establish the orientation of the ICRS axes, with origin at the solar system
barycenter. Typical position uncertainties for the defining sources are of order
0.5 milliarcsecond; the orientation of the axes is defined from the ensemble to
an accuracy of about 0.02 milliarcseconds. As described below, these axes
correspond closely to what would conventionally be described as "the equator and
equinox of J2000.0".
The
International Earth Rotation and Reference System Service (IERS)
monitors the radio sources involved in the ICRF. This
monitoring is necessary because, at some level, most of the sources are variable
in both flux and structure, and the centers of emission can display spurious motions,
which may not be linear on the sky or constant in rate; see the discussion in
Ma et al. (1998), Section 8.
By 2006, it was recognized that a major update of the ICRF was needed to maintain the
accuracy and fixed orientation of the overall frame, and an IAU working group was established
to update the list of sources and coordinates. The working group presented a revised and
extended list of sources and coordinates. The new list was adopted by the IAU in 2009 as the
Second Realization of the International Celestial Reference Frame (ICRF2)
(Ma et al. 2009), superseding
the original in defining the spatial orientation of the ICRS at S and X bands. The ICRF2 has 295
defining sources, chosen from a solution for the positions of 3414 sources. Only 97 of the
defining sources are also defining sources in ICRF1, reflecting the results of the ongoing
analysis of source stability and the working group's goal of mitigating source position variations.
The positional uncertainties have been reduced considerably and the new list is more
evenly distributed across the sky, especially in the south. Typical ICRF2 defining source position
errors, all things considered, are approximately 0.1 milliarcseconds. The
overall orientation of the axes is estimated to be stable within 0.010 milliarcseconds
and is consistent with that of ICRF1.
The ICRF3
(Charlot et al. 2020)
was adopted by the International Astronomical Union at its XXXth General Assembly (20-31 August 2018)
as the replacement to ICRF2 as of 1 January 2019. It contains positions for 4536 extragalactic sources,
as measured in the S/X band, 8.4 GHz. Unlike previous realizations the subset of 303 defining
source are uniformly distributed on the sky. The S/X band positions are supplemented with positions
824 source positions in the K band, 24 GHz, and 678 sources in the X/Ka band, 32 GHz.
A total of 600 sources have positions available at all three frequencies. The positions were determined
independently at each of the frequencies to preserve the underlying astrophysical content in the
positions. The frame is aligned onto the International Celestial Reference System to within the accuracy
of ICRF2. Individual source coordinates have a noise floor of 0.030 mas. The solution for the ICRF3
shows that the solar system is subject to a galactocentric motion of
0.0058 mas yr–1. Thus, the source positions are given for epoch 2015.0, and
they must be propagated for observations at other epochs to preserve the accuracy.
The Frame at Optical Wavelengths
The ICRS is currently realized at optical wavelengths by stars in the
Hipparcos Catalogue of 118,218 stars, some as faint as visual magnitude 12
(ESA 1997). Only stars with
uncomplicated and
well-determined proper motions (e.g., no known binaries) are used for the ICRS
realization. This subset, referred to as the Hipparcos Celestial Reference
Frame (HCRF), comprises 85% of the stars in the Hipparcos catalog. Hipparcos
star coordinates and proper motions are given within the ICRS
coordinate system but are listed for epoch J1991.25. (That is, the catalog
effectively represents a snapshot of the motion of the stars through space taken
on 2 April 1991.) At the catalog epoch, Hipparcos uncertainties for stars
brighter than 9th magnitude have median values somewhat better than
1 milliarcsecond in position and 1 milliarcsecond/year in proper
motion (ESA 1997,
Mignard 1997). The overall alignment
to the ICRF at that epoch
is estimated to be within 0.6 milliarcsecond, with any spurious rotations or distortions
less than 0.25 milliarcsecond/year. Projected to epoch
2015, typical position errors for the brighter Hipparcos stars are
approximately 25 milliarcseconds.
A major reanalysis of the original Hipparcos observations
(van Leeuen 2007a,
2007b)
resulted in a new Hipparcos catalog with substantially improved astrometric data. However,
the IAU never took any action that officially replaced the original Hipparcos catalog as
the basis for the HCRF.
