Feature ArticleEnabling New Operational Scenarios In Mine Countermeasures
Chief Executive Officer
Studio Ing. Banfi SAS
Most of the world's navies are increasingly adopting light autonomous underwater vehicles (AUVs) to extend their mine countermeasures capabilities. A light AUV can be easily shipped to remote operational areas and quickly deployed from any small boat. In this way, mine search capabilities are greatly extended far beyond the reach of any dedicated naval vessel at a fraction of the cost and with reduced risk to human life.
The weak point of this operational concept, however, is that an AUV only provides search capabilities. The final result of an AUV survey is a list of possible targets and their geographical coordinates: Each suspected target still needs to be identified and, if positively recognized as a mine, properly neutralized. Currently, identification and neutralization operations are still conducted with dedicated naval vessels, whose transit time, cost and manning needs greatly limit the operational flexibility enabled by AUVs.
In order to take full advantage of the operational benefits of AUVs, identification and neutralization operations should be carried out from the same small boat used for AUV operations. This could be done with a remotely operated vehicle (ROV), but some conflicting technical requirements apply.
First, the ROV must be sufficiently small and light to allow an easy deployment, with launch, recovery and operational demands comparable to those of the AUV. Despite its size, however, the ROV must also be able to be precisely guided to the specific latitude and longitude target positions identified by the AUV. Finally, the ROV's navigation system should be fully independent of external guidance references, such as acoustic beacons or sonar systems that would still require the presence of a mother ship.
The ROV & Diver TRACKER, recently developed by Studio Ing. Banfi SAS, provides an easy solution to all the above requirements, giving any ROV, even the smallest, precise navigation capabilities independent from any external reference.
Empowering Small ROVs and Divers
The ROV & Diver TRACKER is a self-contained, small and neutrally buoyant navigation instrument that can be easily attached to any ROV. The system is composed of an underwater unit and a surface-operated personal computer that communicate through the umbilical cable of the ROV.
The underwater unit is available in two basic configurations, vertical or horizontal, to fit the installation requirements of any ROV. The underwater unit can also be provided in split, custom-made pieces to be integrated in the ROV, preserving the vehicle's external shape. Whichever configuration is chosen, the ROV only needs to provide a small amount of power and data-transfer capability through its umbilical cable.
The surface computer can be any Windows-based laptop (even the same one used to run the AUV mission) and is used to display the ROV's position in real time, together with any other useful navigation information. Following the ROV on the computer screen, the operator can drive the vehicle to the suspected mine target, perform limited search operations, drop countermine charges and, in general, accomplish the same operations typically carried out from on board a mine-hunting ship.
In addition, when a mission requires human intervention, the TRACKER can be removed from the ROV, outfitted with handles and used by a diver. Just as in the ROV operations, navigation data are transmitted at the surface through the diver's tether and the boat operator can guide the diver just like the ROV.
The navigation principle that allows the ROV & Diver TRACKER to attain good position accuracy without an external reference is Doppler-assisted dead reckoning. With this technique, typically used by AUVs, the TRACKER autonomously acquires a global positioning system (GPS) fix while still at the surface, or, alternatively, receives a manual position entered by the surface operator. With these initial coordinates, once submerged, the TRACKER continuously updates its position by measuring the distance from the starting point, referencing its movements to the seafloor. For this purpose, the TRACKER uses a state-of-the-art Dop'pler velocity log (DVL) that provides precise bottom tracking data.
The described navigation, independent from any external acoustic reference, provides relatively discreet navigation, as the only emissions radiated by the TRACKER are the acoustic pulses of the DVL. These pulses are transmitted with such low power and at such a high frequency that any acoustic emission is completely attenuated a few meters away from the main beams. The TRACKER was also designed to have a minimal magnetic signature.
The TRACKER provides its best navigation precision in shallow and very shallow waters. The incorporated DVL allows the TRACKER to solidly track the bottom at water altitudes ranging between 70 meters and 25 centimeters. Under these conditions, the TRACKER attains a positioning precision equal to one percent of the total distance run from the initial deployment position (the GPS or operator's fix).
This highly accurate shallow-water navigation makes the TRACKER an ideal instrument for military or commercial ROV operations in coastal, harbor or confined waters where, due to high bottom scattering, acoustic positioning systems are not an option.
For depths of more than 70 meters, the TRACKER still provides accurate navigation by referencing its motion to a wide layer of water and automatically applying proper corrections to account for the effect of water currents, if a previous bottom track is available.
Extending Underwater Missions
In either shallow or deep waters, the ROV operator always has the option of surfacing the ROV to bring the real-time navigation error back to zero: As it surfaces, the TRACKER automatically acquires a new GPS fix and instantly resets its position, at which point the ROV can dive again and resume its operations. Alternatively, while maintaining the ROV underwater, the operator can also enter the position of a point of known location recorded by the ROV camera (such as the location of a presurveyed target) to reset the vehicle's position.
These position resets usually introduce a discontinuity in the position track. In order to smooth it out, the TRACKER's software provides the ability to 'renavigate' the track. The available renavigation tools allow all track fixes to be repositioned so as to take into account all of the information that became available after the occurrence of any given position fix.
This tool, besides allowing the TRACKER to be used for mapping purposes, allows the instrument to identify underwater references with extreme accuracy, which can be later used for resetting its position while still underwater. In this way, even in cases of extended operations in an area where no underwater reference was previously known, very few surfacings are required to keep navigation precision within required limits.
By allowing full and accurate ROV guidance from the same small boat that operates AUVs for mine countermeasures, the ROV & Diver TRACKER enables virtually any size ROV to be used for mine identification and clearance purposes.
A light AUV and a TRACKER-equipped ROV can therefore be operated jointly to create a complete and fully operational mine countermeasure unit. As such, the unit can be quickly and easily shipped and deployed in any part of the world, enabling rapid response to mine threats on a global scale.
After earning a master's degree in electronic engineering and working for 16 years as an Italian navy officer, Maurizio Banfi founded Studio Ing. Banfi SAS and developed the ROV & Diver TRACKER.