|U.S. Naval Observatory||Astrometry Department|
Work at the United States Naval Observatory (USNO) in cooperation with the Remote Sensing Division of the Naval Research Laboratory (NRL), has led to the establishment of an astrometric quasi-inertial radio reference frame. This reference frame is based on the radio positions of strong extragalactic radio sources such as quasars and radio galaxies and has been made possible by the stability of the positions of these sources. However, most extragalactic sources display spatial structure on milli-arcsecond scales for the strong radio emission associated with their compact cores. Despite this shortcoming the positions of a large number of these sources can be used to define a quasi-inertial reference frame (i.e. a frame whose basis is inertial). Higher accuracies can theoretically be obtained by taking into account the structure of the sources.
The radio positions of the over 400 extragalactic sources comprising the RORF are as uniformly distributed over the entire sky as possible with available observations. The positional accuracy of the final catalog is at the 0.5 milli-arcsecond level. The final orientation of the catalog has been obtained by a (rigid body) rotation of the positions into the system of the International Earth Rotation Service (1992 IERS Standards), and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy.
Observations used to determine radio source positions are made using the technique of Very Long Baseline Interferometry (VLBI). An ``observation'' basically consists of the difference in the time of arrival of the radio signals from a distant source measured by pairs of radio telescopes. These telescopes can be separated by distances up to the diameter of the Earth and can measure time of arrival differences with accuracies on the order of 10 picoseconds. Observations made over a time period of approximately 15 years using radio telescopes located around the world have been combined in a general solution to solve for the positions of the RORF.