generated according to the nominal scanning law, including the perturbations from DAM, and observation noise. For the latter we conservatively assume that all [Hipparcos]{} and [*Tycho*]{} stars are measured with the same accuracies, per CCD observation, as a star of magnitude $G=13$, independent of the actual magnitude ($\sigma_{\textrm{AL}} = 94~\mu$as, $\sigma_{\textrm{AC}}=489~\mu$as in the astrometric field).

4.  The prior data and simulated observations are processed through the astrometric solution which effectively computes a least-squares estimate of all astrometric parameters together with the parameters describing the instrument attitude as a function of time. The components of the attitude quaternion are represented by cubic splines with a knot interval of 30 s. No special provision is made to handle the rate and angle discontinuities introduced by the use of DAM. The astrometric solution is made for the observation interval 2014.5–2015.0 (0.5 yr), with the reference epoch centred on the observations, i.e., at J2014.75. The reference frame is effectively determined by the positions and proper motions in the [*Hipparcos*]{} subset.

5.  The resulting astrometric catalogue is compared with the ‘true’ catalogue and the statistics of the differences are used to characterize the uncertainties of the solution.

The generation of the simulated ‘true’ catalogue is 