The sampling by GAIA is quite peculiar and introduces aliasing frequencies which must be taken into consideration when trying to describe the temporal behaviour of the underlying continuous signal (for example when trying to find periods). The situation resembles the one encountered with Hipparcos, though the rotation speed is slower with GAIA.

GAIA will perform a continuous scanning with a scan rate of 60 arcsec/s corresponding to a rotation period of the satellite of 6 hours. The preceding and following astrometric fields of view are separated by an angle of 106 deg, therefore the observing pattern will be: 1h46m-4h14m-1h46m-4h14m. Typically, stars will be observed during 4-5 orbits and then gaps of about 30-40 days will separate these grouped measurements (depending of the star position).

The spectroscopic field of view (Medium Band Photometry, MBP, or spectra) will have grouped observations sampled regularly with time gaps of 6 hours. As in the astrometric field, these grouped observations will be separated by large gaps.

The number of measurements over the mission and the large gaps between grouped observations depend mainly on the ecliptic latitude (see Figure 1).

Figure 1: Number of transits for 5 years mission in the medium and broad band photometric system (MBP and BBP)

When a star is temporarily at the node of the great circle of scanning, it is regularly observed for several days. Average, maximum and minimum number of transits considering an effective observing time of 5 years are:

Useful tools have been elaborated by the GAIA Simulation Working Group . They predict the number of transits expected in a given sky direction, and, alternatively, the sky direction of the instrument pointing at any given time during mission operation. It is possible to play with these tools through the SWG web page: