## 3 Experimental Evaluation
### Static PPP  
The precise orbit and clock offset provided by GBM are used to perform static PPP of BDS (B1 CB2a). Figure 5 demonstrates a static PPP error series for 20 days at 17 stations utilizing GBM's precise orbit/clock corrections. All results for the 20 days at 17 stations are plotted in the same figure. The premise is to eliminate the abnormal values in Table 2, where S represents the results for static PPP. It can be seen that all PPP solutions can reach convergence but with significantly different convergence times. In the meantime, the positioning errors in the N and E directions are obviously smaller than those of the U component.  
When performing static PPP, the last epoch of a one-day solution is taken as the result of the static positioning of that day (mark with location point; no explanation is given for  
Figure 4: BDS (B1 CB2a) kinematic PPP positioning error and number of satellite (DOY342,2020).  
Figure 5: Static PPP error series for 20 days at 17 stations utilizing GBM’s precise orbit/clock corrections (the result after the abnormal data are raised).  
the subsequent static PPP position error statistics, and the same processing method is used). Figure 6 shows the distribution of the north (N), east (E), and upward (U) positioning errors of all stations on all days (BDS(B1 cB2a)). It can be seen that the accuracies of BDS (B1 cB2a) static PPP in the three directions of N, E, and U are 6.9 mm, 4.7 mm, and 26.6 mm, respectively. Compared with the horizontal error, the distribution of vertical error is more scattered.  
To further compare and analyze the static PPP performance of BDS-3 (B1 cB2a), GPS (L1L2), BDS-2+3 (B1 IB3I), GPS + BDS-3 (B1 cB2a), and GPS + BDS-2+3 (B1 IB3I), static PPP are also processed. To facilitate the analysis of the gap between the frequency and system combination of BDS and GPS, the RMS values of BDS-3 (B1 cB2a), BDS-2+3 (B1 IB3I), GPS, GPS + BDS-3 (B1 cB2a), and GPS + BDS-2+3 (B1 IB3I) static PPP error in the north, east, upward, and horizontal directions, and in three-dimensions (3D), are calculated.  
The results are shown in Figure 7. The average values of the N, E, and U positioning errors for all the stations on all days are calculated and expressed by AVG, while the RMS of the positioning accuracy for static PPP is indicated by RMS. The results are shown in Table 3.  
Figure 6: Static BDS-3 (B1 cB2a) PPP positioning error.  
Figure 7: Static PPP positioning accuracy histogram.  
It can be seen from Table 3 and Figure 7 that the upward error of BDS-3 (B1 cB2a) is larger than that of GPS and BDS-2+3 (B1 IB3I). Compared with BDS (B1 IB3I) and BDS-3 (B1 cB2a), the horizontal error of GPS is 16.8% and 26.5% better, and the vertical component is 46.3% and 69.5% better, respectively. On the one hand, the precise orbit determination of BDS satellite PCO on-orbit estimation, precise attitude model, solar radiation pressure model, and other aspects of precise orbit determination strategies are not perfect yet, and using the correction values of GPS to correct the PCV and PCO of BDS will lead to significantly lower accuracy than that of GPS, especially in the vertical component. On the other hand, the number of satellites with BIC and B2a signals involved in static PPP is less than that of GPS and BDS-2+3 (B1 IB3I). BDS-3 (B1 cB2a) is significantly different from BDS-2+3 (B1 IB3I) in terms of mean error (M-RMS). The mean error of BDS-2+3 (B1 IB3I) is within 2 mm and there is no large system error, while BDS-2+3 (B1 IB3I) reached -23.2 mm in the vertical direction, indicating a difference between PCO and PCV at different frequencies of BDS. Compared with the BDS-only system, the positioning accuracy of combined GPS and BDS is significantly improved. However, due to the influence of the BDS PCO correction issue, the performance of the vertical error is not ideal, and even slightly lower than the accuracy of GPS-only static PPP. Compared with BDS-3 (B1 cB2a), GPS+BDS-3 (B1 cB2a) increased by 27% and 52% in the horizonal and vertical directions, respectively.