are ignored in the present experiments aiming to demonstrate the basic feasibility of the concept. The simplifying assumptions include (i) that there are no data gaps in the observations; (ii) that all stars are assumed to be single, and their motions thus consistent with the astrometric model represented by the five (or six) astrometric parameters; and (iii) that it is possible to adequately calibrate the gates used to observe the bright stars. In Sect. \[sec:limits\] we briefly discuss the consequences of these simplifications.

![image](stardensity0-99th-neg){width="\columnwidth"} ![image](EQU_depth6_NUM_TRANSITS_FIELD){width="\columnwidth"}

The main steps of the simulations are as follows:

1.  Astrometric parameters and uncertainties are read from the [*Tycho*]{} and [Hipparcos]{} Catalogues and used to set up the prior information as described in Sect. \[sec:priorconstruction\] and in the HTPM paper, Sects. 2.6 and 2.7. These parameters are also used as initial values from which the iterative astrometric solution is started.

2.  An artificial sky (Fig. \[fig:maps-simulations\], left) is created, representing the simulated ’true’ catalogue (see below). This is required in order to generate [*Gaia*]{} observations of the stars, and as a comparison point to evaluate the quality of the astrometric solution.

3.  The [*Gaia*]{} observations (Fig. \[fig:maps-simulations\], right) of the [Hipparcos]{} and [*Tycho*]{} stars are 