Underwater robots and drones such as remotely operated vehicles (ROVs) and autonomous unmanned vehicles (AUVs) are used for oceanic surveys. Underwater acoustic positioning is an indispensable technology for autonomous control system (ACS) in ROVs and AUVs. In long baseline (LBL) acoustic positioning system, multiple distances from the reference points to the positioning target are required. The cross-correlation function between received and reference signals is calculated to measure the distance. In this work, we propose an iterative variable threshold method that is resistant to acoustic reflections. The proposed method has been evaluated in the simulation and field experiment.