SUMMARY

With increased utility of Autonomous Underwater Vehicles (AUVs) for undersea exploration and sensing, it becomes imperative that an efficient navigation system be developed to accurately inform an AUV of its precise spatial location and bearing. While dead reckoning methods using Inertial Measurement Units (IMUs) on-board AUVs are commonly used, these techniques accumulate significant drift error over time, which make them insufficient in providing support for tasks which require accurate positioning, such as AUV docking. To improve AUV positioning accuracy, optical based localization techniques and active acoustic beacons have been used in the past. However, optical methods have very limited range, while active acoustic beacons are expensive, non-covert and require significant calibration and maintenance. This research therefore explores the design and analysis of passive acoustic tags, similar in concept to RFID tags, with engineered acoustic signatures which can be detected using Sound Navigation and Ranging (SONAR) instrumentation on board an AUV. The tags are designed using horizontally stratified materials with different acoustic properties and thicknesses, such that they reflect a unique acoustic signature upon insonification by SONAR, similar in operation to a bar code. In order to obtain tags with an azimuthally invariant scattered response, layered tags with curved symmetry are also proposed. A simulation framework and experimental verification of the operation of both flat and hemispherical tags are presented, along with a discussion of alternate tag designs. AUV triangulation schemes using a constellation of tags are also explored as a part of this research.