GNGTS 2013 - Atti del 32° Convegno Nazionale

pheric delay’ in the travel time of the GPS signal from the satellite to the receiver, with respect to the vacuum model. This delay is modeled using the default model described by Saastamoi- nen (1972), adopting the global pressure and temperature distribution models developed to Boehm et al. (2006). The daily loosely constrained solutions of the 30 clusters obtained after GAMIT processing are combined into a unique solution by the GLOBK software (Herring et al. , 2010b) and aligned into the ITRF2005 reference frame (Altamimi et al. , 2007) by a weight- ed six parameters transforma- tion (three translation and three rotation), using the ITRF2005 coordinates and velocities of the following 16 IGS stations: BUCU, CAGL, GRAZ, IENG, LAMP, MARS, MATE, PENC, SFER, SOFI, TLSE, VILL, WTZR,YEBE and ZIMM At the end of this procedure, the daily time series of the north, east and vertical geographical position components of each site included in the analysis are estimated. These series are analyzed in order to assess the three velocity components for each site, with an observation period longer than 2 yr. The time series component y c ( t ), ( c =1,2,3 for the north, east and vertical component) in the first step of the analysis are modelled by the following relation: (1) where D c and v c are the intercept and constant rate, respectively. The g cj terms are the offset magnitudes for the N identified discontinuities, due to instrumental changes or seismic events eventually occurred at the T j epochs, H is the Heaviside step function. These parameters are estimated using a weighted least square method. Successively, the data (outliers) whose deviation from the mean linear trend, estimated using the previous values, is three time larger than the average WRMS (weighted root mean square) of the position time-series residuals (have been identified and removed. The parameters of Eq. (1) are re-computed without outliers and the residual time series are estimated. These time series are then analyzed with the Lomb- Scargle method (Lomb, 1976; Scargle, 1982), in order to detect the principal periodic signal F with the maximum power value in the spectrum. This research is carried out in the period domain between T /2, where T is the observation time span of the site, and 1 month. The equation 1 has been modified in order to include the terms involving the principal periodic signal F so identified: Fig. 1 – Residual horizontal GPS ve- locities with respect to Eurasia. The fixed Eurasian frame is modelled us- ing the Euler pole proposed to Altami- mi et al. (2007): 56.33 °N, 95.98 °W, ω = 0.261°/Myr. The coloured domains are based on the horizontal interpo- lated velocity field obtained using a D value of 50 Km, that is about three times the average spacing of the net- work (see text and Cenni et al. 2012 for explanations). 30 GNGTS 2013 S essione 2.1