English: Method of operation of a long-baseline (LBL) underwater acoustic positioning system for divers. A diver mounted transponder (A) emits an acoustic signal or 'ping' that travels to baseline transponders (B, C, D). The transponder replies travel back to (A), which measures the two-way signal run time. Using a measured or assumed speed of sound, the corresponding distances are determined. These distance measurements are sent to the diver terminal (E) which uses a search algorithm or geometric computations to establish the diver position. Position data and related information such as position accuracy, depth, distance and bearing to a waypoint are displayed on a screen similar to that of a handheld GPS receiver.