Magnetic anomaly guidance system for mine countermeasures using autonomous underwater vehicles

This paper describes a magnetic anomaly guidance system that, with support from the Office of Naval Research, is being developed for fully autonomous detection, localization and classification of ferrous mines in Very Shallow Water/Surf Zone (VSW/SZ) environments. The magnetic guidance system\´s hardware configurations and magnetic target signature processing methods specifically have been developed for autonomous guidance of small, maneuverable sensing platforms, such as Underwater Bottom Vehicles, (UBVs), to magnetic targets. The prototype magnetic anomaly guidance is a three-axis magnetometer-gradiometer comprised by an array of triaxial fluxgate magnetometers and triaxial accelerometers. This design allows development of magnetic gradient tensor contraction scalars that provide the basis for a unique method for magnetic guidance of UBVs to underwater and buried mines. The triaxial accelerometer array can be used for mitigation of the target-detection-range-reducing effects of large changes in sensor platform orientation. Field test results indicate that a simple magnetic guidance algorithm based on comparison of the magnitudes of contractions of gradient tensor subsets measured by "primary guidance axes" of the sensor array will provide a robust and practical method for fully autonomous guidance of highmobility UBVs to magnetic targets. Target detection range can be enhanced by using rotationally invariant tensor contractions for robust proximity sensing and "pseudo-robust" scalar total field quantities for "homing in on" magnetic targets. Sensor system stability and DLC performance also would be improved by the use of better, temperature-compensated magnetometers. The results indicate that the magnetic anomaly guidance system and its method for target localization are particularly appropriate for homing in on magnetic targets from high-mobility sensing platforms such as small UBVs or Navy Divers.