The astrometric software AGIS can be used to fit arbitrary global
parameters, that is, parameters that depend on all or most of the
data. A flexible software package in AGIS allows one to fit such
parameters in different groups and modes. The primary use of this
block is the determination of physical parameters, like the PPN
parameter . This requires possibly good calibrations and clear
understanding of systematic errors. This will be done in the future
data releases. Another application of the global block is to determine
instrumental (calibration) parameters characterizing Gaia astrometric instrument as a whole. Examples here are the Velocity and
Basic Angle Calibration (VBAC) and Focal Length and Optical Distortion
Calibration (FOC). VBAC is intended to determine, to the largest
possible extent, effective basic angle variations directly from the
astrometric data (see also Section 3.2.3). FOC
determines arbitrary differential distortions of the Gaia astrometric instrument. In Gaia DR2, VBAC and FOC were used in the
astrometric solution to calibrate variations of the basic angle
and differential variations of the astrometric instrument. In this way
the influence of both effects on the astrometric solution is
mitigated. For Gaia DR2 a simple version of both VBAC and FOC was
used, in which the variations were considered as Fourier polynomials
of the heliotropic phase of Gaia satellite (the coefficients
of the Fourier polynomials were fitted as constant or linear functions of
time modulated by a factor depending on the heliocentric distance of Gaia).
For this reason VBAC and FOC calibrations are called
spin-related distortion model in Section 3.4 of
(Lindegren et al. 2018). As an example, Table 3 of (Lindegren et al. 2018)
gives the Fourier coefficients for the basic angle variations as
measured by the BAM and the corrections for those coefficients fitted by
VBAC with the Gaia DR2 data. In the future releases both VBAC and
FOC will be used in more sophisticated versions to verify and possibly
improve the astrometric solutions.