Recent research indicates that the influence of the ionospheric Dregion on the propagation of satellite signals cannot be ignored during the intense X-radiation emitted during solar X-ray flares. In this paper, we investigate the influence of changes in the solar hydrogen Lyα radiation, which is manifested in variations of the D-region electron density and, consequently, the total electron content in this region, on the propagation of the Global Navigation Satellite System (GNSS) signals. We consider changes during a solar cycle and year, represented by the smoothed daily sunspot number and the day of year. The obtained results indicate that the influence of the D-region on these signals is not negligible for positioning with a single signal recorded by a single receiver during the period around the maximum of a solar cycle.

Multi-frequency GNSS satellite signals and their availability enable research in undifferentiated and uncombined models in which a single signal at each frequency is a corresponding independent observation. Such possibilities are reflected by the significant interest in geodetic research as well as in many areas of application of GNSS observation. On the geometric path of the signal between the satellite and the receiver, a significant source of GNSS measurement errors comes from the medium of propagation of electromagnetic waves. Errors attributed to signal propagation are ionospheric and tropospheric signal delay. The mentioned error sources significantly affect the quality of certain pseudorange and, thus, the quality of the receiver's position in the form of three-dimensional coordinates. This paper presents the analysis and determination of the ionospheric delay of GNSS satellite signals based on the determination of Total Electron Content - TEC at five stations (Beograd, Leskovac, Sombor, Zaječar, and Kraljevo) in the territory of the Republic of Serbia. In order to determine TEC values, the software called "GPS-TEC analysis" developed within the Institute for Scientific Research, Boston College, USA was used, which enables the determination of the TEC value on the path between the satellite and the receiver. The data collection was carried out within the year 2022 so that the measurement sessions include characteristic dates when the most significant fluctuations are expected in the context of seasonal variations.