Abstract: Timing aspect of interoperability between GNSS systems is one of the key issues in multi-system GNSS applications.It is important to deal with the system time bias (STB) problem properly especially in unaided receivers.Traditional multi-system receivers solve this problem by adding an additional state in main KALMAN filter.Considering the STBs are relatively slow-varying variables,this paper proposes an innovative real time approach to estimate the system time bias between different satellite navigation systems.With the assistant of estimated system time bias,additional filter states is avoided and a simple and efficient measurement matrix can be constructed.Through several tests in a GPS/BD receiver,we can conclude that the new method has performance comparable with the traditional one.