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gaia early data release 3 documentation

1.3 Spacecraft and ground-segment status applicable to Gaia Early Data Release 3

1.3.1 Time coverage

Author(s): Jos de Bruijne, Gonzalo Gracia-Abril, Neil Cheek, Cian Crowley, Claus Fabricius, Juanma Martín-Fleitas, Alex Hutton, Alcione Mora

For DPAC-internal reasons, the Gaia data flow is artificially split in time segments. The observations accumulated in each of these time segments are the input for the iterative data processing cycles. Each Gaia data release covers an integer number of time segments, always starting from the start of the nominal mission on 25 July 2014. Gaia EDR3 includes the observations taken during Segments 0, 1, 2, and 3 (compared to Segments 0 and 1 for Gaia DR1 and Segments 0, 1, and 2 for Gaia DR2). Table 1.7 shows the start and end dates (UTC at Gaia) and the associated OBMT times of these segments; OBMT stands for on-board mission time(line) and counts the number of six-hour spacecraft revolutions since launch (19 December 2013).

Table 1.7: Time coverage of Gaia data in Gaia EDR3. The gap between segments 0 and 1 is covered by a payload decontamination event (see Table 1.10). Some data products in Gaia DR2 and Gaia EDR3, in particular the astrometric solution, do not include the one-month EPSL scanning (Section 1.3.2) at the beginning of the mission such that, for these data products in these releases, segment 0 effectively starts on 22-08-2014 (OBMT 1192.1). OBMT stands for on-board mission time(line) in units of six-hour revolutions since launch.
Segment Start date End date Start OBMT End OBMT Gaia DR1 Gaia DR2 Gaia EDR3
0 25-07-2014 03-06-2015 1078.4 2330.5
1 04-06-2015 16-09-2015 2335.1 2751.4
2 16-09-2015 23-05-2016 2751.4 3750.6
3 23-05-2016 28-05-2017 3750.6 5230.1

Whereas OBMT is a common time scale used in the data processing consortium, for instance for long-term trend monitoring, and hence appears also in this document, published Gaia data systematically uses barycentric coordinate time (TCB). The approximate relation – with an accuracy of order 1 second – between OBMT (in revolutions) and TCB (in Julian years) at the position of Gaia is:

TCBJ2015.0+(OBMT-1717.6256rev)/(1461rev yr-1). (1.1)

This relation is valid for OBMT>500 rev (3 March 2014).