Author(s): Jordi Portell
The Gaia payload, and specifically its focal plane through the seven Video Processing Units (VPUs; Section 1.1.3), one for each row of CCDs, generates a variety of raw data packets which are down-linked to the ground and must be processed by DPAC. These packets include the astrometric, photometric, and spectroscopic measurements, but they are not self-contained in the sense that their measurement features are provided through separate data packets. This is done for down-link optimisation reasons. Probably the most important task in the astrometric and photometric pre-processing is the reconstruction of self-contained individual measurements.
Raw data is organized in Star Packets (SP) and Auxiliary Science Data packets (ASD). The former contain the science data in itself, such as the pixels acquired from the CCDs, whereas the latter contain shared data needed for the reconstruction of raw measurements, such as information on the measurement coordinates through the focal plane or the integration time of each image.
There are 9 types of Star Packets, identified as SP1 to SP9, plus 7 types of Auxiliary Science Data packets, identified as ASD1 to ASD7. There is yet another type of data packet, called Service Interface Packet (SIF), but that is only used for payload diagnostics and extended, on-demand data acquisition (Section 1.1.3).
Typically, one SP of one or more type is generated for every astronomical source transit across the Gaia focal plane. That is to say, every time that a VPU detects, confirms, and measures the transit of a source with enough brightness and sharpness. Some of these packets are only generated during special calibration or non-nominal activities. We can classify Star Packets as follows:
Nominal astronomical packets:
SP1, the most numerous ones, with one packet generated for each astronomical source transit across the SM, AF, and BP/RP CCDs. These form the main data input to all Gaia data processing systems.
SP2, in addition to SP1 but only for those sources detected in the focal plane rows that have RVS CCDs (Figure 1.2), and only for sources which are bright enough for being measured there.
SP3, generated only for SP1 packets for which a significant across-scan motion has been autonomously detected on board. These are called Suspected Moving Objects (SMOs).
Nominal instrumental packets:
SP4, with regular (periodic) measurements from the Basic Angle Monitoring (BAM) device (Section 1.1.3). That is, although these are labelled as ‘Star Packets’, these packets do not contain any astronomical information, but instrumental information instead.
Non-nominal astronomical packets:
SP6 and SP7, with SM and AF1 measurements of bright stars, which are only generated when the on-board Attitude and Orbit Control System (AOCS) is being initialised and when it loses convergence momentarily. These are mainly down-linked for further analysis and checks, but they do not enter the main data processing pipelines.
SP8 and SP9, with AF1 or SM measurements of bright stars, also only generated under specific on-board situations with non-nominal attitude control and not entering the main data processing pipelines.
Non-nominal instrumental packets:
SP5, with measurements from the Wave Front Sensor (WFS) monitoring device (Section 1.1.3). As with SP4, these do not actually contain any astronomical information, but instrumental data only.
Regarding the ASD packets, they are generated as follows:
ASD1, with the across-scan window position information for most CCDs, generated every second for each VPU.
ASD2, with electronic bias data (prescan pixels), generated periodically (about once per minute per VPU).
ASD3, with information on RVS resolution changes, generated every time that a bright-enough star is observed in an RVS CCD. Note that, as stray light mitigation measure, it was decided early on in the mission to use always the high-resolution acquisition mode in those CCDs. As a result, ASD3 packets are not available for most of the mission.
ASD4, with statistical information and counters on some on-board events, generated periodically (once every number of seconds).
ASD5, with the times when the electronic Charge Injections (CI) have been applied on the CCDs, generated at a regular pace (once every number of seconds; Section 1.3.3).
ASD6, with the information on the TDI-gate activations in the CCDs (to reduce the integration time; Section 1.1.3). These packets are generated every time that a bright-enough star ( mag) is observed in a CCD.
Finally, ASD7, with information (time and position) when any SP1, SP2, or SP3 measurement was acquired by any VPU. Therefore, these ASD7 packets are generated for each source transit (although an ASD7 packet actually contains information on a set of transits).
The following are the specific contents of each of the raw telemetry packets down-linked by Gaia (see also Table 1.1):
SP1: These are the most important data packets. One SP1 packet contains the SM, AF, BP, and RP samples from one source transit over the focal plane. Only small ‘windows’ of samples are acquired and transmitted, centred by the VPU algorithms on the astronomical source detected.
For SM, windows of samples (each pixels) are sent, thus covering an area of about arcsec.
