## 3 Cropos  
CROPOS, the Croatian Positioning System, is a national geodetic infrastructure in Croatia designed for precise positioning and navigation purposes. It comprises a network of 33 Continuously Operating Reference Stations (CORS) strategically distributed across the country at a mutual distance of 70 km. CORS stations are equipped with Global Navigation Satellite System (GNSS) receivers that collect satellite signals to determine accurate three-dimensional coordinates (latitude, longitude, and elevation) for various applications. The  
Figure 2: CROPOS stations used in the analysis: _black dots_ are CROPOS network stations, _red dots_ are the CROPOS stations used in this analysis, and _yellow star_ is the epicenter of the mainshock on 29 December 2020. CROPOS system became fully operational in 2008. It is available 24/7, and until this hazardous earthquake, major system crashes had not been detected since its establishment. The CROPOS system consists of a control center and reference stations. CROPOS serves as the foundation for geodetic and cartographic activities, providing essential data for surveying, mapping, navigation, and scientific research. The system contributes to the development of geospatial information and supports applications in diverse fields, including civil engineering, land management, environmental monitoring, and disaster management. The accuracy and stability of CROPOS are crucial for ensuring reliable geodetic measurements. This infrastructure allows for real-time or post-processed positioning solutions with high precision, enabling users to obtain accurate geospatial information for a range of applications. The stations provide continuous GNSS measurements while CROPOS's control center handles the transfer of these measurements and the remote control of the stations. The correction parameters are being computed based on the measured data. Differentiating the needs of the users, the CROPOS system has three different levels of accuracy (Table 1). Positioning is provided as real-time and post-processing. The real-time positioning is enabled through the differential (DPS) and highly precise positioning system (VPPS) acquiring measurements with an accuracy of 0.3-0.5 m in DPS and 0.02 m and 0.04 m for horizontal and vertical accuracy in VPPS, respectively. A post-processing positioning system (GPPS) provides original measurement data for post-processing at the highest accuracy requirements, yielding the subcentimeter accuracy. The reference frame of the CROPOS system is defined in ETRF2000, epoch 2008.83, with a standard deviation of the reference stations' coordinates of 1.2 mm in the N-S direction, 1.1 mm in the E-W direction, and 3.4 mm along the ellipsoidal height (reference Marjanovic). The values declare the exceptional stability of the CROPOS reference frame. For the realization of the long-term solution of the reference frame and the definition of its stability through the velocities of the stations based on the GNSS measurements, the minimal period of the continuous measurements taken over 2.5 years (Bleweet Lavalle) is needed. In several studies regarding CROPOS's stability, displacement no greater than 1 cm was found at any off the stations, nor in the N-S, E-W, and height direction. Therefore, the CROPOS reference frame was declared to be an emimently stable GNSS reference frame. In this study, a GNSS time series has been analyzed to determine the scale of the displacement of the CROPOS CORS GNSS reference stations due to the earthquake. CROPOS is used in this study because it can contribute significantly to post-earthquake assessment by providing accurate geospatial data for monitoring surface deformations and understanding geological changes. However, we find it essential to consider the potential impact of earthquakes on the infrastructure and data quality of CROPOS, requiring careful analysis and, if necessary, adjustments to maintain its precision and reliability. Overall, 13 stations were included in this study.