# 1.3.1 Time coverage

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

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 DR2 includes the observations taken during Segments 0, 1, and 2 (compared to Segments 0 and 1 for Gaia DR1). Table 1.4 shows the start and end dates (UTC at Gaia) and the associated OBMT times of these segments; OBMT stands for on-board mission timeline and counts the number of six-hour spacecraft revolutions since launch.

The approximate relation between OBMT (in revolutions) and barycentric coordinate time (TCB, in Julian years) at Gaia is:

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