(Note: The following text is mainly taken from Plag, 2006). Box 2.3: The Catalogue of Fundamental Stars is a series of six astrometric catalogues of high precision positional data for a small selection of stars to define a celestial reference frame, which is a standard coordinate system for correlation of star positions. Fourth Fundamental Catalogue (FK4) was measured and published in 1963 to contain 1535 stars in various equinoxes from 1950.0 to 1975.0. Fourth Fundamental Catalogue's Supplement (FK4S) is an amendment to FK4 containing further 1987 stars. Fifth Fundamental Catalogue (FK5) was an update of FK4 in 1988 with new positions for the 1535 stars. It was obsoleted when the quasar based International Celestial Reference Frame (ICRF) was introduced. Fifth Fundamental Catalogue Extension (FK5) was an addition of 3117 new fundamental stars, published in 1991. Sixth Fundamental Catalogue (FK6) is a 2000 update of FK5 correlated with the ICRF through the Hipparcos satellite. It comes in two parts, FK6(I) and FK6(III) containing respectively 878 and 3272 stars. Both are updated and amended versions of FK5, using Hipparcos catalogue data[1]. Hipparcos ... In the past, catalogs of position and proper motion of fundamental stars were defined from dynamical modeling of the Earth's orbital motion. The fixed directions underlying these catalogs were considered conventional for some decades and had to be changed from time to time particularly to take into account advances in the modeling of the motion of solar system objects. The most recent system following these principles is the FK5 \cite[]{fricke++88} and its updated FK6. It is well assessed that the FK5 reference system does not fulfill the demands of modern astrometry \cite[]{kovalevsky97}. As an example, it can be mentioned that the FK5 constant of precession is wrong by 0.3 arcsec/century. Therefore, since 1987, the International Earth Rotation Service (IERS) had determined Earth rotation parameters with respect to extragalactic objects using VLBI. The results were then transformed into the FK5 system, which was the "official" system. Significant progress was made when the {\em International Astronomical Union} (IAU) in 1991 decided to base the realization of its CCRS on kinematic rather than a dynamical definition by using distant extragalactic objects and to adopt directions which would be fixed with respect to a selected set of these objects. One fundamental advantage of selecting extragalactic objects is that they are so distant that their proper motions are not detectable even with the most precise techniques presently available. Moreover, this new concept in the history of the IAU is expected to make coordinate axes fixed with respect to distant matter in the Universe and to ensures that the reference coordinates do not rotate with respect to a large portion of the Universe surrounding our galaxy. In this sense, the new CCRS is quasi-inertial. The related IAU recommendations \cite[see][]{mccarthy92} specify that the origin of the new CCRS is to be at the barycenter of the solar system and the axes are to be fixed with respect to the quasars. The directions would be consistent with their previous realizations, that is the FK5 origin of right ascension and pole (see below), within the uncertainties of the FK5. The IAU resolutions explicitly introduce the Theory of General Relativity as the basis for all theoretical and data analyses related to space and time. With this, it was made sure that the new CCRS would not be detrimental to the analysis of observations from the highly accurate astrometric techniques. The choice of extragalactic objects to realize the fiducial directions was possible due to the availability of a mature and highly accurate observing technique, namely the Very Long Baseline Interferometry (VLBI). Already in 1991, it could be imagined that a realization of the CCRS would become available soon for radio wavelength. However, also in 1991, the IAU decided that such a reference system will not become the actual reference frame for astronomy until it would be completed by a catalogue in the optical range, having in mind the HIPPARCOS Catalogue. Based on the 1991 IAU resolutions, the ICRS is defined in \cite{arias++95}. Based on the definition provided in this article, the IERS published in 1997 the International Celestial Reference Frame (ICRF) \cite[]{ma+fei97}. The ICRF includes the positions of 606 extragalactic radio-sources, out of which 212 are considered to be fundamentally defining the frame. The positions of the other sources are given in the frame, but since they are observed less, their positions are less accurate. The positions of the 212 fundamental sources are determined better than 0.6 mas in $\alpha$ and $\delta$. The ICRF is maintained by high-accuracy observations of extragalactic radio sources by VLBI. This maintenance algorithm ensures stable directions of its axies within $\pm20$ microarcseconds \cite[for more details, see][]{ma+fei97,arias++95} In parallel, the HIPPARCOS stellar reference frame was astrometrically aligned with the ICRF (based on the 212 fundamental sources) to within $\pm 0.6$ mas at the central observation epoch of HIPPARCOS at 1991.25 and in spin/rotation within $\pm 0.25$ mas/year \cite[]{kovalevsky97}. It thus provided the primary realization of the ICRS at the optical wavelengths. Therefore, in 1997, the IAU accepted the ICRS as the new CCRS and the ICRF and HIPPARCOS as its materialization in the radio and optical wavelengths, respectively. The \index{reference systems!ICRS}ICRS is defined and maintained by the IERS. It was adopted by the IAU and the IUGG as the primary celestial reference system, replacing its optical predecessors based on fundamental star catalogs (see Box~2). The observation and analysis aspects related to the realization of the ICRS through the ICRF are today coordinated by the \index{IAG Services!IVS}\ac{IVS}.
