respect to the background stars Gaia itself is measuring.

These data are routinely being collected through the Ground-Based Optical Tracking (GBOT) programme (Altmann et al. 2014) and are critical when the spacecraft is at low declinations (|δ| <∼15◦).

The GBOT uses a network of small-to-medium telescopes (the ESO VLT Survey Telescope, which has contributed 400 hours of time to date thanks to an agreement between ESA and ESO, the Liverpool Telescope, and the Las Cumbres telescopes) and aims to deliver a daily measurement with 20 mas precision.

The GBOT data can only be used in full in combination with Gaia DR1 because including these data in the orbit reconstruction process requires the Gaia catalogue positions of the background stars against which the position of Gaia was measured.

After processing at MOC, the reconstructed orbit is periodically delivered to the Science Operations Centre for inclusion in the science data processing cycles.

5.3.3. Time synchronisation Time synchronisation denotes the establishment of a relation between the onboard time (OBT) reading of the free-running, atomic master clock on board Gaia (Sect. 3.3.10) and a reference timescale such as Universal Coordinate Time (UTC) or Barycentric Coordinate Time (TCB). Two time-synchronisation chains are active for Gaia:
1. MOC maintains a low-accuracy, on-the-fly service that is based on the predicted spacecraft orbit and uses simplified algorithms. This time synchronisation is used for spacecraft operations, for instance for the interpretation of housekeeping data or the definition of onboard command-execution times, and also for initial (first-look) science data processing at the Science Operations Centre. The formal (required) accuracy of this product is 1 ms, but the typical accuracy in practice is 50–100 µs. 2. For the science cases of Gaia to be met, the absolute time accuracy requirement at mission level is 2 µs, which means that the 1-ms MOC service is insufficient. Therefore, a second time-synchronisation and onboard clock calibration chain is operated by DPAC (Sect. 7). This scheme is based on a time-source, packet-based, one-way clock synchronisation scheme (Klioner 2015) and achieves the 2 µs requirement with significant margin during nominal operations. This scheme is fully relativistic, includes onboard delays, propagation delays, and ground-station delays but unavoidably produces results with a delay of several weeks, for instance because the reconstructed spacecraft orbit is one of the inputs.

[Figure: Fig. 8. Throughput evolution of the astrometric fields of view (averaged over all AF CCDs) as a function of time as revealed by the G-band photometric time-link calibration derived in Riello et al. (2016). The curves are Chebyshev polynomials of the actual calibrations. Red lines refer to the preceding field of view; blue lines to the following field of view. The discontinuities and dotted vertical lines refer to the decontaminations (Sect. 6.4). OBMT stands for onboard mission timeline in units of six-hour revolutions since launch (Sect. 5.3.3). Gaia Data Release 1 is based on data covering the interval OBMT = 1078.3795–2751.3518, delimited by the dashed black lines.]