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IERS Message No. 149                                       July 23, 2009
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The Second Realization of the ICRF


On behalf of the IERS/IVS Working Group, we are pleased to announce
"The Second Realization of the International Celestial Reference Frame
by Very Long Baseline Interferometry."

IERS Technical Note 35 (http://www.iers.org/MainDisp.csl?pid=46-25772)
describes the generation of a second realization of the International
Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30
years of very long baseline interferometry (VLBI) observations, by an
international team. ICRF2 contains precise positions of 3414 compact
radio astronomical sources, more than five times the number as in the
first ICRF, hereafter ICRF1. Further, the ICRF2 is found to have a noise
floor of only approximately 40 microarcseconds, some 5-6 times better
than ICRF1, and an axis stability of approximately 10 microarcseconds,
nearly twice as stable as ICRF1. Alignment of ICRF2 with the
International Celestial Reference System (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.


Alan Fey and David Gordon


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