This paper proposes an underwater positioning system (UPS) for remotely operated underwater vehicle (ROV). The system uses two underwater speakers as anchor nodes and a hydrophone as a receiver node. Time-of-arrival (ToA) is locally measured by sensors to identify distances from the hydrophone to the two anchor nodes. Then, using 2-D trilateration algorithm and depth from a pressure sensor, the position of the hydrophone can be identified. In addition, we also find the optimal frequency range that is suitable for the model and use these frequencies in the simulation of the system.