# 4.3.1 Calibration of Astrometry

For Gaia DR2, SSO astrometry has relied upon the image parameters determined by IDU, as for the stars, with the same calibrations. The procedures are extensively described in Chapter 2. This choice, which is the simplest possible option to derive SSO positions, has some obvious limitations, in particular:

• signal smearing due to the apparent motion relative to stars, was not taken into account;

• enlarged PSF and LSF due to finite object size were also not modelled, only standard ones were used;

• colour calibration of PSF and LSF derived from fluxes in RP and BP is degraded, due to object motion.

For these reasons, a slightly degraded astrometric performance was expected for SSOs. However, our verification of post-fit residuals (Section 4.5.2) have shown that this degradation is small and probably is mostly absorbed in the systematic errors. The scatter of single measurements at CCD level was found to be very close to the expected, ideal performance. Only very bright SSOs ($G$ $<$10) exhibited an anomalous, high scatter.