## 1 Introduction  
The development of the GNSS (Global Navigation Satellite System) has always allowed for follow-on improvements in measuring techniques and methods, leading to the need for continuous registration of satellite signals sent to the surface of the Earth. Such physical sensors for continuous registration of satellite signals are permanent reference stations with precise GNSS antennas and receivers. These networks are the underpinnings of global, regional, and local reference frames [1, 2, 3]. This makes research on GNSS time series a relevant issue both in terms of the determination of periodic components [4] and station velocity [5, 6]. Active geodetic networks that realise the spatial reference system in their respective areas are replacing classical geodetic control networks. The contribution of GNSS reference networks also includes ground-based augmentation systems (GBAS), which facilitate relative positioning with real-time kinematic (RTK), network real-time kinematic (NRTK), and the static approach. The European EUREF Permanent GNSS Network (EPN) realises the European Terrestrial Reference System (ETRS89). The next level of active geodetic networks in Europe is national reference networks in individual countries. Most of them are consistent with the guidelines of the European Position Determination System (EUPOS) initiated in 2002 by the Berlin Senate Department for Urban Development and the European Academy of the Urban Development, Berlin. The EUPOS encompasses Central European and Eastern European institutions (Figure 1) which undertook to build and develop national networksof reference stations (national EUPOS GNSS networks) based on unified standards and guidelines [7; 8; 9; 10]. The EUPOS consists of the following member states with their national EUPOS GNSS networks: Slovakia (SKPOS), the Czech Republic (CZEPOS), Hungary (GNSSNet.hu), Estonia (ESTPOS), Poland (ASG-EUPOS), Bulgaria (BULiPOS), Romania (ROMPOS), Latvia (LATPOS), Moldova (MOLDPOS), Macedonia, Germany (SAPOS), Lithuania (LITPOS), and Slovenia (SLOVENIA EUPOS). The total area covered by these national EUPOS GNSS networks is approx. 34% of the EU area and approx. 14% of Europe. The primary principles of the EUPOS, which apply to national networks, are: the network consists of differential GNSS (DGNSS) permanent multifunctional reference stations; the average distance between the stations does not exceed 75 km; the stations' coordinates are determined with the highest precision in a geodetic reference frame, which is an implementation of ETRS89, and in national coordinate systems; and, in order to improve the geometry of the network and ensure sufficient accuracy near borders, the national networks use observations from stations of other positioning systems participating in the EUPOS that are located in border areas to improve network geometry and ensure sufficient accuracy in borderlands. National EUPOS GNSS networks are obliged to offer three services [9]: EUPOS NRTK, EUPOS DGNSS, and EUPOS Geodetic. Therefore, national systems have to offer DGNSS corrections, which makes it possible to support precise positioning and navigation down to metre (EUPOS DGNSS), centimetre accuracy in real time (EUPOS NRTK), and centimetre  
Figure 1: The range of the EUPOS, source: original work based on European Commission, Eurostat (ESTAT), GISCO Countries, 2020—Administrative Units—Dataset. and sub-centimetre accuracy in the online automated post-processing (APPS) of user GNSS observations (EUPOS Geodetic). The EUPOS Geodetic is a sub-service for automatic online post-processing of code and carrier phase static or kinematic measurements based on Receiver Independent Exchange Format 2.11 (RINEX) or 3.0 observation files sent to it. The service should guarantee decimetre or even sub-centimetre accuracy depending on the measuring instrument, the duration of the session, and measurement conditions. EUPOS members took individualised approaches to this matter.