Monitoring of the Garisenda Tower through GNSS using advanced approaches toward the frame of reference stations

The Garisenda tower in Bologna is a symbol of the city and one of the most valuable heritages of the medieval age. The tower is leaning markedly since the XIV century because of a foundation failure and its stability is nowadays under constant monitoring through many sensors. In 2013 a GNSS permanent station was installed on the top of the tower with the aim to test the satellite technology for this particular kind of structural monitoring. Being the leaning of the Garisenda the subject of the investigation and being the sensor placed on its top, one fundamental hypothesis is the stationarity of the ground under the tower with respect to the reference system used for the GNSS measures. This hypothesis has demonstrated to be unreliable considered the high precision of the survey and the Earth crust dynamics, therefore opening interesting issues concerning the reference to be used in such kind of monitoring. The proposed solution rely on a strain model of the area surrounding the Garisenda tower, estimated using data from four other GNSS permanent stations already present in Bologna. The method is described and results are shown in terms of trend over time of the Garisenda's leaning. Nevertheless, the methodology can be generalized for every kind of structural monitoring based on GNSS data for which millimetre level of precision is needed.