The Recomputation of the EUREF GPS Campaigns in Slovenia Sandi BERK 1 , arko KOMADINA 2 , Marijan MARJANOVIĆ 3 , Dalibor RADOVAN 1 , Bojan STOPAR 4 Abstract A new recomputation of the three EUREF GPS campaigns, carried out on the territory of Slovenia, was performed. All the three campaigns were processed in the ITRF 96 at the mean epoch of each individual campaign, starting from raw data. Final IGS orbits and Earth rotation parameters were transformed from their original reference frames to the ITRF 96. Finally, a combined solution at the mean epoch of the three campaigns was computed. A priori velocity model was used for the combined solution. Results were analyzed and compared with the previous solutions. The purpose of the recomputation was to check the accuracy of the official EUREF sites in Slovenia. The results showed the need for their coordinates to be replaced with the new ones. 1 Introduction In order to connect the Slovene national coordinate system to the ETRS, three EUREF GPS campaigns have been carried out on the territory of Slovenia. The first campaign goes back to 1994 (Slovenia and Croatia 94), the second one to 1995 (Slovenia 95), and the third one to 1996 (Croatia 96). The data for each of these three campaigns were processed independently in the period from 1995 to 1997, for each campaign starting from the IGS reference sites. There was a good opportunity to check the results of the first campaign and also to improve them; a densification of the network according to the first campaign was noticeable and the observation time was extended. This also holds true for the campaign in 1996; this time the densification was made for the territory of Croatia, but there was a certain overlapping with Slovenia. Consequently, three sets of coordinates were obtained. The first campaign was processed in the ITRF 92, epoch 1994.4 [Altiner et al. 1995]. The second campaign was processed in the ITRF 93, epoch 1995.7 [Altiner et al. 1997a]. The last campaign was processed in the ITRF 94, epoch 1996.7 [Altiner et al. 1997b]. The official coordinates of the EUREF sites in Slovenia are still those from the first campaign. Comparing the results (in the ETRS 89) after conclusion of the first computations, we obtained coordinate differences of up to 4 cm at certain sites. To meet our expectations, the coordinates from different years should agree at least in order of 12 cm. Meanwhile, a new recomputation of the EUREF GPS campaigns for Slovenia and Croatia has been realized [Altiner et al. 1999]. A new problem connected with this new recomputation was identified. The combined solution was not satisfying, because the sites not observed in all three campaigns had not been treated appropriately. So, a completely new solution was proposed, based on the ITRF 96 [Seeger 2001]. A need for the recomputation of the EUREF campaigns in Slovenia was explained in the EUREF Publication No. 10 [Stopar et al. 2002]. The Surveying and Mapping Authority of the Republic of Slovenia decided for the new recomputation at the beginning of 2001. The majority of work was entrusted to the Geodetic Institute of Slovenia, in collaboration with the Faculty of Civil and Geodetic Engineering at the University of Ljubljana and Mr. Marijan Marjanović from the State Geodetic Administration of the Republic of Croatia. The expert support was kindly offered also by the Federal Agency for Carto- graphy and Geodesy, Frankfurt on the Main, Germany, where the recomputation was initiated. This cooperation was made possible by the financial reimbursement of the costs from EuroGeographics. 2 Campaign overview The first campaign on the territory of Slovenia (Slovenia and Croatia 94) lasted from May 30 to June 02, 1994. The mean epoch was 1994.41. Four daily sessions of 24 hours were completed. There were 14 sites taken into the re- computation; see Table 1. Altogether, 56 daily RINEX files were used, all the sites were observed for 4 days. The second campaign (Slovenia 95) lasted from September 25 to October 01, 1995. The mean epoch was 1995.74. Seven daily sessions of 24 hours were completed. There were 57 sites taken into the recomputation; see Table 1. Altogether, 219 daily RINEX files were used, in average 3.8 per site. Majority of sites were observed for 3 days, which was also the shortest observation time span. Beside 4 reference sites, 8 new sites were observed for all 7 days. 1 Geodetic Institute of Slovenia, Jamova cesta 2, SI-1000 Ljubljana, Slovenia 2 Surveying and Mapping Authority of the Republic of Slovenia, Zemljemerska ulica 12, SI-1000 Ljubljana, Slovenia 3 State Geodetic Administration of the Republic of Croatia, Gruka ulica 20, HR-10000 Zagreb, Croatia 4 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova cesta 2, SI-1000 Ljubljana, Slovenia