A geodetic reference frame for the Virgo interferometer

The Virgo detector, currently in its 2nd generation configuration Advanced Virgo (AdV), is a Michelson interferometer aimed at the gravitational waves research and at opening a new window on the study of the Universe. It is made of two orthogonal arms being each 3 kilometers long and is located at the site of the European Gravitational Observatory (EGO), in the countryside near Pisa, Italy (Figure 1). This paper reports the development of the Virgo Reference System (VRS) that since 2012 was established for the installation of Advanced Virgo [1]. The VRS consists of a wide-scale high precision reference network adopted for the alignment of new equipments and for the displacement of the existing ones. This reference will be adopted for periodic checks and monitoring activity. Considering the weakness of the network geometry, the survey activity was conducted integrating classical and GNSS measurements by applying the necessary corrections to reduce the coordinates into a unique reference system. Tridimensional coordinates were derived from the GNSS observations and merged with the total station (TS) observations, after applying the appropriate corrections. In addition, a rigorous method to define the relation between orthometric and topographic heights, needed for both design/construction and alignment activities, was carried out.