).

Finally, the VPAs govern special features such as user-commanded virtual objects (inserted into the stream of real, detected objects, useful for CCD-health monitoring, background monitoring, etc.), calibration faint stars (a small, user-configurable fraction of faint stars receive full-pixelresolution windows for calibration purposes), and suspected moving objects (objects in a certain, user-defined across-scan speed range receive an extra window in BP and RP to increase the probability of measuring moving objects).

The VPUs generate three different kinds of data packets: auxiliary science data (ASD) packets, star packets (SPs), and SIF packets.

The SPs contain the (generally raw) sample data of all scientific CCDs and form the core of the Gaia science data; they have nine flavours (SP1—SP9), but only SP1 (SM/AF/BP/RP data), SP2 (RVS data), SP3 (suspect-moving-object windows in BP/RP), SP4 (BAM data), and SP5 (WFS data) are produced during nominal science operations.

The ASD packets are essential for interpreting and processing the SPs and come in seven flavours (ASD1—ASD7); they provide pre-scan data, logs of TDI-gate activations, charge injections, object logs to ease onground data processing, etc.
3.3.9.

Payload data-handling unit Science data generated by the VPUs (Sect. 3.3.8) is not directly transmitted to ground but first stored in the payload datahandling unit (PDHU).

The PDHU is a solid-state mass memory with a storage capacity of 61 440 sectors, each of size 2 megabytes, providing ∼120 gigabytes effective in total (after subtraction of Reed-Solomon error-correction bits).

The VPAs (Sect. 3.3.8) contain a FILE_ID function that generates an 8bit file identification (FILE_ID in the range [0, 255]), which is stored in the packet header, for each SP/ASD/SIF science packet.

The FILE_ID is assigned in the VPAs, through user-configurable settings, based on VPA-derived attributes such as VPU number (1–7), packet type (SP, ASD, SIF), field of view (preceding or following), object type (virtual object, calibration faint star, suspected moving object, normal star), magnitude, and/or windowtruncation flags.

This maximises early availability on ground of high-priority data, protects such data from being deleted on board, balances onboard data losses between astrometry plus photometry and spectroscopy, and minimises the latency of astrometric data for bright(er) objects, which is essential for the science alert pipelines (Tanga et al. 2016; Wyrzykowski 2016).

In practice, around 80 different FILE_IDs are in use, allowing us to discriminate between critical data (ASD packets), highpriority data (SIF packets, virtual objects, calibration faint stars, and bright stars, i.e.

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:
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