GNGTS 2016 - Atti del 35° Convegno Nazionale

292 GNGTS 2016 S essione 2.1 ones. The same model could be adopted for defining the probability of earthquake occurrence based upon electrical conductivity. This probability should have similar spurius anomalies due to meteorological activity. Even the study of power supply harmonics could provide a similar contribution to this probability. Finally, sub-ionospheric propagation monitoring could add a probability which is dependent on direction. Recently, from October 2016, a synchronization of VLF signal was attempted at Fermo Station (see Fig. 3) by using a special GPS (NAVMAN, 2002). The sampling system realised by a pc sound card can be synchronized by mixing the signal with an appropriate GPS output clock. NAVMAN output clock is a 10 kHz square wave that is precisely aligned with the UTC second. Synchronization of VLF signal permits to precisely measure phases of radio Broadcasts. Fig. 3 – GPS electronics and connections at the Fermo Station after October 2016.