## 3 Development of the GPSQC Program
Therefore, GPSQC users are required to appropriately define the criteria according to the precision of the GPS data analysis. In this paper, we proposed the criteria and the percentage of quality control parameters to give the initial criteria as shown in Table 2.  
Figure 3 shows the summary report of GPS data quality with more detailed information and includes 7 graphs. Figures 4-7 show the graphs and the time series that represent quality control indicators calculated from GPSQC. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**QC** & \multicolumn{2}{c}{**Skyplots**} & \multicolumn{2}{c}{**Multi-Paths**} & \multicolumn{2}{c}{**Cycle Slips**} & \multicolumn{2}{c}{**Ionospheric**} \\
**Parameters** & _ele (\({}^{\circ}\))_ & _DOP_ & _MP\({}_{1}\)_ (m)_ & _MP\({}_{2}\)_ (m)_ & _cyc_ & _ion_ & _iod_ \\ \hline Criteria & \(>10\) & \(<5\) & \(<1.0\) & \(<2.0\) & \(<\pm 2.0\) & \(<\pm 10\) & \(<\pm 0.3\) \\ Allowance (\%) & 90.0 & 90.0 & 90.0 & 90.0 & 90.0 & 80.0 & 80.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Criteria and allowance of quality control. Figure 2: Screen shot of interactive start window of GPSQC program. Figure 4: Quality control indicators related to geometric arrangement. (**a**) Sky plot of observed satellite. (**b**) Satellite azimuths. (**c**) Satellite elevations. (**d**) _DOP_s. Figure 3: Screen shot of summary report obtained from GPSQC. To evaluate the suitability of GPSQC, we then analyzed the effect of data quality on the results of precise GPS data processing for daily data. To achieve this, we calculated the quality control indicators  
Figure 6: Quality control indicators related to ionospheric effects. (**a**) Time-series plots of _ion_ at each observation epoch. (**b**) Time-series plots of _iod_ at each observation epoch. Figure 7: Quality control indicators related to cycle slip effects (_cyc_) as shown in the time-series plots at each observation epoch. Figure 5: Quality control indicators related to multi-path effects. (**a**) Time-series plots of _MP1_ at each observation epoch. (**b**) Time-series plots of _MP2_ at each observation epoch. using 5 days of GPS observations, from September 1 to 5, 2009 at Suwon station (SUWN), which were registered with the International GNSS Service (IGS). Data processing was carried out by the Precise Point Positioning (PPP) technique from GIPYS/OASIS-II software, which was developed in the Jet Propulsion Laboratory (JPL) and is capable of calculating to a level of processing equivalent to a few millimeters in GPS data processing [21, 22]. Table 3 lists the results of quality control indicators (%) and the precision (significance level of \(\pm 1\sigma\)) of post processing. We used the criteria of quality control given in Table 2, and these criteria represented the difference (length in meters) between the published coordinate and the new coordinate obtained in this study. The published coordinate of SUWN, referred to ITRF2000, was determined with high precision (0.0001 ppm) and officially issued by IGS and the Korean National Geographic Information Institute (NGII). The published coordinates of SUWN were also computed by using the GIPSY/OASIS-II software and the algorithms for determining position developed at the Jet Propulsion Laboratory (JPL). Table 3 shows the lower precision of positioning from the poorer quality of days 1 and 5 than that of days 2, 3 and 4. It also shows that the baseline differences in days 1 and 5 are greater than the other days, while the best result was obtained from day 2 when we calculated the best quality control indicators. From these results, we can see that the data quality clearly affects precise GPS data processing. In particular, it shows that the cycle slip of carrier-phase observed data and the ionospheric effects in days 1 and 5 mainly affect data processing. Therefore, if we perform the quality control for GPS sensing data obtained in the field using GPSQC, GPS users can expect precise positioning and can carry out decisions in advance of field work.