  The Hipparcos mission (1989-1993) resulted in the first space-based stellar
catalogue including measurements of positions, parallaxes and annual proper
motions accurate to about one milli-arcsecond. More space astrometry missions
will follow in the near future. The ultra-small Japanese mission Nano-JASMINE
(launch in late 2013) will determine positions and annual proper motions with
some milli-arcsecond accuracy. In mid 2013 the next-generation ESA mission Gaia
will deliver some tens of micro-arcsecond accurate astrometric parameters.
Until the final Gaia catalogue is published in early 2020 the best way of
improving proper motion values is the combination of positions from different
missions separated by long time intervals. Rather than comparing positions from
separately reduced catalogues, we propose an optimal method to combine the
information from the different data sets by making a joint astrometric
solution. This allows to obtain good results even when each data set alone is
insufficient for an accurate reduction. We demonstrate our method by combining
Hipparcos and simulated Nano-JASMINE data in a joint solution. We show a
significant improvement over the conventional catalogue combination.
