Tech Briefs

AUV uses a magnetometer to detect dangerous submerged munitions.

The objective of Environmental Security Technology Certification Program (ESTCP) Project MR-201002, Autonomous Underwater Vehicle (AUV) Munitions and Explosives of Concern (MEC) Detection System, was to integrate an untethered and unmanned underwater vehicle with a total field magnetometer for underwater munitions detection and upgrade magnetic noise compensation software to reduce interference from electrical and dynamic influences such as vehicle heading, pitch and roll.

Figure 1. Gavia AUV specifications

The integrated AUV MEC Detection System consists of a high sensitivity Geometrics G-880AUV cesium vapor magnetometer integrated with a Teledyne-Gavia AUV and associated Doppler-enabled inertial navigation system, acoustic bathymetric, and sidescan imaging modules. Total field magnetic measurements are recorded with asynchronous time-stamped data logs that include position, altitude, heading, pitch, roll, and electrical current usage. Surveys are performed by using preplanned mission information including speed, height above seafloor or depth, and lane or transect spacing.

Magnetic compensation software was concurrently developed to accept electrical current measurements directly from the Gavia AUV to address distortions from permanent and induced magnetization effects on the magnetometer. Maneuver and electrical current compensation terms can be extracted from the magnetic survey missions to perform post-process corrections.

The AUV MEC Detection System consists of the following primary components: a Teledyne-Gavia model autonomous underwater vehicle, a magnetometer module, and magnetic compensation. The Gavia AUV is a modular underwater robotic system that follows a pre-programmed course, collecting environmental data in situ. Missions are planned using a graphical user interface (GUI) to specify waypoints or survey lines, prescribed depths or altitudes, and desired sensor configurations. The Gavia base vehicle is a mobile sensor platform that can be userconfigured on deck for a particular task or operating condition by the addition of one or more sensor, navigation, or battery modules.

The Gavia AUV is navigated by a Kearfott T-24 “SEANAV” inertial navigation system (INS). While on the surface, a Wide Area Augmentation System (WAAS)-capable Global Positioning System (GPS) in the AUV’s sail provides position fixes to the INS. In addition, when within range of the bottom (< 40 meters [m]), an RD Instruments 1,200 kilohertz (kHz) Workhorse Navigator Doppler velocity log (DVL) measures velocity of the vehicle over the seafloor and provides these measurements to the INS.

The Gavia AUV has a maximum depth rating of 500 m. Additional standard sensors aboard the AUV include speed-of-sound, temperature, salinity (derived), dissolved oxygen, chlorophyll-a, and turbidity, as well as a 900 kHz/1,800 kHz side-scan sonar. Other modules available to the AUV include a sub-bottom profiler, downward-looking camera with strobe, and bathymetric sidescan sonar.

Figure 2. Magnetometer module design

The magnetometer module design schematic is presented in Figure 2. The module flooded section houses the G-880AUV total field magnetometer. The sealed pressure vessel contains the G-880AUV electronics and Applied Physics 539 fluxgate compass (fluxgate). The sensors are interfaced with the magnetometer module circuitry, which is necessary to provide internal electrical power and communication with the vehicle’s control system through the AUV microcontroller “rabbit board.” A relief slot cut in the flooded section is used to facilitate G-880AUV sensor orientation requirements.

The AUV MEC Detection System showed reliable detection of 60 mm mortars and larger munitions at 1.5 m altitudes, and 75 mm projectiles and larger munitions at altitudes over 2 m. Average offsets between the known and measured locations of seed items ranged between 0.7 m and 1.8 m depending on the mission design and is a function of mission planning software. Offsets were less than 0.5 m where survey lines crossed seed item locations.

This work was done by Art Trembanis and Nicole Raineault of the University of Delaware; Val Schmidt of the University of New Hampshire; George Tait and Misha Tchernychev of Geometrics, Inc.; Brian Junck of Weston Solutions, Inc.; and John Kloske of SRI International for the Environmental Security Technology Certification Program. ESTCP-0001

This Brief includes a Technical Support Package (TSP).

Autonomous Underwater Munitions and Explosives of Concern Detection System (reference ESTCP-0001) is currently available for download from the TSP library.

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