# 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).

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:

 $\text{TCB}\simeq\text{J}2015.0+(\text{OBMT}-1717.6256~{}\text{rev})/(1461~{}% \text{rev~{}yr}^{-1})\,.$ (1.1)

This relation is valid for $\text{OBMT}>500$ rev (3 March 2014).