Launched at the end of 2013, the European Space Agency
Gaia mission is now taking astrometric
observations, and the results will replace the Hipparcos data as the most accurate
representation of the ICRS in the optical wavelengths. The spacecraft is in orbit at
the L2 point, 1.5 million kilometers from Earth. A series of data releases
started in September 2016. Gaia data will eventually be complete
for 1 billion stars, down to magnitude 20, and for stars brighter than
magnitude 15, the estimated final accuracies are expected to be better than
25 microarcseconds in position and parallax and 15 microarcseconds/year in
proper motion. This is an unprecedented leap in astrometric accuracy over
all previous observing programs.
Other representations of the ICRS are described in the section
below titled Data in the ICRS.
Standard Algorithms
At its General Assembly in 2000,
the IAU defined a system of space-time coordinates for (1) the solar system,
and (2) the Earth, within the framework of General Relativity, by specifying
the form of the metric tensors for each and the 4-dimensional space-time
transformation between them (IAU 2001).
The former is called the Barycentric Celestial
Reference System (BCRS), and the latter, the Geocentric Celestial Reference
System (GCRS). Since the IAU definitions of the BCRS and GCRS concern only
relativity, they can be thought of as defining two families of
reference systems; the 2000 resolutions did not specify an absolute orientation for
either (although their relative orientation is described by the transformation
between them). To remedy the situation, in 2006, the IAU passed a resolution
(IAU 2008) that
specified that the ICRS defines the orientation of the BCRS. Thus, the
ICRS and BCRS are closely linked and the two terms are often used
interchangeably. A simple way of understanding the connection is that BCRS coordinates
are expressed with respect to the ICRS spatial axes and ICRS data are based on
the BCRS metric.
Also in 2000 and 2006, the IAU adopted new models for the computation of the
Earth's instantaneous orientation within the ICRS. The new models include
new algorithms for precession and nutation, a new
definition of the celestial pole, and two new reference points in the equatorial plane
for measuring the rotational angle of the
Earth around its instantaneous axis. These models are described in detail in the
IERS Conventions (2010) (Petit & Luzum 2010),
in USNO Circular 179 (Kaplan 2005),
and in the 2013 edition of the Explanatory Supplement to the Astronomical Almanac
(Urban & Seidelmann 2013).
These models
are important when the instantaneous coordinates of celestial objects are to be expressed with
respect to the equator and equinox of date, or with respect to a local horizon-based
system.
A collection of computer modules in Fortran and C that implement
these IAU-recommended algorithms for Earth orientation is the
Standards of
Fundamental Astronomy (SOFA)
library. The
collection is managed by an international panel, the SOFA Reviewing Board,
which works under the auspices of IAU Division A (Fundamental Astronomy).
The board solicits code from the astrometric and geodetic community that
implements the IAU models. Subroutines/functions are adapted to established
coding standards and validated for accuracy before being added to the SOFA
collection. The latest version of the U.S. Naval
Observatory Vector Astrometry Software (NOVAS), available in Fortran, C,
and Python, also implements the IAU models.
The new Earth orientation models are, of course, relevant only to
fundamental observations made from the surface of the Earth. Astrometric
observations taken from space platforms, or those that are differential in
nature (based on reference objects that are all within a small field), are
not affected by these models. However, there are other effects that must be
taken into account in analyzing astrometric observations—e.g., proper
motion, parallax, aberration, and gravitational light-bending—and algorithms
for these may be found in Volumes 1 and 3 of the Hipparcos Catalogue
documentation ESA (1997) and in the
2013 edition of the Explanatory Supplement to the Astronomical
Almanac (Urban & Seidelmann 2013).
For analysis of very high accuracy observations from space, see the development by
Klioner (2003).
Finally, IAU-recommended models for the rotation of the planets, satellites, and asteroids,
compiled by the
IAU
Working Group on Cartographic Coordinates and Rotational Elements
, are given
with respect to the ICRS (Archinal
et al. 2018).
Relationship to Other Systems
The orientation of the ICRS axes is consistent with the equator and
equinox of J2000.0 represented by the FK5, within the errors of the latter.
Since, at J2000.0, the errors of the FK5 are significantly worse than those
of Hipparcos, the ICRS can be considered to be a refinement of the FK5
system at (or near) that epoch.
The ICRS can also be considered to be a good approximation (at least as
good as the FK5) to the conventionally defined dynamical equator and equinox
of J2000.0 (Feissel & Mignard 1998).