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Box2: The ICRF The ensemble of distant extragalactic objects constitutes a quasi-inertial reference frame in which the motion and orientation of the Earth can be measured. In practice, this frame is accessed from the Earth through VLBI observations of compact radio sources, for the most part quasars. The red shifts of these quasars are large enough that their physical transverse movement cannot be detected by current radio or optical techniques, and the objects can be treated conceptually as fixed points in the sky. The IAU recognized the utility and accuracy of the extragalactic celestial reference frame by adopting the ICRF effective 1 January 1998. ICRF-Extension.2 is a catalog of some 700 radio source positions (see also Figure ...\ref{f-icrfe}, Fey et al., 2004). The positions and errors of the 212 “defining” sources of the ICRF define (realize) the axes and precision of the ICRS on which all celestial positions are now placed. While the right ascension origin and pole of the ICRF are consistent with the previous FK5 stellar frame within the much larger errors of FK5, the concept of the ICRS/ICRF is fundamentally different in several respects. The defining objects of the ICRF have no real proper motions, and the axes of the ICRS are decoupled from the equator, the ecliptic and any particular epoch. The quasars and other compact radio sources that are included in the ICRF have point-like optical images. Their red shifts indicate great distances so their emissions must be powered by processes different from stars and galaxies, most probably mass inflow onto massive black holes. At the resolution of geodetic/astrometric VLBI using S-band (2 GHz) and X-band (8 GHz), the objects are generally not point-like but have some structure that can also change with time. Such structure changes can be seen as changes in position up to 1 mas (milliarcsecond). The brightest extragalactic radio sources in fact have too much detectable structure to be good astrometric objects. By balancing the competing criteria of source strength, compactness and constancy of structure and position, a set of ~100 geodetic sources has been selected for routine geodetic VLBI observations while the remainder of the ICRF improves the distribution and density over the sky (see Figure~\ref{f-icrfe}). It should be noted that the small number of VLBI stations in the Southern Hemisphere causes the ICRF to be weaker in all aspects in the southern sky. The quasars in the ICRF emit relatively strongly at microwave frequencies while the great majority of quasars are much weaker or radio-quiet. The ICRF now constitutes the fundamental celestial frame for all astrometric and geodetic purposes. This includes both planetary ephemerides and satellite orbits. The former have been related to the ICRF by specialized VLBI observations of transmitters on planets and spacecraft as well as from locations of VLBI stations. Satellite orbit determination requires accurate measurements of the actual rotation angle of the Earth UT1-UTC as a priori information since the rotation of the orbit nodes cannot be modeled over a long period. VLBI observations of GNSS satellites should be feasible in the future as the observing bandwidth for geodetic VLBI is extended. Such observations would directly connect the satellite frames to the ICRF. The motion of the Earth's axis in space, precession and nutation, is also observed using the ICRF. These measurements provide information about the structure of the Earth as it responds to the torques of the Sun, Moon and planets. The ICRF is essential to geodesy as it is the frame for measuring EOP and the ultimate frame for satellite orbits. The ICRF is also the basis for astrometry. In this regard the ICRF has different realizations at various wavelengths, the microwave VLBI realization being the most accurate at this time. The astrometric satellite GAIA is scheduled for launch in late 2011 and has the potential for generating an optical extragalactic realization with an order of magnitude better precision and two orders of magnitude more objects. Other space missions may refine the positions and proper motions of the brightest stars with corresponding improvement of star tracking for satellite orientation. For most geodetic purposes, however, these improvements will not be applicable since no correspondingly precise ground-based observing system exists. An accurate microwave realization for geodetic VLBI will still be needed. |