## 3 Experimental Analysis
### Bias in Code Multipath Combination of BDS  
The above discussion shows that the code multipath combinations of BDS MEO satellite contain remarkable elevation-dependent bias, which is not seen in those of GPS, GLONASS, or Galileo satellites. Considering the difference in orbit types of BDS satellites, we also examine the elevation-dependent bias phenomenon with the BDS IGSO and GEO satellites. Similar phenomenon is also observed in the code multipath combinations of BDS IGSO satellites (see Figure 4a), though the trends for IGSO satellites are not as significant as the MEO ones due to relatively slow variation of IGSO satellite elevation. When it comes to BDS GEO satellites (see Figure 4b), the multipath time series show daily periodicity instead of elevation-dependent bias. This is consistent with the results of Fast Fourier transformation and time correlation analysis which indicate that the principal period for code multipath time series of BDS GEO satellites is roughly equal to a sidereal day [11]. The diurnal periodicity could be exploited to mitigate the code multipath effects of BDS GEO satellites. For example, the low-frequency components of code multipath combinations in history could be extracted and employed as an empirical model to correct the observables. It is proved that the precision of single point positioning could be improved after applying such corrections [11]. Figure 3: The multipath combinations (blue) and satellite elevations (red) for (**a**) GPS satellite C06, (**b**) GLONASS satellite R08, (**c**) Galileo satellite E11 and (**d**) BDS MEO satellite C14 at station F713. As is shown in Figures 3d and Figure 4, the code multipath combination time series on different frequencies of BDS MEO and IGSO satellites show almost the same pattern, while the patterns of the counterparts of GEO satellite are conspicuously different from frequency to frequency. To know more about the between-frequency relationships of multipath, correlations analyses were performed between multipath combinations of every two frequencies. Shown in Figure 5 are the cross-correlations at the four stations F713, F783, GMSD, and JFNG. In addition to the cross-correlation results of all available BDS satellites, those of GPS satellite G06 and Galileo satellite E19 are also displayed for comparison. For GPS satellites, only \(MP_{1}\) and \(MP_{2}\) are available, and the similar case also holds for the following Figures 6 and 7. In Figure 5, the between-frequency correlations of BDS GEO satellites (_i.e._, C01-C05) can have high absolute values, but they seem random considering the differences among satellites and stations. When it comes to the cases of BDS IGSO (_i.e._, C06-C10) and MEO (_i.e._, C11-C14) satellites,the correlations  
Figure 4: The relationship between multipath combination (blue) and satellite elevations (red) for BDS (**a**) IGSO satellite C09 and (**b**) GEO satellite C03 at station F713. Figure 5: Cross-correlations between the code multipath combinations \(MP_{1}\) and \(MP_{2}\) (black), \(MP_{1}\)and \(MP_{3}\) (red), \(MP_{2}\) and \(MP_{3}\) (blue) at stations (**a**) F713, (**b**) F783, (**c**) GMSD, and (**d**) JFNG. behave somewhat systematically. In each panel, the directions of the bars for ICSO and MEO satellites are almost the same, especially for the cross-correlations of _MP\({}_{1}\) vs. MP\({}_{2}\)_ and _MP\({}_{1}\) vs. MP\({}_{3}\)_. When comparing among different GNSS systems, it can be seen that the between-frequency correlations for GPS and Galileo satellites are relatively low and random. When comparing among different stations, it is noted that the between-frequency correlations at stations GMSD and JFNG are generally higher than those at stations F713 and F783. This is probably due to the different antenna types. correlated, although the correlation coefficients are quantitatively different for different satellites or stations. These results, to some extent, validate the existence of elevation-dependent biases in code multipath combinations of BDS non-GEO satellites.