# Considering Inter-Frequency Clock Bias for BDS Triple-Frequency Precise Point Positioning
## 3 Results
### Characteristics of IFCB  
Figure 3 depicts the time series of B1/B2/B3 triple-carrier combinations for BDS-2 satellites as well as B1/B2a/B3, B1/B2b/B3, B1C/B2a/B3 and B1C/B2b/B3 triple-carrier combinations for BDS-3 satellites at station SGG1 on 13 July 2016. The triple-frequency carrier phase combination is actually the difference between two different ionospheric-free carrier phase combinations, and Equation (3) shows an example of B1/B2/B3 combination. Numerical values of the coefficients for each triple-frequency carrier phase combination are given in Table 4. The phase ambiguity in triple-frequency carrier phase combination shown in Equation (3) is determined as the mean value of triple-carrier combination time series over the common continuous ambiguity block for all B1, B2 and B3 carrier phase observations. The phase ambiguity estimate is then removed from the original triple-carrier combination time series. It seems that an alternative way can be used to estimate the IFCB. However, the satellite revisit time for the stations located in different areas is different. The different tracking time of a satellite will affect the magnitude of its IFCB estimates. Bias variations are most evident for the BDS-2 GEO satellites and the oldest spacecraft C06 among the IGSO satellites. As to other BDS-2 satellites, small bias variations can also be recognized but they are generally confined to less than 2 cm. For BDS-3 satellites, in contrast, no significant biased variations can be observed and changes of the triple-carrier combination are often dominated by receiver phase multipath and noise (PMN). After the ambiguity term is eliminated in Equation (3), the residuals mainly include a weighted sum of phase noise, multipath and hardware delay. According to [PERSON] et al. [20], the receiver phase hardware delay should be stable over several hours, and thus it will be included in the phase ambiguity estimate and removed together with the ambiguity term. As to the satellite phase hardware delay, it will also be eliminated together with the ambiguity term since the triple-carrier combination time series of BDS-3 satellites do not show significant trend term. Therefore, the triple-carrier combination variations of BDS-3 satellites mainly show PMN changes. Similar discussions can also be found in [PERSON] et al. [18]. The above noise refers to the measurement noise in carrier phase observations. Figure 2: Geographical distribution of 35 stations. This figure is plotted by the Matlab mapping package M_Map [34]. The green points refer to MGEX stations. The blue and yellow points refer to iGMAS stations. The black points refer to the stations with the capability of tracking the new navigation signals. The results shown in Figure 3 are obtained using a single station, which will be affected by numerous factors, such as the receiver quality, antenna quality and surrounding of the receiving antenna. Therefore, these results can only be roughly used for IFCB analysis. Actually, the triple-carrier combination time series show a comprehensive effect of the measurement noises, multipath as well as the time-varying portion of satellite phase hardware delay, namely IFCB. The satellite internal temperature variations due to varying sun illumination that depends on the relative geometry of the sun-spacecraft-earth may cause the variations of the satellite phase hardware delay, namely IFCB variations [14; 20]. There is a negative correlation between the amplitude of single-day IFCB time series and the sun elevation above the satellite orbital plane. For the satellites with relatively lower sun elevation such as C05 and C06, the time series of triple-carrier combination show evident bias variations. As to BDS-3 satellites, there may be better thermal insulation processing at the signal transmission end. There are some reflectors near the station SGG1, including trees and walls.