An long base line (LBL) system consists of a number of acoustic transponders arranged in a local grid on or near the sea floor. In this type of system, a surface ship with a single interrogation/receive transducer can navigate itself as well as separate, moving targets in relation to the fixed transponder grid. In the 1980s, the ultra-short base line system came into use, and has today almost completely replaced short base line tracking system (SBL) use. The ultra-short base line (USBL) system is a direct result of the computer age and utilizes completely different technology than either the LBL or SBL systems. The ability of the USBL system to track multiple acoustic targets from a single hydrophone fixture gives it several potential advantages over other types of navigation systems. The basic USBL system consists of a command/display module, a single interrogate/receive hydrophone assembly, an interconnect cable, and the small acoustic transponders, responders, pingers that are attached to the target.