Superconducting magnetic gradiometers for underwater target detection

Passive magnetic sensors provide one means to conduct mobile area surveys and search operations for the detection of magnetic anomaly targets. Sensors incorporating Superconducting Quantum Interference Devices (SQUIDs) provide the greatest sensitivity for magnetic anomaly detection available with current technology. A superconducting magnetic-field gradiometer developed in the 1980's has been demonstrated in fusion with acoustic sensors to enhance shallow water sea mine detection and classification, especially for buried mine detection and the reduction of acoustic false alarm rates. This sensor incorporated niobium bulk and wire superconducting components cooled by liquid helium to a temperature of 4 degrees Kelvin (K). An advanced superconducting gradiometer prototype is being developed to increase sensitivity and detection range. This sensor features all thin film niobium superconducting components (replacing the bulk superconducting components used in the preceding generation) and a new liquid helium cooling concept. In the late 1980's, a new class of "high Tc" superconductors was discovered with critical temperatures above the boiling point of liquid nitrogen (77K). The use of liquid nitrogen refrigeration offers new opportunities for this sensor technology, providing significant reduction in the size of sensor packages and in the requirements for cryogenic support and logistics. As a result of this breakthrough, a high Tc sensor concept using liquid nitrogen refrigeration has been developed and a test article of that concept is being fabricated and evaluated. In this paper, these recent advances in sensor development will be reviewed and a current perspective on the role of magnetic sensors for underwater target detection, classification and localization will be addressed.