  Context: The unprecedented astrometric precision of the Gaia mission relies
on accurate estimates of the locations of sources in the Gaia data stream. This
is ultimately performed by point spread function (PSF) fitting, which in turn
requires an accurate reconstruction of the PSF. Gaia Early Data Release 3
(EDR3) will, for the first time, use a PSF calibration that models several of
the strongest dependences, leading to signficantly reduced systematic errors.
Aims: We describe the PSF model and calibration pipeline implemented for Gaia
EDR3, including an analysis of the calibration results over the 34 months of
data. We include a discussion of the limitations of the current pipeline and
directions for future releases. This will be of use both to users of Gaia data
and as a reference for other precision astrometry missions. Methods: We develop
models of the 1D line spread function (LSF) and 2D PSF profiles based on a
linear combination of basis components. We fit the models to selected primary
sources in independent time ranges, using simple parameterisations for the
colour and other dependences. Variation in time is smoothed by merging the
independent calibrations in a square root information filter, with resets at
certain mission events that induce a discontinuous change in the PSF. Results:
The PSF calibration shows strong time and colour dependences that accurately
reproduce the varying state of the Gaia astrometric instrument. Analysis of the
residuals reveals both the performance and the limitations of the current
models and calibration pipeline, and indicates the directions for future
development. Conclusions: The PSF modelling and calibration carried out for
Gaia EDR3 represents a major step forwards in the data processing and will lead
to reduced systematic errors in the core mission data products. Further
significant improvements are expected in the future data releases.
