The wide-field VLBA calibrator survey -- WFCS
  The paper presents the results of the largest to date VLBI absolute
astrometry campaign of observations of 13,645 radio sources with the Very Long
Baseline Array (VLBA). Of them, 7220 have been detected, including 6755 target
sources that have never been observed with VLBI before. This makes the present
VLBI catalogue the largest ever published. Positions of the target sources have
been determined with the median uncertainty 1.7 mas, and 15,599 images of 7179
sources have been generated. Unlike to previous absolute radio astrometry
campaigns, observations were made at 4.3 and 7.6 GHz simultaneously using a
single wide-band receiver. Because of the fine spectral and time resolutions,
the field of view was 4 to 8 arcminutes -- much greater than 10 to 20
arcseconds in previous surveys. This made possible to use input catalogues with
low position accuracy and detect a compact component in extended sources.
Unlike to previous absolute astrometry campaigns, both steep and flat spectrum
sources were observed. The observations were scheduled in the so-called fill-in
mode to fill the gaps between other high priority programs. That was achieved
by development of the totally automatic scheduling procedure.
