系统间偏差(ISB)是全球导航卫星系统(GNSS)精密单点定位(PPP)的重要误差源，也是GNSS PPP时间传递重要误差之一。利用全球均匀分布的MGEX站观测数据构成13条时间链路，评估了GPS、GLONASS、Galileo、BDS单系统和GPS+GLONASS+Galileo+BDS多系统的时间传递性能。结果表明，GPS、GLONASS、Galileo、BDS-2、BDS-3在13条时间链路时间传递钟差差值的RMS均值分别为0.087、0.472、0.085、0.579、0.486 ns,STD均值分别为0.072、0.363、0.071、0.501、0.371 ns;GPS+GLONASS+Galileo+BDS多系统较上述单系统在时间传递钟差差值的RMS均值上分别提升了17.11%、84.81%、15.61%、87.60%、85.22%，在STD均值上分别提升了26.53%、85.44%、25.50%、89.47%、85.72%。当时间间隔为10~3s量级时，单系统的时间链路MADEV值均在10^-13ns量级，而多系统的时间链路MADEV值则在10^-13～10^-14ns量级，说明在PPP时间传递性能上GPS+GLONASS+Galileo+BDS多系统比各单系统有显著提升。

Inter-system bias(ISB) is an important source of precision point positioning(PPP) error in global navigation satellite system(GNSS),and also one of the important errors in GNSS PPP time transfer. We used 13 time links constructed from globally uniformly distributed MGEX station observation data to evaluate the time transfer performance of GPS, GLONASS, Galileo, BDS, and GPS + GLONASS + Galileo + BDS multi-system. The experimental results show that the average RMS values of clock bias differences in the 13 time links of GPS, GLONASS,Galileo, BDS-2 and BDS-3 are 0.087, 0.472, 0.085, 0.579, and 0.486 ns, respectively, and the average STD values are 0.072, 0.363, 0.071, 0.501,and 0.371 ns, respectively. Compared with GPS, GLONASS, Galileo, BDS-2 and BDS-3, the GPS+GLONASS+Galileo+BDS multi-system has improved the accuracy of time transfer clock bias difference in average RMS value by 17.11%, 84.81%, 15.61%, 87.60%, and 85.22% respectively,and in average STD value by 26.53%, 85.44%, 25.50%, 89.47%, and 85.72% respectively. When the time interval is on the order of 10~3s, the MADEV values of GPS, GLONASS, Galileo, BDS-2 and BDS-3 time links are on the order of 10^-13ns, while GPS+GLONASS+Galileo+BDS multi-system time links are on the order of 10^-13ns to 10^-14ns, confirming that the GPS+GLONASS+Galileo+BDS multi-system has significantly improved PPP time transfer performance compared to various GNSS single systems.