Indoor GPS is one of the most popular positioning systems own to its high accuracy, real-time characteristics and multi-task management. In order to simplify the calibration process and extend its application, a single station model was presented recently. This paper proposes a novel ultrasonic ranging method used for the single station model. The traditional high-accuracy ultrasonic ranging method mainly uses a phase detection method by transmitting a multiple-frequency continuous wave. However, this method requires high accuracy of the phase detector and is still limited to small-scale application as a result of applying a continuous wave. Based on the constant time difference between the corresponding zero-cross points, this paper proposes a novel two-frequency pulse wave method, which can estimate the time of flight using the time differences between two received waves. Then a least squares estimation is used to eliminate random errors. Finally, an ultrasonic ranging experiment was conducted to validate its feasibility and stability.