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Case study

Mine-hunting building blocks: Equinox

Improved MCM operations using multi-aperture sonar

August 26, 2021
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Innovators are being challenged by evolving requirements from world navies to improve our mine-hunting capabilities. Sometimes the solution is already there, all you need to do is piece it together, says Head of Defence, Ioseba Tena.

Mine-hunting requirements are well understood. Navies want to improve percentage clearances, minimise risk by removing more sailors from the minefield, and get the job done quicker. Against a backdrop of declining budgets, it’s easier said than done.

Although the approach to solving the challenge differs from navy to navy, the number of uncrewed systems being developed in support of mine countermeasure (MCM) operations is on the increase.

First to deploy these operationally was the US in Operation Iraqi Freedom. The US Navy deployed early versions of the Mk 18 Mod 1 “Swordfish” underwater vehicle in support of mine clearance operations. Since then, many navies have acquired small, man-portable systems to conduct operations in very shallow waters and larger systems for operations in deeper waters. A recent example is the Royal Australian Navy’s SEA 1778 programme which delivered General Dynamics Mission Systems’ Bluefin-9 and the medium-sized Bluefin-12 unmanned underwater vehicles (UUVs). In total, seven systems based at HMAS Waterhen in Sydney are now used to search for, classify and identify sea mines. These systems are all fitted with our Solstice side-scan sonar as standard, as well as an AvTrak 6 from our family of transceivers.

Their performance has demonstrated that ocean robots have a significant role to play in MCM missions. Underwater vehicles succeed because they provide a stable platform in which to mount payload sensors and do a great job of surveying the seafloor, at a constant height from the sea bottom. But, while operating, their data is stored until the system can be recovered and the data uploaded to a computer.

Uncrewed surface vessels (USVs) complement their underwater cousins, as they offer real-time access to data, enabling a faster tempo for operations. Just like the underwater vehicles, to be successful a USV needs to provide a stable platform for its payloads and get them close to the seafloor. And that is where Equinox comes in.

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Equinox

Solstice

SPRINT

Mini-Ranger 2 USBL

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Solstice Multi Aperture Sonar

The first building block used to make Equinox is Solstice. Our Solstice was designed by our sister company, Wavefront, to deliver the ultimate mine-hunting side-scan sonar. Solstice uses a proprietary array design and multiple apertures to improve the signal-to-noise ratio performance, capturing stunning imagery at longer ranges than other sonars at the same frequency. Typically operating at 750 kHz it scans a 200 m swath and delivers a stunning along-track resolution of 0.15°. This makes it the ideal MCM sonar.

Through several experiments Solstice has consistently shown itself capable. Some recent examples in the public domain include a series of exercises conducted by GD Mission Systems. In these, a Bluefin-9 with Solstice on board was used to detect and map the location of lobster pots. These man-made targets are typically smaller than mines so constitute a great target. The data quality from Solstice made it possible to both detect and identify the lobster pots across the whole 200 m swath.

However, to make Solstice work when deployed from a USV several technologies need to come together. First and foremost, it needs a stable platform capable of keeping Solstice at a set height from the seabed as it travels through the water. Introducing ScanFish 3D, the next building block.

EIVA’s ScanFish 3D

Our other sister company EIVA manufactures ScanFish 3D. The ScanFish 3D is a trusted ROTV platform capable of sailing through the water at a constant height from the seafloor. ScanFish 3D has been designed to be both very stable when surveying and highly dynamic when needing to surface at high speed. By sailing through the water while adjusting its trim, there is no need to constantly pay cable in or out to adjust the height. ScanFish 3D is stable enough for Solstice to excel. In addition, it can carry a significant payload, meaning it can be equipped with navigation instruments to accurately geo-reference the data. From its early trials, ScanFish 3D was able to demonstrate performance similar or better to an AUV, keeping an almost constant heading, pitch and roll as the AUV gathered data.

This means that the sonar data can be acquired at the sweet spot for the sensor at 7.5m from the seafloor. Next, the data needs to be georeferenced. This leads us to the next Equinox building block, SPRINT INS.

SPRINT INS minimises position errors

Typically, navies require a 5 m (DRMS) absolute position error for any contact surveyed during mine-hunting operations. The requirement is driven by both the technical limitations of the solutions used to date and the assumption that to re-acquire a target within that position error is achievable. However, in challenging environments with poor visibility, or environments with clutter or significant numbers of contacts, a 5 m error translates as time wasted in the actual process of re-acquiring and identifying the contact.

Equinox uses commercial-off-the-shelf (COTS) instruments to solve this problem. Using a SPRINT INS aided by a Doppler velocity log and a Mini-Ranger 2 ultra-short baseline (USBL) system, Equinox can localise targets within 1 m (DRMS). This means less uncertainty in MCM operations, improved tactics and improved tempo of operations.

Inertial navigation is inherently self-contained, robust and with very good short-term accuracy, but it can drift over time. Therefore, the INS is aided with complementary acoustic positioning, i.e. the DVL and USBL, to provide long-term accuracy and robustness. Conversely, the additional integrity offered by integration of the INS significantly reduces operational delays during periods of challenging subsea acoustic conditions, such as aeration and noise. In addition, the heading accuracy reported by SPRINT minimises lever-arm effects for targets at the furthest range from the sonar. This is a large source of error in towed sonar systems.

We have conducted an extensive trial to test the navigation accuracy achieved by this combination of instruments, forming Equinox. Two bicycles were lowered to the seafloor. Why two bicycles? They make an interesting target for sonar training. They were placed at known positions and were used to establish the navigation accuracy of the system. The results demonstrated the performance to be less than 1 m (1 DRMS), as expected.

To recap Equinox combines Solstice with a ScanFish 3D and a SPRINT INS aided by a DVL and USBL system. There is just one last building block missing, the software to stich it all together.

NaviSuite Kuda

Completing the system is NaviSuite Kuda software, also developed by EIVA, which is used to plan, monitor, and process the data from the Equinox system. NaviSuite Kuda is another COTS system that is used by hydrographers world-wide to run their underwater projects.

To plan a survey, the user defines the sailing route and run lines by simply selecting the area that needs to be surveyed. The software generates a plan that can be fed directly to the USV control software (or to a skipper if using a crewed vessel).

During the mission, NaviSuite Kuda continuously updates the vessel and ScanFish 3D position in real-time. The navigation and Solstice data is processed and displayed on the screen enabling an operator or automatic target recognition (ATR) algorithm to review it as it is being gathered. The information can then be used to inform the next stage of the mission while the survey is taking place.

Equinox also provides real-time geo-rectified waterfall, mosaics and DTM maps and the user interface tools that enable an operator to mark and process objects, including automatic target recognition using AI.

Commercial-off-the-shelf

The critical advantage that Equinox offers over competing systems is that it is made from combining products that retail as COTS. This means its component parts have been deployed in thousands of mobilisations; Equinox selects best of breed COTS to deliver a step up in capability. It delivers improved performance compared to standard side-scan sonar systems, improved navigation accuracy several times better than any other system in the public domain and at a price an order of magnitude lower than competing products that have been specifically designed to conduct mine-hunting operations. For a navy this means an ability to mobilise a capability quickly, using components that can be easily looked after and maintained and feeding into existing training programmes and a community of existing users.

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