Gaia Early Data Release 3: Summary of the contents and survey properties
  We present the early installment of the third Gaia data release, Gaia EDR3,
consisting of astrometry and photometry for 1.8 billion sources brighter than
magnitude 21, complemented with the list of radial velocities from Gaia DR2.
Gaia EDR3 contains celestial positions and the apparent brightness in G for
approximately 1.8 billion sources. For 1.5 billion of those sources,
parallaxes, proper motions, and the (G_BP-G_RP) colour are also available. The
passbands for G, G_BP, and G_RP are provided as part of the release. For ease
of use, the 7 million radial velocities from Gaia DR2 are included in this
release, after the removal of a small number of spurious values. New radial
velocities will appear as part of Gaia DR3. Finally, Gaia EDR3 represents an
updated materialisation of the celestial reference frame (CRF) in the optical,
the Gaia-CRF3, which is based solely on extragalactic sources. The creation of
the source list for Gaia EDR3 includes enhancements that make it more robust
with respect to high proper motion stars, and the disturbing effects of
spurious and partially resolved sources. The source list is largely the same as
that for Gaia DR2, but it does feature new sources and there are some notable
changes. The source list will not change for Gaia DR3. Gaia EDR3 represents a
significant advance over Gaia DR2, with parallax precisions increased by 30
percent, proper motion precisions increased by a factor of 2, and the
systematic errors in the astrometry suppressed by 30--40 percent for the
parallaxes and by a factor ~2.5 for the proper motions. The photometry also
features increased precision, but above all much better homogeneity across
colour, magnitude, and celestial position. A single passband for G, G_BP, and
G_RP is valid over the entire magnitude and colour range, with no systematics
above the 1 percent level.
