## 4 Test Results
### Static Test  
To confirm the improvement in the accuracy of the DGPS of the low-cost single frequency GPS receiver using the modified FKP corrections in the position domain, static and dynamic user tests were conducted. First, the static test is introduced. In the static test, Figure 8a shows the u-blox LEA-6T evaluation kit selected as the low-cost GPS receiver, which is one of the most popular and inexpensive commercial GPS receivers. It can be applied for the DGPS messages in the RTCM version 2.3. The static user is located at Bldg. 312 in Seoul National University in Korea. In addition, to confirm the effect of the FKP corrections corresponding to the baseline lengths which is the distance between the user and the RS, four RSs (YANP, WNJU, YOWL, and WULJ) were selected which are at distances of 50, 90, 140, and 225 km south-east of the user in sequence, as shown in Figure 8b. The test was conducted for approximately 12 h from 4 April 2017 10:30 in local time (UTC+09). To obtain the data from the four RSs simultaneously and let the receivers operate the DGPS and FKP-DGPS, eight receivers were used as the static user. In the test, an RS receiver of Trimble NetR9 and the eight u-blox receivers were connected to a Trimble Zephyr geodetic 2 antenna as shown in Figure 9. Using the measurements from the NetR9 and Zephyr antenna, a precise position was determined. Although the geodetic antenna reduces the noise level of the u-blox receiver compared to the original patch antenna, it does not reduce the bias error due to the spatial decorrelation, which can help in distinguishing the bias error from the noise-related error. In addition, Figure 10a shows the visible satellites in the sky during the test. Figure 10b shows the position dilution of precision (PDOP). The geometry of satellites affects the positioning accuracy and the dilution factor (DOP) depends solely on the geometry. Thus, the PDOP is normally presented with the positioning results in the field of GPS error analysis [28]. Figure 8: (**a**) u-blox LEA-6T evaluation kit; and (**b**) Locations of reference stations. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**DD Residual Errors (RMS 1)** & **PRN \#1** & **PRN \#4** & **PRN \#8** & **PRN \#11** & **PRN \#17** \\ \hline Elevation Angle & 49.9\({}^{\circ}\) & 15.9\({}^{\circ}\) & 14.3\({}^{\circ}\) & 30.6\({}^{\circ}\) & 43.3\({}^{\circ}\) \\ Before Case (\(\alpha\)) & 0.51 m & 0.32 m & 0.63 m & 0.36 m & 0.21 m \\ After Case (\(\beta\)) & 0.36 m & 0.11 m & 0.28 m & 0.37 m & 0.14 m \\ \hline \hline \end{tabular}  
* Root mean square (RMS).  
\end{table} 
Table 3: DD residual errors without and with FKP correction. Figure 11 shows the horizontal and vertical position errors of the DGPS and FKP-DGPS results of the WULJ RS, which is located farthest from the static user in the east-north-up (ENU) coordinate. It is easily confirmed that the horizontal result of the DGPS is slightly biased on the left side. However, the horizontal result of the FKP-DGPS seems unbiased, but slightly noisy. Furthermore, the positive effect of the FKP correction can be confirmed from the vertical result. The blank in the vertical results occurred when the correction data are not received from the NGII FKP server because of the server-side problem. The data in the blank are excluded from the analysis. Figure 10: (**a**) Skxplot in static test; and (**b**) PDOP during static test. Figure 9: Configuration of the static test. Figure 12 shows the reason for the bias in the horizontal result of the DGPS on the west side. Considering the rotation of the Earth, the appearance of satellites is in the east-west direction. Moreover, satellites with low elevation angles more significantly affect positioning results. In Figure 12, it is assumed that the west direction is positive, and the GPS satellite has low elevation angle.