The 2nd realization of the International Celestial Reference Frame: ICRF 2


The International Celestial Reference Frame (ICRF1) was the realization of the International Celestial Reference System (ICRS) at radio frequencies (Ma et al. 1997). It was defined by the very long baseline interferometry (VLBI) positions of 212 defining compact radio sources. Since the adoption of ICRF1 by the IAU in 1997, the work of maintaining the ICRS was given to the IERS, with the International VLBI Service for Geodesy and Astrometry (IVS) having operational responsibility for the VLBI realization. Significant developments and improvements in geodetic/astrometric VLBI sensitivity and quality have been made since the generation of ICRF1. An IERS/IVS Working Group (with members in the USA, France, Germany, Italy, Russia, Ukraine, Australia, and China) was established specifically for the second realization of the ICRF. A complete description of the ICRF2, as well as the procedures used for its elaboration, are given in the IERS Technical Note 35 (2009).

The ICRF2, submitted for adoption by IAU at the XXVII General Assembly (august 2009), contains precise positions of 3414 compact radio astronomical sources, more than five times the number as in ICRF1. Further, the ICRF2 is found to have a noise floor of only 40 micro-arc-seconds (μas) and an axis stability of 10 μas. Alignment of ICRF2 with the ICRS was made using 138 stable sources common to both ICRF2 and ICRF-Ext2. Future maintenance of ICRF2 will be made using a set of 295 new defining sources selected on the basis of positional stability and the lack of extensive intrinsic source structure. The stability of these 295 defining sources, and their more uniform sky distribution eliminates the two largest weaknesses of ICRF1.

The ICRF2 catalogue consists of positions of 3414 sources. Of the total number of sources, 2197 sources are observed only in VCS sessions with most of those observed in only one VCS session. The VCS were a series of six multi-session S/X-band astrometry campaigns designed to map and find precise positions of as many new compact radio sources as possible for use as phase referencing calibrators by the radio astronomical community. The VCS sessions concentrated on making short observations of many new sources. They were not optimized for full sky coverage or atmospheric calibration. In spite of that, many of the VCS source positions are as precise as many non-VCS sources.


Non-VCS sources


1217 objects

among 295 are defining sources


VCS only sources


2197 objects

It should be noted that these positions are not epoch-dependent and hence no epoch is explicitly stated.