G < 16 mag in astrometry and photometry and GRVS < 10.5 mag in spectroscopy), medium-priority data (narrow magnitude bins covering the full magnitude range to cover requirements for First-Look payload health monitoring; Sect. 6.3), and low-priority data (faint stars).

The FILE_ID of a packet determines where it is stored in the PDHU: each FILE_ID uniquely corresponds to a dedicated PDHU file with the same identifier.

At the PDHU level, user-configurable prioritisation of science data is achieved through the following:
Article number, page 13 of 36

– Downlink-priority table: important data are downloaded first.
– Deletion-priority table: in case of PDHU overflow (for instance during Galactic plane scans; Sect. 5.3.1), less important data are deleted first to provide free space for more important data. The default deletion priority is the inverse of the downlink priority.
– Data loss target table: deletion of data from a file, in case of PDHU saturation, is authorised only if the accumulated data loss of that file does not exceed the target; this data loss is estimated as the ratio between the number of sectors deleted in the file and the total number of sectors allocated to the file since the start of the mission (or since the last PDHU reset). Typically, data loss targets are 0% for high- and mediumpriority data and gradually increase to 100% for the faintest objects (G > 20.5 mag and GRVS > 15.8 mag).
A PDHU file can either be dynamic or cyclic in nature:
– A cyclic file has a fixed, user-defined size, meaning that the oldest data are overwritten after the file fills up and wraps around. Cyclic files hence have the property that, after data are transmitted to ground, the data are temporarily left accessible (until overwritten after wrap-around) meaning that data replay is possible, if needed. Cyclic files hence normally store critical data, which means ASD packets. The criticality of these data stems from the fact that one missing packet can inhibit ground processing of thousands of observations.
– A dynamic file grows and shrinks in size as data are added and removed, respectively. Dynamic files have the property that, after data are transmitted to ground, the sector is immediately freed up for new data, meaning that data replay is typically not possible. Dynamic files hence normally store non-critical data, which means SP and SIF packets. For dynamic files, the maximum number of sectors to be transmitted during each access of the file has to be defined by the user.
Because Gaia is not in permanent ground-station contact (Sect. 5.3.1), the PDHU occupancy level typically varies over a day, (partially) filling up outside ground-station contact periods and subsequently emptying during ground-station contacts. During contact, the memory manager cyclically goes through the downlink-priority table, which first contains the cyclic files with critical data, followed by high-, medium-, and low-priority dynamic files with non-critical data. High-, medium-, and lowpriority files have assigned a finite (maximum) number of sectors to download before moving to the next file in the table, with the number reflecting their relative importance. Using a small number of sectors forces rapid multiplexing of all files because, after reaching the end of the priority table and returning to the start, cycling through the cyclic files is rapidly completed because only the data acquired since the files were last visited have to be transmitted. In short, the adopted approach means that all (new) critical data comes down at the start of each ground-station contact period, after which the downlink rapidly multiplexes between non-critical data, taking into account their relative priorities, while keeping up to date with the critical data as it is generated on board; the typical cycle time of the full table is ∼30 min.
3.3.10. Clock distribution unit As explained in Lindegren et al. (2016, see also Sect. 3.1), the fundamental astrometric measurements of Gaia are the observation times at which the image centroids pass the fiducial observation lines of the CCD detectors. Therefore, the architecture Article number, page 14 of 36
of the onboard timing chain has been carefully designed. Central in the time management and time distribution subsystem is the clock distribution unit. This unit maintains a 20 MHz satellite master clock, which is directly derived from an internal, 10 MHz rubidium atomic frequency standard (RAFS) based on the
atomic reference given by the spectral absorption line of the 87Rb isotope. This clock is stable to within a few ns over a six-hour spacecraft revolution and has very small temperature, magnetic field, and input voltage sensitivities. The free running onboard time (OBT) counter, which is the time tag of all science data, is generated from the 20 MHz master clock and is coded on 64 bits; the resolution of this counter is hence 50 ns. The spacecraftelapsed time counter in the central data management unit, which is used for time tagging housekeeping data and for spacecraft operations, is continuously synchronised to OBT using a pulseper-second mechanism.