# BDS-3 Time Group Delay and Its Effect on Standard Point Positioning
## 3 Experimental Data and Processing Strategies  
In this experiment, the observation data were selected from six stations provided by iGMAS, namely, BRCH, GUA1, LHA1, WUH1, XIA1, XIA5. The data set is from the days of the year (DOYs) 1 to 41, 2019. The sampling rate is 30 s. All iGMAS stations can track all BDS-3 open signals. The BDS-3 old frequency B1I and B3I broadcast ephemeris are provided by MGEX, and the new frequency B1C and B2a broadcast ephemeris are provided by iGMAS. The data processing strategies of single- and dual-frequency for BDS-2, BDS-3 and BDS-2 + BDS-3 are detailed in Table 3. In Table 3, \"non-corr\" means code without TGD correction, and \"tgd-corr\" means code bias correction with TGD parameters in the navigation message. Note that B1I/B3I represents the B1I and B3I ionospheric-free combination SPP model. B1I refers to the single-frequency (B1I) SPP model. The other combination shows similar characteristics. During data processing, the parameter estimation method uses the least squares method, and the troposphere model adopts the Saastamoinen model. The ionospheric delay of the BDS-2 B1I and B3I signals is corrected by the K8 model. With the new signals (B1C and B2a) released by BDS-3, the ionospheric delay correction model adopts the BDSGIM. For dual-frequency data processing, ionospheric delay is used for the dual-frequency ionospheric-free combination. The summaries of the stations are shown in Table 4. Figure 1 shows the distribution of all the stations. The station coordinates in the SINEX file provided by iGMAS ([[http://112.65.161.230/download/index.php](http://112.65.161.230/download/index.php)]([http://112.65.161.230/download/index.php](http://112.65.161.230/download/index.php)); [[ftp://222.240.181.170/products/](ftp://222.240.181.170/products/)]([ftp://222.240.181.170/products/](ftp://222.240.181.170/products/))) are used as references values to assess the positioning accuracy [31]. It is noteworthy that the difference  
note that the positioning accuracy of the dual-frequency SPP based on B1I/B3I is even worse than that of the single-frequency based B1I and B3I signals. This phenomenon can be explained by the fact that the noise amplification factor is 3.5 [10] in the B1I/B3I dual-frequency signal based on the SPP scheme. Figure 3: Vertical positioning error scatter plots of B1I SPP with and without TGD correction on the selected stations. In each plot, the horizontal and vertical axes indicate the universal time (h) and the Up (U) component error, respectively (unit: m). Figure 2: Horizontal positioning error scatter plots of B1I SPP with and without time group delay (TGD) correction at the selected stations. In each plot, the horizontal and vertical axes indicate the East (E) and North (N) components errors, respectively (unit: m). Figure 4: Positioning error scatter plots of B3I SPP without TGD correction for the BRCH station. In each plot, the horizontal axis indicates the universal time (h), and the vertical axes indicates the E, N and U component errors, respectively (unit: m). Figure 5: Horizontal 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 E and N component errors, respectively (unit: m). Figure 7 depicts the box-whisker diagrams of the distribution of three-dimensional (3D) positioning errors in the \"tgd-corr\" and \"non-corr\" schemes for the BRCH, GUA1, LHA1, WUH1, XIA1 and XIA5 stations for the period of 41 days. In these diagrams, values are considered outliers if they are less than \(Q_{1}-1.5\cdot IQR\) or greater than \(Q_{3}+1.5\cdot IQR\), where \(Q_{1}\), \(Q_{3}\) and \(IQR\) denote the first quartile, the third quartile and inter-quartile range of the distribution, respectively. It is intuitively shown in Figure 7 that the number of outliers is very small for all the frequencies at each station.