In fact, the equator is well determined fundamentally from the VLBI observations
that are the basis for the entire ICRS, and the ICRS pole is within
20 milliarcseconds of the dynamical pole. As previously mentioned, the zero point of
VLBI-derived right ascensions is arbitrary, but traditionally has been set by
assigning to the right ascension of source 3C 273B a value derived from
lunar occultation timings—the Moon's ephemeris thus providing an indirect
link to the dynamical equinox. The ICRS origin of right ascension was made
to be consistent with that in a group of VLBI catalogs previously used by the
IERS, aligned in this way. The difference
between the ICRS origin of right ascension and the dynamical equinox has been independently
measured by two groups that used different definitions of the equinox, but in both cases
the difference found was less than 0.1 arcsecond.
Because of its consistency with previous reference systems, use of the ICRS
would be transparent to any applications with accuracy requirements
that are not more stringent than about 0.1 arcseconds. That is, for
applications of this accuracy—which is good enough, for example, for
telescope pointing—the distinctions between the ICRS, FK5, and dynamical
equator and equinox of J2000.0 are not significant. However, as mentioned above,
implementation of the latest IAU Earth orientation models (precession and nutation)
is needed to express most accurately (and to avoid systematic errors in) the
apparent positions of celestial objects with respect to the equator and equinox
of date, regardless of which catalog or ephemeris is used for the source data.*
For a concise review of the ICRS adoption and its implications, see the paper by
Feissel & Mignard (1998).
* FK5 data should not be used for current applications.
The FK5 proper motions are based on the previous value for the rate of precession, and
their use may cause a very small spurious rotation in the coordinate system defined by
the computed star positions.
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Data in the ICRS
Although the ICRF3 and HCRF are currently its basic radio and optical realizations,
the ICRS has been extended to fainter magnitudes and other
wavelengths. An ever-increasing amount of fundamental astronomical data is
being brought within the system. Some examples (not a complete list) are:
The VLBA Calibrator Survey (VCS) is a list of radio sources,
with positions in the ICRS, to be used as calibrators for the Very
Long Baseline Array and the Very Large Array. Some of the VCS sources are
part of ICRF2. See
Gordon et
al. (2016). The ICRS is also being established at radio frequencies
higher than S- and X-band; see, for example, the reports on
the ICRF3.
At optical wavelengths, the Tycho-2
Catalogue (Høg et. al. 2000)
incorporated a re-analysis of observations from the Hipparcos "star mapper" instrument
with data from 144 earlier ground-based star catalogs. Tycho-2 contains data on 2.5 million stars,
going fainter than the main Hipparcos catalog, and combines the accuracy of the Hipparcos
position measurements with proper motions derived from a time baseline of almost a century.
Also in the optical band, the U.S. Naval Observatory CCD Astrograph Catalog (UCAC)
provides ICRS-compatible positions and proper motions for 113 million stars over the
entire sky as faint as red magnitude 16. Star position accuracies are similar to
Hipparcos and Tycho-2 accuracies at the current epoch for the stars in common, although
UCAC extends to fainter magnitudes. UCAC4, the final pre-Gaia release, was distributed
in 2012 (Zacharias 2013).
UCAC5, a reanalysis using reference star data from the first Gaia data release, with
improved proper motions, was released in 2017
(Zacharias 2017).
The Large Quasar Reference Frame (LQRF) (Andrei
et al. 2009) is another representation
of the ICRS at faint optical magnitudes. It contains the coordinates of 100,165 quasars,
well distributed around the sky, accurate to about 100 milliarcseconds.
The ICRS was extended to the near infrared through the Two Micron All Sky Survey
(2MASS) (Cutri et al. 2003,
Zacharias et al. 2005). This
ground-based program provided positions for 471 million point sources, most of which
are stars, observed in the J, H, and Ks infrared bands. The 2MASS catalog is a
single epoch survey without proper motions; positions are listed for J2000.0,
which is within the 4-year span of observations. Astrometric accuracy at J2000.0
is around 80 milliarcseconds in the Ks magnitude range 9–14,
with larger errors at both brighter and fainter magnitudes.
All modern (post-2000) high precision planetary and lunar ephemerides produced
by three institutions have been aligned to the ICRS:
This means, in practice, that the apparent coordinates of the planets and Moon
computed from any of these ephemerides for a specific time and place will be
comparable to (in the same coordinate system as) the apparent coordinates of stars computed
for the same time and place—providing that the positions and proper motions of the stars
are taken from an ICRS-compatible catalog, and the standard algorithms described
above are used for both the solar system objects and stars.
