Abstract: A high resolution underwater acoustic navigation system is provided by a combined pulse and continuous-wave or Doppler system. Locations of the object being tracked are periodically determined by the pulse subsystem and are used to initialize the Doppler subsystem. The Doppler Subsystem tracks the location from the fix obtained by the pulse subsystem. Both subsystems are interfaced with a central processing unit or digital computer and the data rate input to the computer is substantially reduced by the use, in the Doppler subsystem, of a phase angle quadrant change counter whose accumulated count is periodically provided as an input to the computer.