GNSS data processing
The OL01 station records, with 1 second sampling, the data coming from the satellites belonging to the GNSS (Global Navigational Satellite System) that orbit the Earth. OL01 data are recorded as RINEX (receiver independent exchange) format files and are processed together with those of other stations of interest to obtain daily estimates of the coordinates of the network stations, called time series, according to two reference systems: the international system ITRF08 (International Terrestrial Reference Frame 2008) and the European system EURA. The calculation, which includes the estimation of the relative positions of GNSS stations and satellite orbits, is performed with the GAMIT/GLOBK software package developed by the Massachusetts Institute of Technology, Harvard University and the Scripps Institution of Oceanography.
In the first stage of elaboration, through the GAMIT package, RINEX files are created and processed, which are used for the estimation of the weakly constrained daily solutions: station coordinates, orbits, atmospheric delays, ambiguities and variance covariance matrices of the solutions themselves. Subsequently, (with the GLOBK package), the calculation of daily solutions is performed, including some stations belonging to the EPN (EUREF Permanent Network) and IGS (International GNSS Service) in addition to OL01 and the stations of the Lodi GNSS network.
The daily solutions obtained are then framed in the ITRF08 and EURA reference systems, and the time series are obtained.
The time series represent the trends of the locations of the analysed site. The following factors are taken into account in the analysis of the time series:
• the time series have seasonal trends;
• in the time series, there are annual random signals related to the movements of the GPS monument, signals determined by the hydrology of the site, atmospheric pressure, and temperature variations.
In addition, it is necessary to consider two types of noise: uncorrelated noise (white noise), which introduces random fluctuations with a Gaussian distribution in the solutions, and time-dependent noise (random walk), which includes the effects of small random movements of the monument (usually a pillar that attaches the GNSS antenna to the rock substrate), uncertainties in the parameters of satellite orbits, atmospheric effects and local phenomena.