As for the accuracy of the measured quantities, a clear distinction must be made between the epoch accuracy and the all-mission accuracy. The former is the accuracy of a measured quantity obtained from only one observation of the object, while the latter is the accuracy obtained at the end of the mission, when all the observations of the same object have been put together and all attitude, calibration and sky data are available. Actually, due to the short exposure times, single observations are obtained with a relatively low signal-to-noise ratio, and the all-mission accuracy is then about times better, where is the total number of observations. The single observation accuracy is however sufficient to determine proper motions and parallaxes from multi-epoch positions and light curves for variable stars from multi-epoch brightnesses.
From a statistical standpoint, the accuracy of both astrometric and photometric measurements is basically determined by the number of detected photons emitted by the object. As for GAIA, since the one-scan exposure time is fixed and the number of observations is approximately the same all over the sky, the main paramenter determining the all-mission astrometric and photometric accuracy is therefore the brightness of the observed object. A weaker dependence on the object's position on the sky and spectrum can then be seen, arising from the scanning law followed by the satellite and the spectral response curve of the detectors, respectively.
The all-mission astrometric accuracy averaged over the sky is given in
Table 2.2 for a G2V star of different magnitudes.
On the basis of Galaxy models the accuracy of parallaxes and proper motions
can be translated into relative errors on distances and tangential velocities.
It is estimated that 21 million distances will be determined to better than
1 per cent, 46 million better than 2 per cent, 116 million better than 5 per
cent and 220 million better than 10 per cent.
As for proper motions, 44 million will be determined to better than
0.5 km/s, 85 million better than 1 km/s, 210 million better
than 3 km/s, 300 million better than 5 km/s and 440 million
better than 10 km/s.
Expected accuracies of epoch photometry in the band and all-mission photometry in the bands are given in Figure 2.9. Accuracies of a few hundredths of magnitude are expected for stars of most spectral types of . When combined with the medium-band photometric measurements performed by the spectrometric instrument, paramenters such as the spectral class, effective temperature, surface gravity, metallicity, interstellar extinction and color excess will be determined for most observed objects. Distances and absolute magnitudes of objects whose parallax is too small to be measured astrometrically will also be obtained from photometric measurements.
The accuracy of radial velocity measurements is expected to be in the 1-10 km/s range, the former value being achieved for stars brighter than and the latter for stars of .