# GPS Results from Long Time Monitoring of Geodynamic Processes in South-Western Bulgaria
## 2 GPS Data and Estimation  
At the end of 2019, the project \"Monitoring of geodynamic processes in the region of Sofia\" of the Department of Geodesy, the National Institute of Geophysics, Geodesy and Geography at BAS, funded by the Bulgarian National Research Fund, was launched [11]. This allowed for a new comprehensive measurement of the geodynamic network. The campaign was concluded in the summer of 2020 [10; 12]. In the next summer, of 2021, three additional points were measured--BELM, SATO, and LOZ2 (Figure 1). Point BELM was previously measured in 1997 and 2000, and point SATO was measured in 1996 and 2003. The original point LOZE was destroyed after the measurements in 1997 and 2000; therefore, in 2021, we measured the duplicating point LOZ2, which was measured in 1997 but with a shorter observation period. For this reason, the obtained result for velocity of this point has greater error but is still reliable [13]. Point PLA1 is a CORS station in EPN Densification network from 2012 [14].  
The measurements were processed/reprocessed in a two-step procedure using the GAMIT/GLOBK software v10.71 [15; 16] to ensure the quality and homogeneity of the solutions. In the first step, loosely constrained estimates of station coordinates, Earth orientation, orbital parameters, and atmospheric zenith delays were determined using GAMIT. Major models and parameters used in GAMIT GPS data processing are given in Table 1. In the second step, a global Kalman filter was applied using GLOBK, to the combined, loosely constrained solutions and associated covariances, in order to estimate a consistent set of station coordinates and velocities. A six-parameter transformation was estimated by minimizing the horizontal velocities of 10 globally distributed IGS stations with respect to the IGS14 realization of the ITRF2014 reference frame [17]. The epochs of all GPS measurements included in this study is shown in Table 2.  
\begin{table}
\begin{tabular}{l l} \hline Elevation cutoff angle and data weighting & \(10^{\circ}\), data weighting depending on the elevation angle \\ \hline Data sampling and data weighting & 30 s for data editing, and 120 s for parameter estimation \\ \hline Antenna phase left & IGS ANTEX files are used to correct absolute PCVs of satellite and receiver \\ \hline Ionospheric refraction & Ionosphere-free linear combination \\ \hline \multirow{3}{*}{Troposphere refraction} & VMF1 for dry delay and parameter estimation in 2-h intervals for wet delays. \\  & Troposphere horizontal gradients in 24-h interval are estimated. \\ \cline{1-1}  & Atmospheric tidal loading corrections VMF1 [18]. \\ \hline Ocean tide & FES2004 model [19] with correction for the left-of-mass motion \\ \hline Solid Earth tide, pole tide & Models recommended by IERS Conventions 2010 \\ \hline \end{tabular}
\end{table}
Table 1: Major models and parameters used in GAMIT GPS data processing.  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Point ID**} & \multicolumn{8}{c}{Year of Measurements} & \multicolumn{1}{c}{Number} \\ \cline{2-10}  & **1996** & **1997** & **2000** & **2001** & **2002** & **2003** & **2004** & **2012** & **2017** & **2020** & **2021** & **of Epochs** \\ \hline BANK & * & * & & & * & & * & 4 \\ BELI & * & * & & & & * & & 3 \\ BELM & * & * & & & & * & & 4 \\ BOXS & * & * & * & * & & * & & 5 \\ BONN & * & * & & * & & * & & 5 \\ BUHO & * & * & & & & * & 3 \\ CARV & * & * & & * & & * & 4 \\ DELA & * & * & * & & * & & 4 \\ DOB1 & * & * & * & & & * & 4 \\ DSEC & * & & & & & * & 3 \\ FROL & * & & & * & & * & 2 \\ CURM & & & & * & * & & 5 \\ KRAL & * & & & * & * & & 2 \\ LOZ2 & * & & & & & * & & 3 \\ MALC & * & & & & * & & 2 \\ MECH & & & * & & & * & 2 \\ MUHO & * & * & & & * & & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Epochs of GPS measurements. The asterisk indicates the presence of measurements.