## 4 Experiments and Results
### Results and Analysis  
Figure 9 shows the solution sequence of the original baseline and the corrected baseline series of ASF and SD-MHM. We find that the multipath affects the accuracy of the original baseline, while both ASF and SD-MHM significantly mitigate the multipath. Except for the background period, the ASF corrected result is consistent with SD-MHM. However, in the background period, the series after ASF correction is still affected by the multipath. Therefore, it is necessary to analyze the multipath model and satellite visibility. The observation times of some satellites are different in the two ORTs. For example, the C21 and C24 observation times of DOY 296 and DOY 303 are different. Therefore, the ASF method cannot mitigate the multipath in some satellites, and the corrected series remains multipath. The SD-MHM method can ignore the impact of satellite operations. Based on the multipath spatial correlation, the SD-MHM method can search for the multipath model values of neighboring locations to mitigate the current multipath.  
Dilution of precision (DOP) reflects the strength of satellite spatial geometry and positioning accuracy. Figure 10a shows the DOP value, elevation angle and number of visible satellites of DOY 303. Figure 10b shows the correction results of the coordinate series in the east, north, and vertical directions obtained by the SD-MHM method. The fluctuation of HDOP has almost no effect on the east and north directions. Moreover, the series after the correction of the east and north directions fluctuates within 2 mm. For the vertical direction, VDOP has a significant correlation with the vertical direction correction series, which is more evident in the VDOP fluctuation, but it still performs well in positioning accuracy. Furthermore, the average number of visible satellites per epoch is 12.5. It provides sufficient observation data for BDS positioning.  
Figure 8: (**a**) DOY303 C29 single-difference residuals and ASF and SD-MHM multipath models and their mitigated series; and (**b**) the power spectral density of the mitigated series and the single-difference residuals.  
Figure 11 shows that the proposed SD-MHM method is slightly better than ASF for east, north, and vertical directions and the baseline positioning accuracy. For inconsistent observation time of some satellites, the SD-MHM positioning accuracy is significantly better than the ASF method. Among them, the nine-day average positioning accuracies of the original coordinate series in the east, north, and vertical directions are 1.81, 2.49, and 8.70 mm, respectively. The positioning accuracies of the series corrected by ASF in the east, north, and vertical directions are 0.88, 1.05, and 3.13 mm, and the positioning accuracy improvement rates are 51.4%, 57.8%, and 64.0%, respectively. After correcting by SD-MHM, the positioning accuracies of the series in the east, north, and vertical directions reach 0.79, 0.90, and 2.84 mm, and the positioning accuracy improvement rates reach 56.4%, 63.9%, and 67.4%, respectively. The SD-MHM method is better than the ASF method for BDS multipath mitigation. Moreover, the average accuracy of the original baseline is 1.97 mm. The average accuracy after ASF and SD-MHM corrections are 0.95 and 0.84 mm, and the average improvement rates are 51.8% and 57.4%, respectively. Notice that, for DOY 310 and  
Figure 10: (**a**) HDOP, VDOP, and elevation of DOY303; and (**b**) the corrected series by the SD-MHM in the east, north, and vertical directions.  
Figure 9: The original baseline series, ASF, and SD-MHM corrected series and C21 single-difference residuals at the adjacent periods.  
311, the BDS constellation has repeated two ORTs compared to the established SD-MHM multipath model, and the SD-MHM method still effectively mitigates the multipath.