## 3 Experimental Analysis
### Comparing CNR and Code Multipath Combinations of Different Systems  
GPS, GLONASS, Galileo, and BDS are considered the main four global navigation satellite systems at present. Although they share the principle of distance resection to provide service, they are different in some aspects, such as the constellation, frequency choice, multi-access techniques, and so on. The space segment of BDS comprises GEO, IGSO, and MEO satellites. Unlikely, the constellations of the other three systems contain MEO satellites only. The technology of frequency division multi-access (FDMA) is employed by GLONASS, while that of code division multi-access (CDMA) by the other three. In addition, the frequencies of signals are different from system to system, and triple-frequency observables are already available for BDS and Galileo systems. Considering the above differences, behaviors of carrier-to-noise ratios and code multipath combinations for the four different systems are compared. Carrier-to-noise ratio (CNR) can indicate the precision of carrier phase observations so it is utilized to evaluate the quality of ranging signals. Taking the station F713 for instance, observables of GPS, GLONASS, Galileo, and BDS were simultaneously collected during the same period. The CNRs for observables of GPS satellite (G06), GLONASS satellite (R01), Galileo satellite (E11), and three BDS satellites (C03, C08, and C11) are illustrated in Figure 2 for the station F713. BDS C03 and C08 are GEO and IGSO satellites, respectively, while the C11, as well as GPS and Galileo satellites, are all MEO ones. Different colors represent different frequencies of each system. As is shown in Figure 2, the maximum CNRs for all GNSS satellites are around 50 dB-Hz, and the CNRs vary significantly with the elevations for all satellites except the BDS GEO ones, due to their relatively stable elevations. For GPS satellite G06 and GLONASS satellite R01, the CNR of measurement \begin{table}
\begin{tabular}{l l l} \hline \hline
**Stations** & **Antenna Types** & **Receiver Types** \\ \hline
**CUT0** & TRM59800.00 SCIS & TRMBLE NETR9 4.70 \\
**F711** & TRM57971.00 NONE & TRIMBLE NETR9 \\
**F713** & TRM55971.00 NONE & TRIMBLE NETR9 \\
**F714** & TRM55971.00 NONE & TRIMBLE NETR9 \\
**F721** & TRM57971.00 NONE & TRIMBLE NETR9 \\
**F783** & TRM57971.00 NONE & TRIMBLE NETR9 \\
**GMSD** & TRMB59800.00 SCIS & TRIMBLE NETR9 4.80 \\
**JFNG** & TRM59800.00 NONE & TRIMBLE NETR9 4.70 \\
**JOG2** & JAV\_RINGANT\_GST NONE & JAVAD TRE\_G3 TH DELTA3.4.7 \\
**NNOR** & SEPCHOKE\_MC NONE & SEPT POLARX4 2.3.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The types of antennas and receivers equipped at the GNSS stations. Figure 1: The distribution of the GNSS stations. on the first frequency is higher than that on the second, provided the same elevation. However, it is not always true for all GLONASS satellites, some of which, not displayed in the figure, have almost the same CNR for measurement on both frequencies. For BDS satellites, the B1 measurement has the lowest CNR among all the three frequencies while the B3 has the highest CNR. The CNR of B3 measurement of satellite C11 reaches almost 55 dB-Hz at high elevation angles (>60\({}^{\circ}\)). No apparent difference is found in the CNRs of triple-frequency observables of Galileo satellite. When the elevation of satellite is relatively low, the CNR of GPS L2 observables is more sensitive to the elevation than all the other three systems. The CNR of GPS L2 observable is only about 20 dB-Hz at elevation below 10\({}^{\circ}\). Figure 2 shows that the CNRs of all satellites, except C03, increase significantly with the increase of elevations. This is consistent with previous studies about GNSS measurement CNR and the elevation of corresponding satellite [10; 11; 13]. The stable CNR of satellite C03 is due to its unique GEO satellite orbit, which is designed to appear stationary above the equator.