## 2 Geodetic Surveys and Data Processing  
The use of the GPS measurement technique in the Republic of Croatia began in the late 1980s and early 1990s as part of several domestic and international scientific collaborative projects: CROATIA'91, TYRGEONET'91, TYRGEONET'92, AGREF'92, IGS'92, ZAGORJE'92, IGS'92, BRZA PRUGA'93, SLAVONIJA'93, ADRIATIC MICRPALATE'93, and TYRGEONET'95 [40]. The first reference GPS measurement campaign in the Republic of Croatia, EUREF-CRSLO-1994 (CRSLO'94), was organized in 1994 in collaboration with the Republic of Slovenia [41; 42]. This campaign aimed to connect the national coordinate networks of both countries to EUREF and adopt the reference coordinate system ETRS89. Later, in 1995, Croatia and Slovenia established national reference GPS networks within the CROREF'95 and SOLOVENIA'95 [42] GPS measurement campaigns. In 1996, during the second EUREF GPS measurement campaign, CROREF'96 [43] was performed in the territory of the Republic of Croatia, with simultaneous collaborative measurements in Slovenia and Bosnia and Herzgeovina. The last reference GPS measurement campaign in Croatia, CROREF'05, was  
Figure 1: Low coverage of GNSS-derived velocities in the area of the Republic of Croatia represents a scientific gap (modified from [3]. performed in 2005 [44]. The campaign was again conducted in collaboration with Slovenia and Bosnia and Herzegovina. Parallel to these national reference GPS measurement campaigns, in 1994, the Republic of Croatia (Croatian State Geodetic Administration) started the so-called Croatian Geodynamic project, CRODYN [45], in collaboration with the former IfaG (today BKG). The purpose of CRODYN was (and still is) to track and monitor the displacements of the Adriatic microplate (AMP) using the GPS campaign measurements performed in 1994 (CRODYN'94), 1996 (CRODYN'96), 1998 (CRODYN'98) [29], and finally in 2013 (CRODYN'13) [46; 47]. The Republic of Croatia also participated in international geodynamic projects by extending the project networks to Croatian national (GPS) ones. Croatia participated in CERGOP (in 2002, followed by CERGOP-2; [48], which is in CERGN ([49], with eight GPS measurement campaigns (1994, 1995, 1996, 1997, 1999, 2001, 2003, and 2005) [50]. In 1997, the BKG started the EUVN [1] ([PERSON] et al., 1998) project to unify the different European height data from GPS/levelling stations. Croatia participated in EUVN with the GPS measurement campaign EUVN'97 [51]. In 2002 and 2003, the Republic of Slovenia was active in an AMP geodynamic study within the PIVO project [52] through GPS measurement campaigns in the territory of Slovenia, as well as Croatia. The last international geodynamic research project of AMP in which the Republic of Croatia took part was RETEAT [53], with measurement campaigns in 2003, 2004, and 2005. Details on the Croatian national networks and project results are available in [54]. The most crucial milestone in future geodynamic studies of AMP over the territory of Croatia in the 20 th century was the establishment of a national network of continuously operating (GNSS) reference stations (CORS) called the Croatian positioning system (CROPOS) in December 2008 by the Croatian State Geodetic Administration within the PHARE-2005 project [55]. It included the exchange of CORS data with the positioning systems (POSs) of bordering countries, including Slovenia (SIGNAL), Hungary (GNSSnet.hu), and Montenegro (MontePOS). The geodetic observation data and obtained results used in this paper were a part of the geodynamic studies in the territory of the Republic of Croatia received from [47; 56; 57; 58]. The data from the 17 selected GPS measurement campaigns for 20 years (1994-2013) listed in Table 1, as well as the GNSS data from the CROPOS and POSs of bordering countries (2008-2013), were used to obtain the combined solution for the campaign data and POS data using the Bernese 5.0 GPS software [4].