Initial evaluation of the new real-time tracking gradiometer designed for small unmanned underwater vehicles

The shallow water localization of buried mines places increased emphasis on sensor and sensor platform size and maneuverability. The Office of Naval Research (ONR) has funded a number of projects to develop efficient buried minehunting capabilities. In particular, they are supporting the development of two Unmanned Underwater Vehicles (UUVs) designed specifically for shallow water minehunting. In addition, the ONR has funded the development of two magnetic sensors for use on these UUV's. This paper documents the recent progress that has been made in operating one of these sensors, the uuv-Real-time Tracking Gradiometer (uuv-RTG) underwater and in motion. The new uuv-RTG, with a 165 mm baseline, consists of four, 3-axis fluxgate magnetometers, each located within a 3-axis Helmholtz coil. The coils on the fourth magnetometer are not connected to allow its use as a reference sensor that provides magnetic feedback to the other three sensors. The feedback developed by the reference sensor is used to null the signals caused by motion, allowing the three primary sensors to operate at low noise levels in large magnetic fields. These three magnetometers are then used to develop six gradients, five of which are independent. With the appropriate software algorithms, this system is able to provide useful range and bearing to ferromagnetic targets, as well as their magnetic moment. Initial evaluation of RTG technology underwater was accomplished by installing a field prototype RTG (RTG-fp) with a 305 mm baseline, onboard a towed, low mag sled designed by Florida Atlantic University (FAU) for their Buried Object Scanning Sonar (BOSS II). The RTG-fp operating in unison with the BOSS II system on the BOSS sled, provided limited but useful data regarding its performance and ability to be fused with a sonar, such as the BOSS II. Prior to their planned integration in late 2005, the new generation uuv-RTG and the BOSS III are currently undergoing performance testing on individual AUVs called Bluefin 12s. Operating a magnetic sensor onboard a small AUVs such as the 324 mm diameter Bluefin 12 is especially difficult. This is due, not only to the close proximity of the magnetometers to multiple magnetic interference sources that are integral to the vehicle, but also to magnetic interference generated by other sensors that are present and share the same payload section. An extensive noise mitigation package consisting of three, 3-axis magnetometers and 2 current sensors were installed on the Bluefin 12 to mitigate this problem. The uuv-RTG is currently being magnetically characterized. Once the characterization is completed, it will be integrated with BOSS III on a winged version of the Bluefin 12. Here it will be evaluated both independently and in fusion with the BOSS III.