The objective of this project 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. The system was to also achieve data density, positional accuracy, and compensation improvement ratios necessary for wide area assessment (WAA) and detailed characterization surveys.
The integrated Autonomous Underwater Vehicle (AUV) Munitions and Explosives of Concern (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 side-scan 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 pre-planned 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.
In March 2012, the system was demonstrated in Tampa Bay near St. Petersburg, Florida. Two 100-m by 100-m test plots were established approximately 3 miles from shore in water 30 ft deep. Each test plot was seeded with inert munitions ranging from 60mm mortars to 155mm projectiles. Data were collected with the AUV MEC Detection System at both test plots at 1.5-m, 2-m, and 3-m altitude above the sea floor and at 2-m line spacing.
The AUV MEC Detection System showed reliable detection of 60mm mortars and larger munitions at 1.5-m altitudes, and 75mm 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 at the time of the demonstration. Offsets were less than 0.5-m where survey lines crossed seed item locations. No net drift of the navigation solution was observed during survey missions, thus confirming target positional accuracy of less than 1-m is achievable. Vehicle dynamic performance objectives for bottom keeping, pitch, roll, and along-line data density were achieved.
Considerable suppression of system noise was realized using upgraded compensation software. The most prominent magnetic distortions in the survey data correlated with vehicle pitch and heading. Post-process corrections yielded improvement ratios from 5.1 to 7.6 in the calibration grid and 11 to 12.4 in the blind grid.
Daily operational costs for this demonstration totaled approximately $5,300/hectare of survey data collection and processing. There was a daily recurring cost of $400 for instrument setup and preparation.
Several advantages were attained as a result of the modular design, autonomous capabilities, and rapid deployment of the AUV MEC Detection System, including ease of use and application in a broad range of environments as compared to current towed array marine detection systems. This autonomous and self-contained system can provide cost savings over current systems by reducing the mobilization/demobilization effort, requiring less manpower for operation, and reducing the need for a large surface support vessel altogether. This commercial off-the-shelf AUV MEC Detection System shows improved efficiency, safety, and cost savings compared to current systems for WAA and detailed characterization surveys.
The components used to develop the magnetometer module are primarily commercially available; however, their integration and operation was customized for the purposes of this demonstration. Issues including excess survey coverage to achieve full coverage requirements and across-line spacing limitations associated with commercially available mission planning software need careful consideration when selecting this technology and developing missions. A quality control program specific to underwater surveys that verifies navigation accuracy, detection capabilities, and system operation will need to be created for regulatory approval.
Due to the operational complexities associated with the operation of the Gavia AUV, personnel require specialized training in properly assembling, configuring, and operating the equipment to perform detection surveys. Mission plan creation, data transfer, and communication with the AUV, as well as monitoring of the AUV during surveys are all tasks that require specialized training.