A main difference is that the auxiliary stars in HTPM are replaced by [*Tycho*]{} stars, for which prior positions are used as described in the previous section.

Another difference is that we make the more conservative assumption that calibration errors contribute a constant RMS noise of 300 $\mu$as and 1000 $\mu$as per individual CCD observation, in the along-scan and across-scan direction, respectively. Finally, we use a (more realistic) dynamical attitude model (DAM; ). DAM includes a detailed modelling of the attitude perturbations caused by a number of effects such as micro-propulsion thruster noise and micro-meteoroid hits. The observations are simulated using the so-called ‘astrometric attitude’ , which is the physical attitude averaged over the time required for a source to cross a CCD. Most of the stars are bright which implies that observations are gated [@2012SPIE.8442E..1PK] and use a shorter integration time, resulting in a less smoothed attitude. However, we use the attitude computed for the full CCD integration time of 4.4 s, since the additional noise contribution of shorter integration times is less than 12.7 $\mu$as, see Table 1 in , and therefore negligible in the present context.

The real TGAS must cope with a number of complications which 