## 3 Discussion
### The UNIPA GNSS CORS Network Monitoring  
With an area of 25,711 km2, Sicily (Italy, Europe) is the largest island in the Mediterranean Sea and occupies a strategic position that has made it, over the centuries, a centre of attraction and a crossroads of people and artistic, cultural, and scientific traditions. Until the last decade, the presence of GNSS CORS was limited exclusively to deformation monitoring and strain control activities. Two main CORS networks managed by the INGV (_Istituto Nazionale di Geofisica e Vulcanologia_, Italian National Institute of Geophysics and Volcanology) and the Italian Institute for Environmental Protection and Research (_Istituto Superiore per la Protezione e la Ricerca Ambientale_, ISPRA) have been implemented in the last decades. In Sicily, the INGV integrated network, named _RING_, has been located in the most seismogenetically relevant areas of the eastern part of the island [105; 106], due to the presence of Mount Etna volcano and with higher seismological interest. More generally, this network consists of about 200 stations located throughout Italy, and it is one of the most important CORS networks in Europe and the world. Differently from the RING, another GPS network, called _SiOrNet_, was aimed at monitoring deformation cycles along some faults belonging to the oriental coast of Catania (Timpe) [107]. The _SiOrNet_ GPS network was located in a more restricted area around the Acicatena and S. Tecla-Linera faults (Catania). This network provided valuable support to ISPRA after the volcanic and tectonic events of Etna in 2002 and is a helpful addition to the _Ithaca project_ database. Since 2008, the _UNIPA GNSS CORS_ public network (Figure 2) has been developed and implemented by researchers at the University of Palermo, with Topcon instrumentation, in the framework of the national research project _PRIN2005_ (Networks of GPS permanent stations for real-time surveying). It consists of nine permanent stations (SPs), framed in the RDN (_Rete Dinamica Nazionale_, powered by IGM) ETRF2000 datum, and managed by Geo++ software version 2004 and Nearest (Near), _Flachen Korrektur Parameter_ (FKP), and Virtual Reference Station (VRS) streams [108]. Topcon Positioning Italy (TPI) as a commercial activity and as a result of the collaboration with the university has then expanded the network by installing additional stations in the eastern part of Sicily; thus, nowadays, the network has 17 SPs, framed in the IGS05 datum, managed by Geo++ software version 2004, with VRS and Near available streams. TPI has managed all CORS on the Topcon Live network since 2012 [109]. The papers of [PERSON] et al. [108; 109; 110] showed all details concerning the _UNIPA GNSS CORS_ network design, preliminary analyses with statistical analysis, data availability, and the used geodetic framework, with the coordinates, time series, and displacements from 2008-2012. Table 1 shows the coordinates of the _UNIPA GNSS CORS_. During the first design phase, an RFI was the subject of study regarding the CORS in Palermo, and the radio frequency interferences (RFI) affecting the CORS activities were verified in another application of research [110] through electromagnetic field measurements and GNSS analyses with scientific software (TEQC by UNAVCO, QC2 SKY by the Polytechnic of Turin, and Network Deformation Analysis (NDA) by Galileian Plus) to evaluate RFI influences on tropospheric and ionospheric models. Specifically, the RFI on the Palermo permanent station was analysed during the period 2007-2008. In 2006, three weeks of preliminary electromagnetic monitoring were conducted with a PMM 8055 before permanent stations started their monitoring activities. In particular, the permanent station is equipped with a Net-G3 Topcon receiver using double GPS/GLONASS constellations, is Galileo-ready, and is placed in a plastic rack on a building.