The LOST2 system is a new accurate underwater positioning system that nonlinearly combines parts of dead-reckoning, acoustic-based positioning, and terrain-based positioning. The system is composed of two major subsystems, a system observer and a constrained extended Kalman filter. Inputs to the system are as follows: (1) high resolution bathymetry, (2) measured ocean depth at the position of the vessel, (3) measured or estimated vessel velocity, (4) slant range to and position of a known point, and (5) an initial prediction of the vessel's location. The system development, simulation studies, results from sea trials and some suggestions for future work are presented. The system is capable of providing position estimates with the same degree of accuracy as present methods, with significantly less hardware. These results prove the concept of the system as a new method to position underwater vessels
