The purpose of this research project was to design, implement, and evaluate a prototype underwater positioning system which extends the reach of the terrestrial Global Positioning System (GPS) underwater. The GPS does not function underwater because the high-frequency low-power signals used by the GPS are not able to penetrate more than several meters in water. The Underwater Global Positioning System (UGPS), presented in this work, provides underwater position data to an unlimited number of underwater assets, such as autonomous vehicles. The user requirements are discussed and a design is presented that incorporates a topside surface buoy (satellite) and a subsurface receiver. The satellite is responsible for receiving GPS data and relaying the data, via acoustic signals, to the subsurface receiver. The receiver calculates its position using the coded acoustic signals. The implementation of the prototype UGPS satellite and subsurface receiver are discussed in detail; the custom electronics, software, data acquisition systems and mechanical housings are described. The key operating characteristics of the UGPS are investigated both experimentally and through the analysis of a model describing the entire UGPS. Employing the prototype UGPS, a series of sea-trials were performed that provides essential design data for developing the next version of the system. The main characteristics that were experimentally investigated were: the long and short-range accuracy; the repeatability; and the resolution. The experimental data was also employed to confirm the UGPS model performance. The prototype system demonstrated the feasibility of the UGPS concept and showed that a position accuracy of 6.5m should be attainable for an unlimited number of underwater receivers operating within a one square kilometer workspace. The accuracy can be enhanced to sub-meter by employing more accurate GPS receivers in the satellites and using a sound velocity meter to measure the sound velocity profile of the acoustic workspace.