Preliminary evaluation of the laser scalar gradiometer for buried minehunting

Fusion concepts using a magnetic sensor in combination with acoustic and optical sensors operating onboard an unmanned underwater vehicle (UUV) are under consideration to reacquire and confirm buried contacts detected in an initial sonar search. Two magnetic sensors are currently being developed by the Office of Naval Research for buried minehunting (BMH): Polatomic's Laser Scalar Gradiometer (LSG) and Quantum Magnetics' Realtime Tracking Gradiometer (RTG). The LSG is a multi-channel electron-spin resonance scalar magnetometer/gradiometer, while the RTG is a multi-channel tensor gradiometer using fluxgate technology. In this paper, we will describe progress in the development and testing of the LSG. The operation of the LSG is based on the opto-magnetic properties of helium-4 gas in accordance with the Zeeman effect. The LSG and its predecessor, the P-2000, have attained increased sensitivity over comparable sensors by the use of a laser in place of incoherent light for optical pumping. The LSG employs four helium sense cells configured in a volume-filling arrangement to measure four independent channels of information: the scalar field magnitude and admixtures of the three components of the scalar-field gradient vector. The P-2000 electronics consists of a mixture of discrete analog and digital circuits that requires three rack-mounted units. To satisfy the size and power requirements for operation in small UUVs and the requirement for autonomous sensor operation, the LSG electronics features integrated digital surface mount technology. In initial testing, preliminary LSG performance has been measured by acquiring data with the sensor stationary and in motion in a magnetically quiet environment. In addition, a tracking experiment has been conducted to validate algorithms for target detection, classification and localization (DCL). Work is in progress to optimize the performance of the LSG. When this work is completed, the LSG will be integrated into the REMUS 600 developed by Woods Hole Oceanographic Institute (WHOI) and experiments over target fields at sea will be conducted. This paper reviews the LSG design, presents results from the initial land-based testing, and describes plans for system integration and underwater experiments conducted with the LSG onboard REMUS 600.