## 3 Results
### Characteristics of ISB Estimates  
The characteristics of ISB estimates derived from GPS/GLONASS/Galileo/BDS-2/BDS-3 integrated SPP under a cutoff elevation angle of 10\({}^{\circ}\) are rigorously investigated in this section. Figure 2 illustrates the epochwise ISB estimates at stations KIRU and PTBB on DOY 41 of 2020. It is seen that all the four groups of ISB estimates are relatively stable with a varying range of approximately 10 ns. Most importantly, the ISB estimates for BDS-2 and BDS-3 with respect to GPS show obvious differences by several nanoseconds, indicating that the arrangement of only an ISB parameter for both BDS-2 and BDS-3 satellites is unreasonable. \begin{table}
\begin{tabular}{l c c} \hline
**Receiver Types** & **Number** & **Stations** \\ \hline \multirow{3}{*}{JAVAD TRE\_3} & \multirow{3}{*}{29} & SUTM, WIND, WUH2, POTS, URUM, ULAB, NYA2, ZAMB, MBAR, QUIN, BOGT, CUSV, ARHT, CRO1, GUAM, SOD3, FFMJ, WTZZ, WARN, HUEG, LEIJ, GODN, GODS, STHL, POL2, BSHM, (SGOC, SGPO, LPSG) \\ \hline \multirow{3}{*}{JAVAD TRE\_G3 TH} & \multirow{3}{*}{12} & MADR, GOLD, MCM4, PALM, GLPS, PIMO, PIE1, HRAO, FAIR, KOKV, AREQ, UNSA (DOY 41–48, 2020) \\ \cline{1-1} \cline{3-3}  & & MIZU, OUS2, UNSA (DOY 49–70, 2020) \\ \cline{1-1} \cline{3-3}  & & MOBS, SYDN, AGGO, OP71, BRUX (DOY 41–56, 2020) \\ \hline \multirow{3}{*}{SEPT POLARX5} & \multirow{3}{*}{45} & MAW1, MAR6, VIS0, USDJ, JPLM, ARUC, DGAR, IISC, FALK, ABPO, SUTH, DARW, HAL1, MKEA, VACS, KIR0, MAO0, MAL2, FAA1, MAS1, REUN, PTGG, AJAC, CHPI, KIRU, REDU, VILL, JOZE, KOUR, (CEDU, HOB2, TID1, STR1, POHN, YAR3, KAT1, TOW2, GRAZ, YARR, ABMF, AREQ, DJIG, NKLG, KITG, PADO) \\ \hline \multirow{3}{*}{SEPT POLARXSTR} & \multirow{3}{*}{19} & ONSA, GODE, BREW, KOKB, NLIB, SPT0, MGUE, CEBR, NNOR, AMC4, BRUX (DOY 57–70, 2020), (HERS, THTG, KOUG, PTBB, STJ3, YEL2, USN7, HARB) \\ \cline{1-1} \cline{3-3}  & & KIR8, GOPE, CKSV, BRST, LMMF, RGDG, GANP, (KRGG, UNB3) \\ \hline \end{tabular}
\end{table} 
Table 1: Receiver types of the selected stations. Figure 1: Geographical distribution of the 120 selected MGEX stations. To investigate whether the receiver firmware versions can affect the ISB estimation, Figure 3 exhibits the time series of daily ISBs (i.e., the average value of the epochwise ISB estimates over a day) at 29 stations, which encounter a change of receiver firmware versions. Different stations are identified by the distinct colors. When the curves turn into dotted lines, it means that the receiver firmware versions are changed. To be reliable, the epochwise ISB estimates with a visible satellite number below 3 are discarded, and the daily ISBs are calculated only when the number of available epochwise ISB estimates is more than 500 (i.e., 250 min). As shown in Figure 3, the daily ISBs are partially absent for several stations, which can be attributed to the absence of observations or the insufficient epochwise ISB estimates. As we can see, the daily ISBs do not present obvious fluctuations when the receiver firmware versions are changed. Thus, the effects of receiver firmware versions on the ISB estimation are not significant. In addition, the time series of daily ISBs between Galileo and GPS at two stations (KRGG and UNB3), between BDS-2 and GPS at a station (KRGG), and between BDS-3 and GPS at two stations (KRGG and UNB3) show obvious differences with respect to those at the other stations, in view that the ISB estimates are dependent on the receiver types, which will be validated in this section, and both the two stations are equipped with the TRIMBLE ALLOY receiver. There may be other influencing factors for the distinct ISB series between GLONASS and GPS at a station (CEDU), such as the temperature and other environmental factors, which needs to be investigated further. Figure 2: Epochwise ISB estimates at stations KIRU and PTBB on DOY 41 of 2020. To investigate whether the receiver types can affect the ISB estimation, Figure 4 shows the time series of daily ISBs at BRUX (red) and UNSA (blue) stations, which encounter a replacement of receiver types.