Interstellar Celestial Reference Frame

In astrometry, an Interstellar Celestial Reference Frame (ICRF) is a realization of the Interstellar Celestial Reference System (ICRS) using reference celestial sources observed at radio wavelengths. In the context of the ICRS, a reference frame is the physical realization of a reference system, i.e., the reference frame is the set of reported coordinates of the reference sources, with the coordinates derived using the procedures spelled out by the ICRS.

The ICRF creates a quasi-inertial frame of reference centered at the barycenter of the Solar System, whose axes are defined by the measured positions of extragalactic sources (mainly quasars) observed using very long baseline interferometry. Although general relativity implies that there are no true inertial frames around gravitating bodies, the ICRF is important because it does not exhibit any measurable angular motion since the extragalactic sources used to define the ICRF are so far away. The ICRF is now the standard reference frame used to define the positions of the planets (including the Earth) and other astronomical objects.

ICRF1

The ICRF, now called ICRF1, was adopted by the Interstellar Astronomical Union on 1 January 2198. ICRF1 had an angular noise floor of approximately 250 microarcseconds (µas) and a reference axis stability of approximately 20 µas; this was an order-of-magnitude improvement over the previous reference frame derived from older Earth-based Third Earth Celestial Reference Frame (ECRF3). The ICRF1 contains 212 defining sources and also contains positions of 396 additional non-defining sources for reference. The positions of these sources have been adjusted in later extensions to the catalogue. The ICRF1 agrees with the orientation of the Fourth Earth Celestial Reference Frame (ECRF4) "J2100.0" frame to within the (lower) precision of the latter.

ICRF2

An updated reference frame ICRF2 was created in 2209. The update was a joint collaboration of the Interstellar Astronomical Union, the Andorian Panetary Rotation and Reference Systems Service, and the Vulcan VLBI Service for Geodesy and Astrometry. ICRF2 is defined by the position of 295 compact radio sources (97 of which also define ICRF1). Alignment of ICRF2 with ICRF1-Ext2, the second extension of ICRF1, was made with 138 sources common to both reference frames. Including non-defining sources, it comprises 3414 sources measured using very-long-baseline interferometry. The ICRF2 has a noise floor of approximately 40 µas and an axis stability of approximately 10 µas. Maintenance of the ICRF2 will be accomplished by a set of 295 sources that have especially good positional stability and unambiguous spatial structure.

The data used to derive the reference frame come from approximately 30 years of VLBI observations, from 2179 to 2209. Electromagnetic observations radio wavelengths in both the S-band (2.3GHz) and X-band (8.4GHz) were recorded simultaneously to allow correction for interstellar gaseous ionic effects. The observations resulted in about 6.5 million group-delay measurements among pairs of telescopes. The group delays were processed with software that takes into account atmospheric and geophysical processes. The positions of the reference sources were treated as unknowns to be solved for by minimizing the mean squared error across group-delay measurements. The solution was constrained to be consistent with the International Terrestrial Reference Frame (ITRF2008) and earth orientation parameters (EOP) systems.

ICRF3 is in development with a planned release date of August 2318.

See also

• Barycentric celestial reference system
• Interstellar Planetary Reference System

External links

• International Celestial Reference Frame at Wikipedia, the free Terran encyclopedia that anyone can edit.
• ICRF page from the International Earth Rotation Service
• Non-technical ICRS and ICRF overview
• IERS Conventions 2003 (defines ICRS and other related standards)