# BDS-3 Time Group Delay and Its Effect on Standard Point Positioning
## 3 Experimental Data and Processing Strategies
Since the BDS-2 codes are affected by TGD parameters, the \"tgd-corr\" scheme exhibits better positioning results, which results in reduced positioning errors compared to the \"non-corr\" scheme. This result is reflected in the distribution of the positioning errors from the \"tgd-corr\" scheme, exhibiting median and 75 th percentile values below 10 m, except at the BRCH station, for all frequencies, as depicted in Figure 7. Furthermore, it can be concluded that the median and 75 th percentile values in the Asia-Pacific region are less than those at the BRCH station. We will explain this phenomenon further below. Moreover, the median and 75 th percentile values of the \"tgd-corr\" scheme are less than those of the \"non-corr\" scheme. Figure 6: Vertical positioning error scatter plots of B1I/B3I SPP with and without TGD correction at the selected stations. In each plot, the horizontal and vertical axes indicate the universal time (h) and the U component errors, respectively (unit: m). To analyse the BDS-2 positioning accuracy from different schemes based on the SPP more clearly, the root mean square (RMS) of the single- and dual-frequency SPP are calculated, and we plot the RMS of each day for the different stations (Figure 8). In addition, the improvements in the E, N, U components of the \"tgd-corr\" scheme compared to the \"non-corr\" scheme at different stations are shown in Figure 9. In Figure 9, we first obtain the mean RMS values of the 41-day SPP solutions at each station. The improvements from the \"tgd-corr\" scheme are then calculated and compared to those from the \"non-corr\" scheme. In addition, the mean values at the six stations are summarized in Table 5. Note that the outliers appearing in SPP are treated as follows. The pre-test and post-test residuals are checked in our program. Observations are deleted when the absolute value of residual is greater than 30 m. The main conclusion to be drawn from Figures 8 and 9, in conjunction with Table 5, is straightforward. It is obvious that both the single- and dual-frequency SPP of the BDS-2 can achieve metre-level positioning accuracy. As we can see from Figure 8, the results based on B1I and B1I/B3I SPP without TGD correction deviate from the true position. From Figure 9, we can conclude that the accuracy of B1I/B3I- or B1I- based SPP with TGD correction, in addition to that at the BRCH station, in the E and N components increased by more than 50%, and the U component increased in the range of 25% to 50%. The reason for the slight improvement at the BRCH station is that BRCH is in Europe, so the number of observed BDS-2 satellites is relatively small, which results in  
Table 4.0 Box-whisker diagrams of the distributions of the three dimensional (3D) positioning errors in the tgd-corr (red) and non-corr (blue) schemes at the BRCH, GUA1, LHA1, WUH1, XIA1 and XIA5 stations for the 41-day period (tgd: tgd-corr, non: non-corr) using the regional BeiDou system (BDS-2) satellites. The box heights and the bars inside the boxes denote the inter-quartile ranges (IQRs) and the medians of the distributions, respectively. The whiskers’ lengths represent the maximum and minimum values of distributions (unit: m). Outliers are identified with plus signs (see the text). an unsatisfactory equation structure. We can further explain from Figure 10, which shows the average global position dilution of precision (PDOP) on DOY 10, 2019, with an elevation cut-off angle of 5\({}^{\circ}\). The PDOP is a spatial geometric intensity factor of the satellite distribution; the better the general satellite distribution is, the smaller the PDOP value. From Figure 10, we can conclude that the BDS-2 PDOP presents obvious regional characteristics. The PDOP value of the BRCH station in Europe is greater than that of the Asia-Pacific region where the other stations are located, which support our previous findings. In addition, some BDS-2 satellites have a high degree of noise at lower elevation angels in the European region.