## 2 Materials and Methods  
### GNSS Quality Management  
In this study, a standardized quality control (QC) protocol was applied to approximately 17,000 RINEX (receiver independent exchange format) files collected from the unified geodetic control points (UGCPs) across South Korea between 2008 and 2021. The QC procedure was implemented prior to the geodetic network adjustment phase to ensure that only high-quality and reliable observation data were used in subsequent computations. To maintain consistency and reliability throughout the dataset, a set of key quality indicators was extracted and evaluated for each GNSS observation file. These indicators were carefully selected to assess not only the completeness and temporal continuity of the data but also the signal environment and receiver performance--both of which are essential for achieving accurate and stable geodetic positioning. Low-quality data resulting from signal loss, multipath interference, or equipment malfunction can introduce significant errors during network adjustment. Therefore, early detection and exclusion of such data through a rigorous QC process is a critical step in ensuring the integrity of the adjustment results [10]. Key quality indicators were derived from raw GNSS observation data, including the following:  
* Total observation time (h),
* Data acquisition intervals (dt),
* Estimated vs. actual number of observations,
* Multipath effect estimates for L1 and L2 signals (MP\({}_{1}\), MP\({}_{2}\)),
* Cycle-slip ratios (slps, non_slps, o/slps). These quality indicators were selected based on international GNSS data standards and methodologies established in previous studies. Beyond fundamental QC metrics, GNSS data assessment can incorporate a range of advanced analytical techniques, including coordinate time series analysis, detection of noise and outliers in carrier-phase and code observations, signal-to-noise ratio (SNR) evaluation, and multipath-effect modeling [11]. To further improve positioning accuracy, precise error modeling is often necessary. This involves applying corrections such as phase center offset (PCO) and phase center variation (PCV), as well as using atmospheric delay models for tropospheric and ionospheric effects [12]. Moreover, software packages such as GAMIT/GLOBK incorporate these corrections to enable precise positioning. They are particularly effective in evaluating the relative positioning accuracy between base stations in network-based processing approaches [13]. Table 1 summarizes the QC metrics and their functional descriptions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Index** & **Function** \\ \hline first EPOCH & GNSS observation start time (year/month/day/hour) \\ last EPOCH & GNSS observation end time (year/month/day/hour) \\ hrs & Total observation duration [unit: hours] \\ dt & Data reception interval [unit: seconds] \\ expt & Estimated total number of observations (elevation angle applied) \\ have & Actual number of observations \\ MP\({}_{1}\) & Average multipath length of L1 signal [unit: m] \\ MP\({}_{2}\) & Average multipath length of L2 signal [unit: m] \\ o/slps & (Actual number of observations)/(number of cycle slips) \\ slps & Number of cycle slips \\ non\({}_{slps}\) & Percentage of observations without cycle slip [unit: \%] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of GNSS observation quality indicators. In addition to the acquisition rate and multipath criteria applied in this study, global GNSS standards--such as those established by the International GNSS Service (IGS)--off essential benchmarks for data quality assurance. According to ICS guidelines, GNSS stations are required to maintain a data acquisition rate exceeding 99% to qualify for official registration, underscoring the need for robust equipment and minimal signal obstructions [14; 15]. Stations exhibiting data loss greater than 1% may be disqualified from registration, emphasizing the importance of maintaining a well-optimized antenna environment and consistently reliable instrumentation.