Demonstration of the Real-Time Tracking Gradiometer for Buried Mine Hunting While Operating From a Small Unmanned Underwater Vehicle

In many undersea conditions, optical sensors and sonar can be used to discriminate against sea mines. However, there are many conditions where these sensors are insufficient. For example, when a mine is fully buried these sensors are of little help. Under these conditions, additional sensor technologies are required. Since it is not affected by the medium, a technology of choice is magnetics. In the late 1990s a "T-shaped" gradiometer with a 12-inch baseline was developed. It became known as the RTG. Measurements performed in the nonmagnetic facility at the Naval Surface Warfare Center Panama City (NSWC-PC) demonstrated good localization capabilities and it was selected to become part of an ONR initiative to replace the human diver with Unmanned Underwater Vehicles (UUVs) using custom designed payload modules in high-risk mission areas. In the early 2000s the land-based RTG was refitted for underwater applications and integrated with the Florida Atlantic University\´s Buried Object Scanning Sonar (BOSS). Both were operated from a towed nonmagnetic sled where they demonstrated the ability to localize on buried undersea magnetic targets. The collection of simultaneous magnetic and acoustic data provided the opportunity to apply sensor fusion. While the towed nonmagnetic sled was an ideal magnetic platform, it was unsuited for the shallow water operations required by the Navy. In response to those requirements, both RTG and BOSS were redesigned to fit on newly developed UUVs, such as the 12.75"-diameter Bluefin 12. As expected the UUVs magnetic platform noise level was considerably higher due to the increased number of magnetic noise sources on an active autonomous vehicle and the closer placement of the RTG to these noise sources. To mitigate this increased noise, a magnetic noise cancellation system using magnetometers and current sensors, strategically placed within the control section of the UUV, was implemented. The initial underwater shake down of this entirely n- - ew system occurred in August 2005. This demonstrated, for the first time, autonomous control of the RTG by the Bluefin 12. Sea tests continued during 2006, collecting simultaneous data from the RTG, BOSS and a simple optical camera. These co-registered data have been used to demonstrate the common detection and localization of buried targets. This paper focuses on the 2006 sea testing of the system and the initial analysis of the data from the fluxgate-based RTG