6$%. Additionally, individual observations may be lost due to cosmic rays, CCD defects, charge injection, telemetry losses, etc. Such losses are less damaging to the astrometric solution as they do not create gaps in the attitude determination and sky coverage, although they do affect the results in a statistical way. For bright stars the additional observation dead time is estimated to be about 5%, resulting in a total dead time of 11%.

To explore the robustness of the TGAS solution to acquisition dead time we apply the actual time sequence of data gaps obtained during six months of the early [*Gaia*]{} operations[^3] to the nominal simulation described in Sect. \[sec:sim\]. While most of the applied gaps are shorter than 10 min, the two longest ones are 5.0 and 2.6 days. The total length of the gaps is 15.6 days, corresponding to 8.5% of acquisition dead time. Compared to the nominal value (6%) this simulation is therefore conservative, although we ignore the additional observation dead time.

Removing all observations corresponding to these gaps we find that a stable solution is still possible. Compared with the solution without gaps some stars are observed less, resulting in larger formal errors. About 1000 stars 