done slightly differently for the [Hipparcos]{} stars and the [*Tycho*]{}-only stars (those that are not in the [Hipparcos]{} Catalogue). For the [Hipparcos]{} subset, ‘true’ astrometric parameters are simulated by perturbing the prior data (i.e., the [Hipparcos]{} Catalogue) by amounts that are consistent with the prior covariances. This subset is further described in Sect. 3.2.1 of the HTPM paper. For the [*Tycho*]{}-only stars, the ‘true’ positions are similarly obtained by perturbing the prior values according to their assigned uncertainties and correlations. For the proper motions we regard the values given in [*Tycho-2*]{} as ‘true’ for the present purpose; this is acceptable as they are not used anywhere in the solution, not even as priors. As the [*Tycho-2*]{} Catalogue does not contain parallaxes, we simulate their true values based on the apparent magnitudes, neglecting extinction and assuming that the absolute magnitudes have a normal distribution with mean value $+5$ mag and standard deviation 3 mag (see HTPM paper, footnote 4). Although it would have been possible to make the parallax distribution dependent on the proper motion of the individual star, we do not consider the added complication worthwhile, as the results are rather insensitive to the assumed distributions. Radial velocities are simulated 