The Gaia-GBOT Astrometric Reduction Pipeline has been developed in the framework of the Gaia Ground Based Optical Tracking Project (The Gaia-GBOT project) by Sebastien Bouquillon (SyRTE/Observatoire de Paris) with the help of Gerard Francou (SyRTE) and all the Gaia-GBOT members (Gaia-DPAC, CU3, Auxiliary Data): C. Barache (SyRTE), T. Carlucci (SyRTE), F. Taris (SyRTE), M.Altmann (ARI/Heidelberg), A. H. Andrei (MCTI/Rio de Janeiro), R.Smart (INAF/Torino), I. A. Steele (ARI/Liverpool-JMU) and S.Els (ESAC/Villafranca).

This pipeline allows to derive high precision astrometry for moving celestial bodies with optical CCD detectors. Its main features are: the possibility to choose from a large set of centroiding algorithms well-adapted to moving objects, robust estimators for the error of positions, and a unique framework for all the parts of the reduction process (specically, the control of the all of the parameters are gathered in a unique monitor file to avoid the choice of some no-coherent set of parameters between each part of the process).
Another special feature of this pipeline is to not consider the images independently, but to automatically group together similar images. The notion of similar images is determined by the values of specific parameters selected by the user in the monitor file (for instance the parameters GroupDistMax or GroupDayMax which allow to group together all the images with center distances and observation time differences below a fixed value).By default, the images considered as similar are the ones taken for the same target, with observation time delay below 1 day, with image center distance below 20 minutes of arc, taken with the same telescope, the same CCD detector and the same filter.

The pipeline is based on two hundred and fifty subroutines written in GNU fortran-95, available under CeCILL free software licence (see below for the donwload conditions) and deals only with FITS images. It is divided into eight main routines:

• The HeaderModif program which homogenizes the headers of the images and groups together similar images.
• The FindSources program which detects in each image all the sources brighter than a threshold value and for each of them extracts the related data in pixel and ADU (position, scattering, flux, ...).
• The CatalogueMaker program which downloads the reference catalog stars corresponding to the field of view of each image.
• The AstroReduc program which identifies among the sources detected into each image, the stars belonging to the reference catalog and performs the astrometric (and photometric) calibration.
• The CentImprover program which improves the determination of the centroid of the sources (only when the centroiding method selected by the pipeline user needs a first preliminary astrometric calibration).
• The TargetFinder program which finds the target among the sources detected into each image and improves the determination of its centroid and other characteristics.
• The MakeTiff program which creates TIFF previews of the image reduction results.
• The MakeReductionOverview program which gathers all the reduction parameters, results and statistical data for each set of observations (i.e. for each group of similar images).

Figure below gives a schematic overview of the links between these different routines (for more details concerning this pipeline and others GBOT tools, please read the following PDF documents)

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