Smart Tether for Relative Localization of Moored and Towed Bodies

Determining a geo-reference of an underwater object is a critical requirement for marine applications such as mine-hunting and marine surveying. Although a GPS fix can be obtained at the surface, the watch circle of a tether connecting a surface and underwater body can impose positional errors that are far greater than GPS errors. Here we propose to develop a technology to automatically determine the geodetic location of an underwater body by acoustically measuring the relative position vector to a surface expression, equipped with an acoustic transmitter. The proposed system makes use of vector hydrophones, allowing determination of the bearing of a surface expression relative to a moored body with a single, relatively small sensor. Significantly, the angle of arrival accuracy of vector sensor is nearly independent of frequency, facilitating a compact, low frequency localizing system. This is an important consideration in very shallow water or surf zone regions where bubbles and suspended sediment can have impose enormous high frequency acoustic attenuation. The proposed system is envisioned to provide a low cost, general purpose solution to a host of underwater positioning applications. One important potential application to be investigated is the enabling of a sea-based Differential Global Positioning Base Station.