The GMRT 150 MHz All-sky Radio Survey: First Alternative Data Release
  TGSS ADR1
  We present the first full release of a survey of the 150 MHz radio sky,
observed with the Giant Metrewave Radio Telescope between April 2010 and March
2012 as part of the TGSS project. Aimed at producing a reliable compact source
survey, our automated data reduction pipeline efficiently processed more than
2000 hours of observations with minimal human interaction. Through application
of innovative techniques such as image-based flagging, direction-dependent
calibration of ionospheric phase errors, correcting for systematic offsets in
antenna pointing, and improving the primary beam model, we created good quality
images for over 95 percent of the 5336 pointings. Our data release covers
36,900 square degrees (or 3.6 pi steradians) of the sky between -53 deg and +90
deg DEC, which is 90 percent of the total sky. The majority of pointing images
have a background RMS noise below 5 mJy/beam with an approximate resolution of
25" x 25" (or 25" x 25" / cos (DEC - 19 deg) for pointings south of 19 deg
DEC). We have produced a catalog of 0.62 Million radio sources derived from an
initial, high reliability source extraction at the 7 sigma level. For the bulk
of the survey, the measured overall astrometric accuracy is better than 2" in
RA and DEC, while the flux density accuracy is estimated at ~10 percent. Within
the scope of the TGSS ADR project, the source catalog as well as 5336 mosaic
images (5 deg x 5 deg) and an image cutout service, are made publicly available
online as a service to the astronomical community. Next to enabling a wide
range of different scientific investigations, we anticipate that these survey
products provide a solid reference for various new low-frequency radio aperture
array telescopes (LOFAR, LWA, MWA, SKA-low), and can play an important role in
characterizing the EoR foreground. The TGSS ADR project aims at continuously
improving the quality of the survey data products.
