are more stars per pixel. A larger scatter is seen in the more sparsely populated areas of the sky, where the statistical uncertainty of the calculated RSE values is higher. The global RSE value is 1.03, the global RMS value 1.09. This shows that TGAS, under the given assumptions, provides formal standard errors that essentially correctly characterize the actual errors.

Simulation including data gaps\[sec:deadtime\]
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![image](EQU_depth6_NUM_TRANSITS_FIELD-DT){width="\columnwidth"} ![image](formalerror_varpiMEDIAN-DT.png){width="\columnwidth"}

The results presented so far and in Table \[tab:results\] are based on a simulation which includes all observations according to the nominal scanning law over the assumed period of 0.5 yr. The real mission has numerous data gaps of varying lengths, caused for example by orbit maintenance manoeuvres, eclipses by the moon, and solar activity. With no observations acquired at these times, the attitude modelling cannot take advantage of the physical continuity of the attitude across the gaps. The result is a globally weakened astrometric solution, which potentially could make the TGAS solution infeasible for a dataset as short as 0.5 yr. We investigate this in a separate simulation including data gaps.

The acquisition dead time for [*Gaia*]{} (the fraction of time during which no observations are acquired) is estimated to be $\simeq 