The paper deals with large-scale crustal deformation due to hydrological surface loads and its influence on seasonal variation of GPS estimated heights. The research was concentrated on the area of Poland. The deformation caused by continental water storage has been computed on the basis of WaterGAP Hydrological Model data by applying convolution of water masses with appropriate Green’s function. Obtained site displacements were compared with height changes estimated from GPS observations using the Precise Point Positioning (PPP) method. Long time series of the solutions for 4 stations were used for evaluation of surface loading phenomena. Good agreement both in amplitude and phase was found, however some discrepancies remain which are assigned to single point positioning technique deficiencies. Annual repeatability of water cycle and demanding procedure for computing site displacements for each site, allowed to develop a simple model for Poland which could be applied to remove (or highly reduce) seasonal hydrological signal from time series of GPS solutions.

Research activities of Polish research groups in a period of 2015–2019 on reference frames and reference networks are reviewed and summarised in this paper. The summary contains the results concerning the implementation of latest resolutions on reference systems of the International Union of Geodesy and Geophysics and the International Union of Astronomy with special emphasis on the changes in the Astronomical Almanac of the Institute of Geodesy and Cartography, Warsaw. It further presents the status of the implementation of the European Terrestrial Reference System 1989 (ETRS89) in Poland, monitoring the terrestrial reference frame, operational work of GNSS permanent IGS/EPN stations in Poland, operational work of the laser ranging station in Poland of the International Laser Ranging Service (ILRS), active GNSS station network for the realization of ETRS89 in Poland, validation of recent ETRS89 realization, expressed in ETRF2000 in Poland, and maintenance of the vertical control in Poland (PL-KRON86-NH). Extensive research activities are observed in the field of maintenance and modernization of gravity control not only in Poland, but also in Sweden and in Denmark, as well as establishment of gravity control in Ireland based on absolute gravity survey. The magnetic control in Poland was also regularly maintained. The bibliography of the related works is given in references.

The article presents the reviewed and summarised research activities of Polish research groups on reference frames and reference networks in a period of 2019–2022. It contains the results on the implementation of latest resolutions on reference systems of the International Union of Geodesy and Geophysics and the International Astronomical Union focusing on changes in the consecutive issues of the Astronomical Almanac of the Institute of Geodesy and Cartography, Warsaw. It further presents the status of the implementation of the European Terrestrial Reference System 1989 (ETRS89) in Poland, monitoring the terrestrial reference frame, including research on global terrestrial reference frames, GNSS data analysis within the EUREF Permanent Network, research on GNSS receiver antenna phase centres, research on impact of non-tidal loading effects on position solutions, and on station velocities. Then the activities concerning the realization of ITRS and ETRS89 in Poland are discussed, including operational work of GNSS IGS/EPN stations as well as operational work of the laser ranging station of the International Laser Ranging Service, with special emphasis on the Polish active GNSS network for the realization of ETRS89 and maintenance of the vertical control network. Extensive research activities are observed in the field of implementation of the International Terrestrial Gravity Reference Frame in Poland, maintenance and modernization of gravity control network in Poland but also in Sweden, establishment of gravity control network in Ireland based on absolute gravity survey as well as maintenance of the national magnetic control network in Poland which is traditionally performed on a regular basis.