GPS and GLONASS have adopted independent system times, which will cause an offset between the GPS and GLONASS system timescales. This GPS-GLONASS system time difference will result in a bias between GPS and GLONASS measurements in combined GPS/GLONASS data processing. Therefore, the users must cope with two clock offsets: one is the clock offset with respect to the GPS time and the other is the clock offset with respect to the GLONASS time. The system time difference between GPS and GLONASS may be interpreted as the offset between the GLONASS receiver clock and the GPS receiver clock, which can be obtained by taking the difference between the two receiver clock offsets. Alternatively, this system time difference could be estimated directly along with the position coordinates instead of solving a GLONASS receiver clock offset unknown. The time offset between the GPS and GLONASS systems is an important parameter in combined GPS/GLONASS processing. To obtain a better understanding of its temporal variation characteristics, the precise point positioning (PPP) method is used to estimate the system time difference. Since PPP could provide centimeter-level position accuracy, high-accuracy system time difference estimates can be expected. The estimation results show that the system time difference is very stable according to its standard deviation and variation within one day, and the estimates are quite receiver-dependent due to the existence of inter-system hardware delay biases. After analyzing its short-term variation characteristics, a proper stochastic model of the system time difference parameter has been obtained and applied to combined GPS and GLONASS PPP processing.
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