## 4 Conclusions  
GNSS satellite clocks are a key factor in GNSS time transfer and precise positioning. Periodic effects present in satellite clock data may transfer to the GNSS products, giving false signals in areas such as geodynamical analysis and time transfer. This is of great importance, especially in Precise Point Positioning (PPP) applications with zero-difference data processing. In this paper, the authors prove the existence of identifiable effects connected with satellite orbital periods and common for the same satellite type (MEO, GEO, and IGSO). Other effects, present in all of the analysed signals, and giving a similar spectral structure, may be connected with the processing strategy applied for clock correction determination or maintenance works in the time scale-reference clock switch. Although the jumps caused by switching the reference clock are relatively small (ca. 2 ns), it is worth noting that such effects are not clearly visible and are not documented in final products in any way. Moreover, the usage of the L-S method allows for a determination of the periods not connected with satellites orbital movement nor with the time scale maintenance in several BDS-2 satellite clocks. This kind of phenomenon needs further, extended investigation. The time\(-\)frequency analysis shows that the satellite clock behaviour may vary in time, and determined periods have variable amplitudes over the analysed time span. The analysed data may also include periodic components related to the sampling interval, but this phenomenon was not analysed in this study. The presented analysis was carried out only for the BeiDou system. In future research, it is necessary to study this type of phenomenon also for other GNSS systems, possibly with the use of special software packages developed for analysis unequally spaced data (e.g., [50]) to verify potential limitations of the methodology used.  
Conceptualization, J.K. and K.M.; methodology, J.K. and K.M.; software, J.K. and K.M.; validation, J.K., K.M. and M.J.; formal analysis, J.K., K.M. and M.J.; investigation, J.K. and K.M.; resources, J.K. and K.M.; data curation, J.K. and K.M.; writing--original draft preparation, J.K. and K.M.; writing--review and editing, J.K., K.M., D.P. and F.Z.; visualization, J.K. and K.M.; supervision, J.K., K.M., D.P., M.J. and F.Z.; project administration, J.K., K.M. and D.P.; funding acquisition, J.K. and K.M. Percentage author's contribution: J.K. 60%, D.P. 5%, F.Z. 5%, M.J. 5% and K.M. 25%. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the National Science Centre, Poland (Project No. 2016/23/D/ST10/ 00498); by the Initiative for Excellence Research University grant at AGH University of Science and Technology; and by the research subvention no. 16.16.150.545.  
Not applicable.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**PRN** & **IGS-SVN** & **NORADDI** & **SVN** & **Satellite Type** & **Clock Type** & **Manuf** & **Launch Date** & **SatSutus** & **Healthy** & **Service Signal** \\ \hline  
udidium & CASC & 23 September 2019 & Operational & Healthy & B1/B3/B1C/B2/B3 \\ \hline
46 & C104 & 46938 & ICSO-25 & RD5-3 & Rudidium & CASC & 30 September 2015 & Experiment & – & – \\ \hline
57 & C102 & 46749 & MEO-18 & RD5-38 & Rudidium & CASC & 25 July 2015 & Experiment & – & – \\ \hline
58 & C100 & 46788 & MEO-25 & RD5-85 & Rudidium & CASC & 25 July 2015 & Experiment & – & – \\ \hline
59 & C217 & 43663 & GBO-1 & RD5-3 & Hydrogen & CASC & 1 November 2018 & Operational & Healthy & B1/B3/B1 \\ \hline
60 & C229 & 45344 & GBO-2 & RD5-3 & Hydrogen & CASC & 9 March 2020 & Operational & Healthy & B1/B3/B1C/B3 \\ \hline
61 & C230 & 44807 & GBO-3 & RD5-3 & Hydrogen & CASC & 23 June 2020 & Testing & – & B1/B3/B1 \\ \hline \end{tabular}
\end{table}
Table 1: Cont.