4.1.6 Time scales

Author(s): Sergei Klioner

As explained in Section 4.1.3, Gaia data processing uses the rigorous relativistic definitions of reference systems including time scales as their integral parts. The coordinate time of BCRS — TCB (Soffel et al. 2003) — is used throughout data processing and parametrizes the final Gaia catalogue.

Another important technical time coordinate used in Gaia data processing is the On-Board Mission Timeline (OBMT). OBMT represents the readings of the on-board Gaia atomic clock plus a constant chosen for each continuous time interval between Gaia clock resets in such a way that OBMT remains increasing with physical time. Strictly speaking, OBMT is not a time scale since it is not necessarily continuous. OBMT is a purely technical time coordinate that is however unique for any event on board of Gaia. Although the raw observations of Gaia are parametrized by OBMT, for various purposes (e.g. interrogating the solar-system and Gaia ephemerides), OBMT should be related to TCB. This is done by creating the Gaia time ephemeris — a model of Gaia’s clock fitted to the special time synchronization data — the one-way time transfer from Gaia to the ESA ground stations (Klioner 2015; Klioner et al. 2017).