4.2.2 Ground Based Optical Tracking (GBOT)

The Ground Based Optical Tracking campaign (GBOT) was founded to provide optical astrometric data to be used to aid the reconstruction of Gaia’s orbit to a precision required to fully correct for aberration and to determine precise baselines for observations of small solar-system objects by Gaia. The requirements are to know the velocity to 2.5 mm s${}^{-1}$, and the position to 150 m. This translates to a requirement of one daily data set (daily = over the course of 24 hours) with a positional error of 20 mas, which is the commitment of Gaia. Despite the fact that Gaia turned out to be 3 mags fainter than the assumed 18 mag, subsequent studies, both theoretical and based on observations, have proven that GBOT can reach those aims in terms of precision. In terms of accuracy, we are limited by the accuracy of the current reference frame, therefore the commitment can only be reached after the data has been re-reduced with Gaia data from the first or another early release.

GBOT utilises a small network of 2 m telescopes, the 2.5 m VST+OMEGACAM on Mount Paranal in Chile, the 2 m Liverpool telescope on Roque de los Muchachos, La Palma, Spain, and the 2 m Faulkes North and South which are based on Maui island (Hawaii, USA) and Siding Spring in Australia. Generally data is obtained on a daily basis, with a 5–7 night Full Moon gap. The GBOT data are reduced and stored at the Observatoire de Paris in a Saada database. The GBOT programme is described in detail in Altmann et al. (2014) and the reduction method and pipeline software in Bouquillon et al. (2014).

The reduced data are collected and delivered to ESOC (Darmstadt) on a monthly basis. The delivery consists of two files, one containing the observations and the other one the information regarding the observatories.

Even if the latter is not updated regularly (only when the need arises, i.e. a new site is added or something regarding an existing one has changed), it is part of the delivered packaged every time. This file contains the topocentric coordinates of the observing site, and identifier and flags which indicate changes.

The other file contains all observations which have passed GBOT’s quality control. These consist of an identify code, an observatory code, a timestamp, coordinates in right ascension and declination and their errors together with some error flags.

In turn the GBOT groups obtains orbit reconstruction data from the MOC group at ESOC, which are then converted to ephemerides, and made available for the observatories on a dedicated GBOT server.

The GBOT data will be used for Gaia orbit reconstruction once the data itself has been re-reduced using astrometry from the first and second Gaia release, ironing out the zonal errors in the current earth based reference catalogue data, the lack of proper motions in Gaia’s first release and other residual systematic effects. For the first two releases GBOT data is not used.