# 3.3.7 Global parameters

Author(s): Sergei Klioner

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 $\gamma$. 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.