A newly developed, compact long‐baseline (LBL) acoustic transponder system, used in several experiments for submarine and sea surface positioning, has proven to be easy to operate and reliable. Relocation of the ocean bottom transponders can be carried out to minimize vertical component errors of ship's positions. A transponder net is usually located via Loran‐C navigation in the western Pacific. Accuracy of positions acquired after relocation of the transponders is estimated to be a few meters at a slant range of several thousands of meters. Because several experiments have shown the difficulty of pinpointing the movement of an instrument towed from a ship, a transponder system is indispensable to precise underwater experiments.