Indoor positioning technology and its accuracy are crucial research topics for ubiquitous computing. While the GSM-based approach has always been used to provide outdoor positioning to compensate the lost of GPS in urban area, we seldom see systems that utilize the GSM-based approach for indoor positioning. On the other hand, the WLAN-based approach is widely used to provide indoor positioning service. However, with its Ad hoc layout and signal fluctuation, it is hard to provide a good performance based on the WLAN-based approach. In this paper, we develop an indoor positioning system that makes use of both GSM and WLAN signals to do location estimation such that the resultant system is more accurate and more stable and thus enhancing the performance of the whole system. Experimental results show that our system is stable and can reach centimeter-level accuracy, which outperforms other existing indoor positioning systems that utilizes a single network only.