VLBI monitoring of compact extragalactic radiosources started in the 1970's as a secondary objective of major projects such as the solar system exploration, the determination of plate tectonics and crustal deformations or the Earth's rotation and deformations (see Sovers et al. 1998 for a review of experiments, models and results in astrometry and geodesy with VLBI). The sets of accurate coordinates of the observed radiosources materialized the quasi-inertial reference frame to which the motions of interest were referred. The first sets of objects selected for these purposes were both bright and compact at S and X bands with the existing observing capabilities. The scientific interest in the celestial reference per se increased with the continuous progress of VLBI technology (hardware and software) and the progressive extension of the observing network to a worldwide one. The set of monitored sources grew from a few tens to currently over 600.
The complete data set (2.6 millions observations to date) is organized in 24-hour multi-baseline observing sessions in the S and X bands (corresponding to respective frequencies 2.3 GHz and 8.4 GHz, and wavelengths 13 cm and 3.6 cm). It is periodically reanalyzed to take advantage of the progress in data analysis capabilities and in modelling of the complex set of phenomena that are to be dealt with for astro-geodetic research. These phenomena concern local and global atmospheric and oceanic effects, the influence of ice sheets or of the fluid core on the Earth's rotation, solid Earth and oceanic tides, plate tectonics, etc. The results used in this work are a part of the recent re-analysis for the ICRF, the so-called ICRF-Ext.1 (Ma et al. 1998; IERS 1999; http://hpiers.obspm.fr/webiers/results/icrf/README.html ) which materializes the IAU-recommended International Celestial Reference System (ICRS) (see Feissel & Mignard 1998). Table 1 summarizes the general characteristics of the 667 sources in ICRF-Ext.1.
Table 1. Physical characteristics of the 667 ICRF Ext-1 fiducial objects
The radiosource coordinates in ICRF and ICRF-Ext.1 are derived from the complete set of observations over 1979-1999. They are qualified in two ways, 1. by ascribing realistic uncertainties that take into account both the random and systematic errors, and 2. by categorizing them, in decreasing order of confidence, as "defining", "candidates", and "other". This complex assessment scheme reflects the existence of variabilities in the apparent directions of the sources, a nuisance for astrometry that may become a signal for astrophysics, if detailed information is available. This study is based on the estimation of session-per-session coordinates in a fixed reference frame for many sources of ICRF-Ext.1. We select sources with a strong observational history after 1988.0 - a time when the global astro-geodetic VLBI program reached a first maturity stage. We then study the source variability in a local frame related to the direction of maximum variance of the series of positions.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000