The coordinates of the extragalactic radiosources in the International Celestial Reference System (ICRS) are derived from a single analysis of all VLBI observations available in 1995, that is about 2 millions observations taken on a worldwide network since 1979. The catalogue of coordinates is known as the International Celestial Reference Frame (ICRF).
The data analysis is based on the state of the art modeling. Parts of the modeling that are relevant to the accuracy of the source coordinates are the parallel estimation of corrections to the standard precession and nutation models, and accounting for azimutal structure in the tropospheric delay. Residual systematic errors are expected to be well under 0.001" (one milliarcsecond).
The tie of the coordinates to the ICRS is set by means of a no-rotation condition with respect to a previous IERS realization of the ICRS, involving the best known sources. The accuracy of the tie is ±0.02 mas. The quoted position accuracies are based on the formal uncertainties derived from the analysis, inflated by additive variance and scaling factor to account for possible effects of mismodeling and and data deficiencies.
The median final uncertainty of the best 253 sources is 0.4 mas. An additional list of 357 less well known sources have a median final uncertainty of 0.8 mas. The selection of best sources involves the consideration of source structure as well as statistical assessment of the astrometric quality of the derived coordinates.
While the current ICRF is understood to be the best realization to date, VLBI observations and monitoring of the sources are continued, and modelling is making progress. Meanwhile, as was experienced in the past, sources with compact structure may be subject to flares and other physical variations that dilute their astrometric suitability. Conversely, some sources currently little observed may prove useful for the definition of the frame. In addition, the ties of the Hipparcos stellar reference frame and of the ephemerides of the solar sytem bodies to the ICRS will have to be monitored. Thus it is necessary to continue the type of studies that led to the building of ICRF.
Whenever the amount of new information and understanding will justify it, updated coordinates will be computed and made available for some or all of the sources. Note that in all future updates of coordinates, the direction of the coordinate axes will be maintained consistent with the ICRS.