| Underwater mine detection and removal is of great importance for both military and commercial fleets. Currently, trained dolphins locate possible mines in the water column and attach a marker buoy. Divers then deploy satchel charges at the suspected sights at obvious personal risks. Autonomous underwater vehicles (AUVs) are currently being tested to replace divers in this effort, and they would track acoustic pingers to reach the possible mine location. Beacon tracking by autonomous underwater vehicles is clearly a vital component in this application. In this research we designed and constructed an ultra short base-line (USBL) passive sonar used to provide a method of bearing estimation. An acoustic pinger and a passive sonar were designed to operate at 10:5 kHz. The sonar employs a unique three-element, L-shaped array with a 1=4 wavelength hydrophone separation. The acoustic pinger and sonar also provide a noncoherent 4-FSK downlink to the AUV for command and control applications. The power consumption of the passive sonar including the DSP processor is less than one-half Watt and has a form factor of less than 50 cm3, enabling power-efficient, compact deployments. A maximum-likelihood based estimator was developed to estimate the bearing angles of the acoustic pinger. The estimator is immune to multipath for delay spreads not exceeding 250ms. Angle estimates are produced by a coarse initial grid search followed by several Newton-Raphson iterations. Simulation results indicate that this system can estimate bearing angles to less than 1 degree of standard deviation with a received signal-to-noise ratio of 6 dB. |
