those in [*Tycho-2*]{} (which incorporate century-old ground-based observations) could reveal not only systematic errors in the [*Tycho-2*]{} proper motions but also some long-period astrometric binaries.

#### Bright-star performance and calibration issues:

Like any other AGIS solution, TGAS will use the generic calibration model described in , Sects. 3.4 and 3.5, which takes into account the actual geometry of the optics and detectors as well as calibrations linked to chromatic image displacements, basic-angle variations, and radiation-induced image displacements. However, a specific complication of TGAS is that it almost exclusively uses stars brighter than magnitude $\sim$12, for which [*Gaia*]{} employs CCD gates [@2012SPIE.8442E..1PK] to avoid saturation. The gated observations need a separate calibration for each gate, but with the limited amount of [*Gaia*]{} data in a TGAS solution there may not be sufficient observations of bright stars to do so. Gate 4 is used for the brightest stars with magnitudes $G\la 8.84$. If it turns out that this gate cannot be reliably calibrated with half a year of [*Gaia*]{} data, we would in the worst case lose all stars brighter than $G\simeq 8.84$ in the TGAS solution, or about 2.3% of the [*Tycho*]{} stars. The reduced number of stars degrades the solution 