While the initial reason for developing a ultrasonic positioning system is to be used for the Neptune project there are many other environments that could benefit from the use of an accurate positioning system.  The Neptune project is the initiative to create a large scale deep water observatory located on the Juan de Fuca tectonic plate. The use of a accurate positioning system is required to aid in monitoring the position of a ROV tethered to the base of the observatory.  This system would allow the navigation of the ROV around the observatory at a depth of 2500m and a radius of 100m.

Acoustic Navigation Systems for Underwater Vehicles
Ultra Short Baseline (USBL) acoustic positioning systems use a single group of close-spaced receivers unlike long baseline (which use long lines of receivers on the ocean floor) and short baseline (several receivers mounted across the hull of the tether vessel) systems. Commercially available USBL systems are generally high priced (on the order of $100,000) and have accuracy of 10 cm at 100m.  This proposed solution, once modified for underwater use, could have a much higher accuracy and a lower cost.  The accuracy of the system is dependant on the phase detection accuracy, usually on the order of degrees.  With a sound wave length in water of 1.5 cm and with a pessimistic 10 phase accuracy, the error would be within 0.41mm. 

Positioning Systems
Various vehicles traveling at differing velocities could use our system to aid in knowing exact position and calculating future positioning, velocity or other changes.  Jet positioning during mid air refueling, traffic on highways during rush hour, and cranes moving equipment in the dark are just a few of the areas where an accurate positioning device could be used.

Other Designs
If the medium is know that the sound wave from the transmitter is traveling through then humidity, and temperature calculations could be added to the receiver side of the positioning system.  This could be used to aid in the Neptune project by allowing temperature and current measurements to be obtained without the need for additional instruments.  In addition to this, robotics could aid in the development of this technology.  Using robotics in conjunction with temperature calculations would allow exploration of inhospitable places like volcanoes or other worlds.