Asynchronous ranging is one practical method to implement a locating system that provides accurate results. However, a locating system utilizing asynchronous ranging generates a large number of messages that cause transmission delays or failures and degrades the system performance. This paper proposes a novel approach for efficient congestion control in an asynchronous range-based locating system. The proposed method significantly reduces the number of messages generated during the reader discovery phase by eavesdropping on other transmissions and improves the efficiency of ranging by organizing the tags in a hierarchical fashion in the measurement phase. Our evaluation shows that the proposed method reduces the number of messages by 70% compared to the conventional method and significantly improves the success rate of ranging.