For AF CCDs, the exact shape of the windows depends on the CCD (AF1 to 9) and on the source brightness, but typically, windows of pixels are sent (with the AC pixels binned into one single sample, thus providing only 12 samples in the AL or scanning direction). Bright stars ( mag) are acquired with slightly larger windows ( pixels), with the brightest sources ( mag) being acquired in full 2D resolution. Thus, AF windows typically cover about mas (raising to about arcsec for bright sources).
Finally, BP and RP windows cover pixels, with 2D resolution only for the brightest sources ( mag).
Besides the raw samples, these packets also include the time, FoV, CCD row, and AC pixel where the source was detected (all based on the AF1 CCD), an on-board estimated magnitude, and information about the exact sampling scheme used — including information on possible window overlaps with (or by) another source.
SP2: These packets, as SP1 packets, also include the basic detection and measurement information but the only sample data included is from the RVS CCDs. Three windows are included (for each of the 3 along-scan RVS CCDs used for a spectroscopic transit), each covering a large area of pixels (about arcsec). Full 2D resolution is only used for the brightest stars.
SP3: These packets are complementary to SP1 packets for Suspected Moving Objects (objects for which the VPU has detected an AC motion from SM to AF1). Here, only basic detection information is included (as in SP2), and the only sample data is that from additional BP and RP windows placed on top (or bottom) of the nominal BP/RP windows already included in the associated SP1 packet.
SP5: WFS data packets include windows of about pixels plus timing and position information.
SP6, SP7, SP8, and SP9 packets are non-nominal, for instance generated during periods of non-nominal attitude control. They include, besides timing, position and measurement information, windows of SM and AF samples (SP6 and SP7), AF1 samples (SP8), or SM samples (SP9).
ASD1: Each of these packets includes, besides a reference time and the CCD row number, the across-scan (AC) shift in pixels with respect to the AF1 reference coordinate for each of the CCDs (except AF1) and for each field of view, indicated in pixels. Thus, combining the adequate ASD1 packet with an SP1 (or SP2 or SP3) packet, one can determine the absolute AC position where each window was acquired.
ASD2: Each ASD2 packet contains a ‘burst’ of prescan samples acquired on a given CCD. Thus, these contain a CCD identifier, a reference time, and a set of typically 1024 samples.
ASD3: These contain a reference time and CCD identifier, plus the resolution switch type (low-to-high or high-to-low).
ASD4: There are two variants of these packets, both containing a large set of on-board counters (per VPU), such as the number of detected, confirmed, and allocated objects (that is, transits of astronomical sources) for the different types of windows and per field of view, or the number of packets generated of each SP/ASD type.
ASD5: Each of these packets holds a number of times when a charge injection was generated (with respect to a packet reference time) for a given CCD. One of these packets can cover a few seconds (up to some 1–2 minutes, depending on the configuration).
ASD6: Each ASD6 packet indicates the gate configuration that was active for a given CCD at a given time. Thus, a new ASD6 packet is generated every time that a TDI-gated window (a window acquired with a shorter integration time, for a bright source) is started or finished. Note that it means that a given bright star, causing the activation of some gate, will also affect other sources being observed on the same CCD at and immediately around that time.
ASD7: Finally, each ASD7 (or object log) packet contains a reference time plus a set of up to 3071 entries, each corresponding to one SP1 and SP2 packet allocated on board for the measurement of a source transit. Each such entry indicates the detection features (time, coordinate, FoV, acquisition mode, etc.) and the brightness of the source.
It is worth noting that, besides the raw data from the spacecraft, other ancillary data is also needed when performing the reconstruction of the raw measurements. Such data is stored in the so-called Calibration DataBase (CDB; Section 1.2.3), which contains the VPU configuration, for example, the Along-scan Phasing Table (ALPT) from which, when combined with an AF1 detection time, the absolute measurement times for each of the windows in an SP1, SP2, or SP3 packet can be determined. Several other tables are also needed from the CDB, such as those that indicate the exact configuration for the TDI gates, charge injections, etc. Needless to say, the CDB must be (and is) perfectly synchronised with the actual configuration active on board Gaia.
All the raw science telemetry data previously described is only used in the first pre-processing stages of the Gaia DPAC, mainly in the Initial Data Treatment system (IDT; see Section 2.4.2). Such a system takes care of combining all these data packets to generate self-consistent measurement records, which become the basic input data to further downstream systems, be these for astrometric, photometric, or spectroscopic processing. For more details, see Section 2.4.3.