## 5 Conclusions  
GNSS Continuously Operating Reference Stations (CORS) are used for determining the positions of objects in professional/engineering/scientific applications or establishing new control stations in geodetic and engineering surveying. Any errors at the CORS \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Date** & **Time Period Per Day** & **File Length** & **Interval** & **Data Format** \\ \hline
1–10 July 2020 & & & & \\ \hline
1–10 August 2020 & & & & \\ \hline
1–10 September 2020 & 0000–2400 (UTC) & 15 min & 1 s & RINEX 2.11 \\ \hline
1–10 October 2020 & & & & \\ \hline
1–10 November 2020 & & & & \\ \hline
1–10 December 2020 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Observation/data file information of the GPS data quality assessment of the selected IGS stations in Canada/USA. stations will be propagated to the rover/unknown stations, and they will also affect the network RTK methods for raw data/corrections to be applied at rovers. Currently, data quality information on carrier-phase measurements of CORS stations is not provided by station administrators, and the approach to estimating carrier-phase data quality at CORS stations is missing in the literature. Therefore, the motivation of this paper was to propose a comprehensive and practical approach to estimate the GNSS data quality at CORS stations, and the data quality information should be useful for different applications/users, such as single-frequency users. A GNSS CORS data quality assessment approach was proposed and demonstrated with CORS networks in Hong Kong and some selected IGS stations in the USA/Canada. The implementation methods (software and computer tools) of the assessment metrics have been described. Case studies of the CORS stations of the Hong Kong Satellite Positioning Reference Station Network (SatRef) and 14 IGS stations in the USA/Canada were carried out. The data quality metrics of the CORS stations estimated by the proposed approach are the carrier-phase measurement noise, pseudorange, and carrier-phase multipath errors; no validation of the proposed approach is possible because the \"true\" errors and noise are never known. Note that the magnitudes of multipath errors and noise are not absolute. However, the relative error and noise levels of the CORS stations can serve as the purpose of quality checks among CORS stations. In the case studies, some stations have significantly greater carrier-phase measurement noise than other stations in the same CORS network. This may indicate that there are problems with the instruments and/or the environments of the stations. In such cases, the station administrator should check the hardware and software at the station(s); for example, the data quality of SatRef's HKLT station is poor. The estimated pseudorange and carrier-phase multipath errors can be used as indicators of the multipath severity of CORS stations for DGNSS and RTK services, respectively. DGNSS and RTK service providers/users can select low multipath-severity CORS stations as the reference station(s) in single-station-based or network-based DGNSS and RTK approaches. All CORS stations ought to provide data quality information to users. Conceptualization, L.L. and K.-W.T. ; methodology, K.-W.T. and L.L. ; software, K.-W.T. ; validation, K.-W.T. ; formal analysis, K.-W.T. and L.L. ; investigation, K.-W.T. and L.L. ; resources, L.L. ; data curation, K.-W.T. and L.L. ; writing--original draft preparation, K.-W.T. and L.L. ; writing--review and editing, L.L. ; visualization, K.-W.T. and L.L. ; supervision, L.L. ; project administration, L.L. ; funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the Startup funds of the Hong Kong Polytechnic University, grant number 1-BD6G.