In this paper is described the impact of ionospheric activity on the performance of GNSS systems, starting from a description of the structure, physical phenomena and parameters of the ionosphere. It is explained in detail the cause of the formation of ionospheric errors (time delay of GNSS radio signals) and the possibility of measurement and correction of the same through the various models of the ionosphere and the types of GNSS receivers. The most important ionospheric parameter TEC (total electron content of the ionosphere) is defined, an extensive analysis of the various methods of measuring TEC is carried out and the factors that influence its spatial-temporal variability are listed. The occurrence of ionospheric scintillation and the effects that it can have on the operation of GNSS receivers are also described. The parameters and measurement of ionospheric scintillation are detailed. The analysis of the capabilities to monitor the status of the ionosphere using special GISTM GNSS receiver was performed. There were also conducted measurements of the value of TEC and ionospheric scintillation. There are detailed characteristics and application programs made with PolaRxS_PRO GNSS receiver. Calibrated values of the VTEC are graphicaly presented and analyzed and so are the values of the parameters of ionospheric scintillation.