are a collection of software routines configurable through a set of parameters that can be changed by telecommand.

The seven VPUs are fully independent although each one runs the same set of VPAs albeit (possibly) with different parameter sets.

Parameter updates are possible but require a transition from VPU operational mode to VPU service mode, which means a loss of science data of a few dozen seconds.

The VPUs and VPAs have a large number of functions such as CCD command generation, including deriving the TDI-line signals from the spacecraft master clock (Sect. 3.3.10) for the synchronisation of the CCD sequencing.

The CCD TDI (line) period is defined as 19,656 master-clock cycles and hence lasts 982.8 µs.

The VPAs are also responsible for the detection, selection, and confirmation of objects.

The detection algorithm uses full-frame SM data to discriminate stars from spurious objects, such as cosmic rays and solar protons, autonomously using PSF-based criteria; the parameter settings adopted for operations guarantee a high level of completeness down to the faint limit at G = 20.7 mag (Sect. 8.4) at the expense of spurious detections in the (diffraction) wings of bright stars essentially passing unfiltered (de Bruijne et al. 2015, ; in May 2016, a new set of parameters was uploaded that accepts fewer false detections at the expense of a reduced detection efficiency of objects beyond 20 mag).

After detection in SM, (the brightest) accepted objects are allocated a window from the pool of available windows.

A final confirmation of each detection is enabled by the CCD detectors in the first AF strip (AF1); this step eliminates false detections in SM caused by cosmic rays or solar protons.

Whether a detected object is actually selected or not for observation, i.e. receives a window, depends on a number of factors.

Several limitations exist, for example in dense areas or when multiple bright stars, each requiring singlepixel-resolution windows, are present in the same TDI line(s); in particular this is caused by the fact that the total number of samples in the serial register that Gaia can observe simultaneously per CCD is 20 in AF, 71 in BP and RP, and 72 in RVS (Sect. 3.3.2).

In case of a shortage of windows, object selection (or resource allocation, where resource refers to serial samples) is based on object priority; the latter is a user-defined attribute which, in practice, is only a function of magnitude, where bright stars have higher priority.

The VPAs assign windows based on the onboard measured position and brightness of the object propagate windows through the focal plane, along-scan in line with the spin rate and across-scan to follow the small, across-scan motion of objects induced by the scanning law (Sect. 5.2).

The window management, meaning the collection of CCD sample data, the truncation of samples in case windows of nearby sources (partially) overlap, and packetisation and lossless compression of the science data is also driven by the VPAs.

In addition, the VPAs feed the (closed) attitude control loop with rate measurements based on the measured transit velocities of 13–18-mag objects between SM and AF1 (Sect. 3.2.1).

They also govern the activation of TDI gates for the along-scan duration of brightstar windows in AF, BP, and RP, and the periodic activation of charge-injection lines in AF, BP, and RP (Sect. 3.3.2).

The VPAs collect health and housekeeping data, such as pre-scan data for CCD-bias monitoring, detection-confirmation-selection statistics, object logs to enable CCD-readout reconstruction for
PEM non-uniformity calibration (Sect. 3.3.2), etc., collect BAM and WFS data (Sects. 3.3.4 and 3.3.3), and collect serviceinterface-function (SIF) data.

The SIF function provides direct access to the synchronous dynamic random-access memory of the VPU, allowing monitoring, debugging, or extracting data that is not available in the nominal telemetry (for instance post-scan pixels or full-frame SM data).

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.