Physics

Frame (networking)

Archaeology

Ephemeris

Galaxy

Astronomy

Geodesy

Stars

Magnitude (astronomy)

Astrometry

Calibration

Proper motion

Satellite

Parallax

Geography

Reference frame

Context (archaeology)

Epoch (astronomy)

Computer science

Astrophysics

Quantum mechanics

Telecommunications

Astronomy and Astrophysics

Context. Gaia null Data Release 2 (Gaia null DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia null satellite during the first 22 months of its operational phase.Aims. null We describe the input data, models, and processing used for the astrometric content of Gaia null DR2, and the validation of these resultsperformed within the astrometry task.Methods. null Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. These data were calculated in two steps. First, the satellite attitude and the astrometric calibration parameters of the CCDs were obtained in an astrometric global iterative solution for 16 million selected sources, using about 1% of the input data. This primary solution was tied to the extragalactic International Celestial Reference System (ICRS) by means of quasars. The resulting attitude and calibration were then used to calculate the astrometric parameters of all the sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion.Results. null For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G null null null = 17 mag, and 0.7 masat G null = 20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas yr−1 , respectively.The optical reference frame defined by Gaia null DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas yr−1 . From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr−1 null in proper motion are seen on small ( null DR2 astrometry are given in the appendices.

Gaia Data Release 2: The astrometric solution