# Characterization of GNSS Signals Tracked by the iGMAS Network Considering Recent BDS-3 Satellites
## 4 Discussion  
The carrier-to-noise density ratio, pseudo-range noise and multipath, satellite-induced code bias and inter-frequency bias measured by the iGMAS stations for the different BDS-3 signals are characterized and compared with BDS-2, GPS, and Galileo. In addition to the old B1I and B3I signals inherited from the BDS-2 satellites, the BDS-3 satellites are capable of transmitting several new open service signals, namely B1C, B2a, and B2b. For the carrier-to-noise density ratio, the BDS-3 B1I signal is higher than that of BDS-2, and the B3I signals are at almost the same level. The new BDS-3 B2a/B2b signals have approximately similar signal strength to the GPS L5 and Galileo E5a/E5b signals, while BDS-3 B1C signals exhibit the lowest signal strength of all the analyzed signals, which is consistent with the results of analysis of BDS-3e satellites in [PERSON] et al. (2018). For the pseudo-range noise and multipath errors, the BDS-3 B1I/B3I signals show smaller errors compared with BDS-2, and B1C/B2a/B2b signals have no significant differences compared to GPS L1/L5 and Galileo E1/E5a/E5b signals.  
Differently from other GNSS satellites, the BDS-2 code measurements are affected by satellite-induced biases larger than 1 m from horizon to zenith, which will inevitably affect the precise applications which involve the BDS-2 code measurements, such as single-frequency PPP ([PERSON] et al., 2018) and PPP ambiguity resolution ([PERSON] et al., 2018; [PERSON] et al., 2018). We use observations collected from different receivers/antennas to analyze the existence and magnitude of the satellite-induced code biases for the BDS-3 satellites. No obvious elevation-dependent and receiver-dependent code biases are identified for all of the  
Figure 9: Variations of triple-frequency carrier phase combinations of (B1I, B3I, B2b) and (B1C, B2b, B2a) for C20, C22, C27, and C29 satellites during a pass at station HMNS.  
available BDS-3 signals. Similar results have been found in previous experimental satellites [5; 33]. This indicates that BDS-3 satellites have improved satellite internal hardware device design and navigation unit assembly to avoid the problem of satellite-induced code biases present in BDS-2, providing better performance.  
For the IFCB variations, which have been identified to exist in BDS-2 and GPS Block IIF satellites [24; 26], the small bias variations with peak amplitudes of about 1 cm can been found in BDS-3 triple-frequency carrier phase combinations. However, no apparent bias variations were observed for BDS-3e satellites in [PERSON] et al. [5] and [PERSON] et al. [27]. Differences between the BDS-3 and BDS-3e satellites may be a cause of different results. In addition, it must be kept in mind that the triple-frequency carrier phase combinations show a comprehensive effect of the carrier phase noise, multipath, and the variations of the satellite phase hardware delay, namely IFCB. The results shown in Figure 9 might be affected by several factors, including the quality of the receiver and antenna, surroundings of station, and satellite internal hardware devices. The effect of such IFCB variations of BDS-3 satellites on multi-frequency data processing, e.g., triple-frequency PPP, need to be further investigated.  
It should be noted that previous BDS-3e satellites, for the purpose of experiment validation, will not serve as a part of the BDS-3 satellite network. At present, the test task has been completed and these experimental satellites have been supplemented to the BDS-2 constellation. Although the signal characteristics of BDS-3e satellites have been analyzed in previous publications [5; 13], it is still necessary to evaluate the performance of authentic BDS-3 satellites. The results of the satellite-induced biases are in line with the experimental satellites, and the results of IFCB variations are slightly different.