For a review and comparison of the JPL, IAA, and IMCCE ephemerides, see the papers
from Session 2 of the Journées 2010 conference proceedings
(Capitaine 2011).
The tabulations in
The Astronomical Almanac
are based on ICRS-compatible data sources, including the JPL DE430
planetary and lunar ephemerides (prior to the 2015 edition, DE405/LE405). The almanac is
prepared using the IAU-recommended algorithms for Earth orientation.
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Authorizing IAU Resolutions
The construction and implementation of the ICRS was authorized and
supported by the IAU. Resolution B2, passed by the 23rd General Assembly of the
IAU in August 1997 (IAU 1999), states that
- from 1 January 1998, the IAU celestial reference system
shall be the International Celestial Reference System
(ICRS) as specified in the 1991 IAU Resolution on reference
frames and as defined by the International Earth Rotation
Service (IERS);
- the corresponding fundamental reference frame shall be the
International Celestial Reference Frame (ICRF) constructed
by the IAU Working Group on Reference Frames;
- the Hipparcos Catalogue shall be the primary realization
of the ICRS at optical wavelengths;
- the IERS should take appropriate measures, in conjunction
with the IAU Working Group on Reference Frames, to maintain
the ICRF and its ties to the reference frames at other
wavelengths.
The “1991 IAU Resolution on reference frames” referred to above was
Resolution A4 passed by the 21st IAU General Assembly
(IAU 1992). It recommended that “the space
coordinate grids with origins at the solar system barycentre and at the centre
of mass of the Earth show
no global rotation with respect to a set of distant extragalactic objects” and that
“the principal plane of the new conventional reference system be as near as possible
to the mean equator of J2000.0 and that the origin in this principal plane be as near
as possible to the dynamical equinox of J2000.0.” It also recommended that an IAU
working group establish a list of extragalactic radio sources that would be
“candidates for primary sources defining the new conventional reference frame.” Thus,
the ICRS as established in 1997 was based on specifications defined by the IAU in 1991.
At the subsequent IAU General Assembly in 2000, Resolution B1.2
(IAU 2001) restricted the number of
Hipparcos stars that would be considered part of the optical realization
of the ICRS. The relevant part of this resolution states that
- Resolution B2 of the XXIIIrd IAU General Assembly (1997)
be amended by excluding from the optical realization of the ICRS all
stars flagged C, G, O, V and X in the Hipparcos Catalogue;
- this modified Hipparcos frame be labeled the Hipparcos
Celestial Reference Frame (HCRF).
Effectively, this change eliminated about 15% of the stars in the
Hipparcos catalog, leaving those with well determined linear proper
motions. The flags referred to are given in Hipparcos data field H59.
Resolutions B1.3. B1.4, B1.5 of the 2000 General Assembly defined the
Barycentric Celestial Reference System (BCRS), the Geocentric Celestial Reference System
(GCRS), the transformation between them, and the time scales appropriate for each system.
Resolutions B1.6, B1.7, and B1.8 of the same General Assembly defined
the IAU 2000A precession-nutation model, the celestial pole, points on
the celestial and terrestrial equators from which the rotational angl
of the Earth is measured, and the expression for the Earth rotation angle
as a function of Universal Time (UT1).
At the IAU General Assembly in 2006, Resolution 2
(IAU 2008) completed the definition
of the Barycentric Celestial Reference System (BCRS) with the words
For all practical applications, unless otherwise stated,
the BCRS is assumed to be oriented according to the ICRS axes.
So the fundamental celestial reference system is
actually defined by both the BCRS (relativistic metric) and ICRS (orientation).
Two resolutions were adopted at the XXX General Assembly of the IAU. Resolution B1
adopted the
International Terrestrial Reference System (ITRS)
(Petit & Luzum 2010)
as the preferred GTRS for scientific and technical applications. And resolution B2 adopted the ICRF3
(Charlot et al. 2020).
Texts of all IAU resolutions,
listed by year of the General Assembly at which they were adopted, can
be found at the
IAU
web site
. Extended explanations of the resolutions mentioned
here, as well as formulas for their practical implementation, can be found in USNO Circular 179
(Kaplan 2005).
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References
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