A fundamental requirement for groups of unmanned underwater vehicles to work cooperatively together is the ability for each vessel to know the relative position of its neighbors. While external communication and positioning infrastructures can be used for this purpose, a more flexible approach is to give each vessel the ability to independently discern the location of its neighbors using its own communication and sensor systems. Recent developments in acoustic modems have included the ability to measure range between two vessels as well as the Doppler imparted on the signal by the relative motion between vessels. The measured Doppler shift can be used to compute the instantaneous range rate between two vessels; researchers at NRL are presently working to validate these measurements using GPS. Approaches to vessel relative positioning and navigation using successive range and range-rate measurements, along with directed vessel maneuvers, will be presented.