Navigation solution cannot be completed due to insufficient observable satellite signals blocked in urban canyons, deep pit mines, the surrounding forest and other application fields. This paper proposes a novel clock modelassisted positioning method, which uses pseudorange and Doppler observations to achieve realtime estimation of the receiver clock bias and clock drift parameters through an Extended Kalman Filter (EKF). The clock model is built and used to assist positioning in the absence of navigation signals. Simulation and vehicle test results show that this method can complete the realtime positioning solution when satellite navigation signals are missing, which makes the system availability significantly improved.